An integral field spectroscopic survey for high redshift damped
  Lyman-alpha galaxies
Astronomy & Astrophysics manuscript no. ms6410 c© ESO 2018
October 22, 2018
An integral field spectroscopic survey for high redshift damped
Lyman-α galaxies ⋆
L. Christensen1 , L. Wisotzki2, M. M. Roth2, S. F. Sánchez3, A. Kelz2, and K. Jahnke4
1 European Southern Observatory, Casilla 19001, Santiago 19, Chile, e-mail: lichrist@eso.org
2 Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
3 Centro Astronómico Hispano Alemán de Calar Alto, Calle Jesús Durbán Remón 2,2 E-04004 Almerı́a, Spain
4 Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
Received/accepted
ABSTRACT
Aims. We search for galaxy counterparts to damped Lyman-α absorbers (DLAs) at z > 2 towards nine quasars, which have 14 DLAs
and 8 sub-DLAs in their spectra.
Methods. We use integral field spectroscopy to search for Lyα emission line objects at the redshifts of the absorption systems.
Results. Besides recovering two previously confirmed objects, we find six statistically significant candidate Lyα emission line objects.
The candidates are identified as having wavelengths close to the DLA line where the background quasar emission is absorbed. In
comparison with the six currently known Lyα emitting DLA galaxies the candidates have similar line fluxes and line widths, while
velocity offsets between the emission lines and systemic DLA redshifts are larger. The impact parameters are larger than 10 kpc, and
lower column density systems are found at larger impact parameters.
Conclusions. Assuming that a single gas cloud extends from the QSO line of sight to the location of the candidate emission line, we
find that the average candidate DLA galaxy is surrounded by neutral gas with an exponential scale length of ∼5 kpc.
Key words. galaxies: formation – galaxies: high redshift – galaxies: quasars: absorption lines
1. Introduction
Galaxy counterparts to Damped Lyman-α systems (DLAs) seen
in quasar (QSO) spectra have been suggested to be (proto)-disk
galaxies with line of sight clouds of neutral gas with column
densities N(H i) > 2 × 1020 cm−2 (Wolfe et al. 1986). Analyses
of absorption line profiles have indicated that rotational compo-
nents with velocities of ∼200 km −1 can be involved in these
systems suggesting that DLAs reside in large disk galaxies
(Prochaska & Wolfe 1997; Ledoux et al. 1998a). On the other
hand, numerical simulations show that in a hierarchical forma-
tion scenario merging proto-galactic clumps can also give rise to
the observed line profiles (Haehnelt et al. 1998).
A large fraction of the neutral hydrogen present at z > 2 is
contained in high column density DLA systems (Lanzetta et al.
1995; Storrie-Lombardi & Wolfe 2000; Péroux et al. 2001). In
addition to the classical DLAs, clouds with column densities
1019 < N(H i) < 2 × 1020 cm−2 also show some de-
gree of damping wings, which is characteristic of DLA sys-
tems. It is suggested that sub-DLA systems contain a signifi-
cant fraction of the neutral matter in the Universe (Péroux et al.
2003). Metallicity studies have shown that the properties of
the sub-DLA systems are similar to those of DLA systems
(Dessauges-Zavadsky et al. 2003), apart from the latter category
having large ionisation corrections (Prochaska & Herbert-Fort
2004).
Send offprint requests to: L. Christensen
⋆ Based on observations collected at the Centro Astronómico
Hispano Alemán (CAHA), operated by the Max-Planck Institut für
Astronomie and the Instituto Astrofisica de Andalucia (CSIC).
The association of DLAs with galaxies has been a subject of
much study. Originally, either space-based or ground-based deep
images were obtained to identify objects near the line of sight to
the QSOs (Steidel et al. 1995; Le Brun et al. 1997; Warren et al.
2001). To confirm nearby objects as galaxies that are respon-
sible for the DLA lines in the QSO spectra, follow-up spectra
are needed to find the galaxy redshifts. At z < 1, confirmations
of 14 systems exist to date (Rao et al. 2003; Chen & Lanzetta
2003; Lacy et al. 2003; Chen et al. 2005, and references therein),
while at z & 2 only 6 DLA galaxies are confirmed through spec-
troscopic observations of Lyα emission from the DLA galax-
ies (Møller & Warren 1993; Djorgovski et al. 1996; Møller et al.
1998; Leibundgut & Robertson 1999; Møller et al. 2002, 2004),
three of which are located at the same redshifts as the QSOs
themselves. Other techniques to identify DLA galaxies involve
narrow-band imaging (e.g. Fynbo et al. 1999, 2000) or Fabry-
Perot imaging. A Fabry-Perot imaging study of several QSO
fields did not result in detections of emission from DLA galax-
ies (Lowenthal et al. 1995), while recently the same method was
used to identify a few emission line candidates (Kulkarni et al.
2006).
Integral field spectroscopy (IFS) presents an alternative that
provides images and spectra at each point on the sky simultane-
ously. This technique can be used to look for emission line ob-
jects at known wavelengths, but unknown spatial location. This
technique is ideally suited to look for Lyα emission lines from
the galaxies responsible for strong QSO absorption lines. At the
Lyα wavelength corresponding to the redshift of the DLA sys-
tem, the QSO emission has been absorbed, enabling us to lo-
cate emission line objects very near to the QSO line of sight.
Because of the large column densities in DLAs and the res-
http://arxiv.org/abs/0704.0654v1
2 L. Christensen et al.: An IFS survey for high-z DLA galaxies
onant nature of Lyα photons the corresponding emission line
may be offset in velocity space relative to the DLA line (e.g.
Leibundgut & Robertson 1999), but such an offset is not always
observed (e.g. Møller et al. 2004).
IFS searches for emission from DLA galaxies towards
two QSOs have resulted in upper limits for their fluxes
(Petitjean et al. 1996; Ledoux et al. 1998b), while a sub-DLA
galaxy previously known to be a Lyα emitter was confirmed
with IFS (Christensen et al. 2004). Here we present a survey us-
ing IFS to look for Lyα emitting DLA galaxies. Section 2 de-
scribes the sample of QSOs included in the survey, which are
known previously to have DLAs and sub-DLAs in their spec-
tra. Section 3 describes the observations and data reduction. In
Section 4 the method of detecting emission line candidates is
described. Section 5 presents the results and comments on each
object. Properties of the Lyα emission candidates detected in
the survey in relation to the six previously known Lyα emitting
DLA galaxies are presented in Section 6. Section 7 summarises
our findings. A flat cosmology with H0 = 70 km s
−1 Mpc−1,
Ωm = 0.3, and ΩΛ = 0.7 is used throughout.
This study, as well as previous ones that try to identify the
host galaxies of DLA systems, can be biased since the galaxy
observed at the right redshift likely belongs to the brightest emis-
sion line object close to the line of sight. In the case that the
host galaxy is a much fainter galaxy in a group, it will not be
identified correctly. In the remaining part of the paper, an ‘iden-
tified’ DLA galaxy refers to observations that show (line) emis-
sion from independent observations, while the ‘candidates’ are
only reported in these IFS observations. Although extensive tests
are done on the data to distinguish the candidates from potential
artifacts, independent observations are needed to prove them as
Lyα emitters connected with the DLAs.
2. Sample selection
We selected a number of DLA systems without previous de-
tections of associated Lyα emission. The selected QSOs with
known DLAs were chosen based on the following criteria
1. N(H i)> 2 × 1020 cm−2
2. DLA redshift (2 < z < 4)
3. Northern hemisphere object
The first criterion includes only classic DLA systems. Many
QSO spectra show additional sub-DLA systems. Although their
relationship to DLAs is still debated, these systems were in-
cluded in the survey because of their probable physical associa-
tion with galaxies.
To increase the sample size with a minimum number of
pointings we preferentially selected QSOs with multiple DLAs.
IFS covers a range of wavelengths, and correspondingly Lyα
emission at a large range of redshifts in the line of sight for each
QSO. However, in retrospect, this can affect the emission line
detections, because extinction in foreground DLAs could ob-
scure emission from background ones when the galaxies lie in
the same line of sight. Hence, upper limits on detections of the
higher redshift systems can be biased.
From the list of DLA systems compiled by S. Curran1,
we found 66 QSOs matching these criteria in 2003. More re-
cently, detections of DLAs in the Sloan Digitized Sky Survey
QSO spectra have greatly increased the number of known DLAs
(Prochaska & Herbert-Fort 2004; Prochaska et al. 2005). A sys-
tematic survey of all 66 objects would require a large amount
1 http://www.phys.unsw.edu.au/∼sjc/dla/
of time with present instruments, so we selected a few systems
based on their observability during the allocated observing runs.
We avoided DLAs with Lyα absorption lines close to sky emis-
sion lines.
The total sample consists of 9 QSOs with a total number of
14 DLA systems as listed in Table 1. These QSOs have an ad-
ditional 8 sub-DLAs which are included in the survey. Because
of the small number of DLAs involved in the survey, a proper
statistical study is not the aim of this paper. Instead we focus
on a few systems to exploit the benefits of IFS for this kind of
investigation.
To study the applicability of IFS in identifying DLA galax-
ies we initially observed DLA galaxies where Lyα emis-
sion had been reported previously in the literature. Two of
these systems could be observed during our runs; Q2233+131
and PHL 1222, originally identified by Steidel et al. (1995),
Djorgovski et al. (1996) and Møller et al. (1998). Both objects
are reported to have extended Lyα emission (Fynbo et al. 1999;
Christensen et al. 2004). Table 1 includes these two previously
known Lyα emitting DLA and sub-DLA galaxies, although the
criteria listed above are not satisfied. The absorption system to-
wards Q2233+131 has a column density that classifies it as a
sub-DLA. Unless otherwise noted, these two objects are kept
separate from the detection of candidate emission line objects in
the remainder of the paper.
Most of the DLAs in the IFS study lie towards bright QSOs
(R < 19). This ensured that the PSF variations as a function of
wavelength could be determined, which was necessary for the
subtraction of the QSO emission. Bright QSOs had larger resid-
uals from the subtraction of the continuum emission, which po-
tentially affected out ability to recover emission line objects that
were offset in velocity space and located closer than 1′′ to the
QSO line of sight. However, tests with artificial objects showed
that this was a minor problem for the data set (see Sect. 5.3).
3. Observations and data reduction
Using the Potsdam Multi Aperture Spectrophotometer (PMAS)
mounted on the 3.5m telescope at Calar Alto we observed the
objects listed in Table 2 during several runs from 2002–2004.
The PMAS integral field unit (IFU) was used in the standard
configuration where 256 fibres are coupled to a 16×16 element
lens array. During the observations each fibre covered 0.′′5×0.′′5
on the sky giving a field of view of 8′′×8′′. Each fibre resulted in
a spatial element (spaxel) represented by a single spectrum. The
256 spectra were recorded on a 2k×4k CCD which was read out
in a 2×2 binned mode. With a separation of 7 pixels between in-
dividual spectra, the fibre to fibre cross-talk was negligible (less
than 0.4% for an extraction of all 7 pixels). Detailed overviews
of the PMAS instrument and capabilities are given in Roth et al.
(2000, 2005).
For individual exposures a maximum time of 1800s was used
because of the large number of pixels affected by cosmic ray hits.
Furthermore, because of varying conditions such as the atmo-
spheric transmission and seeing, the total exposure time for each
object was adjusted, or sometimes an observation was repeated
under better conditions. The photometric conditions during ob-
servations were monitored in real time with the PMAS acquisi-
tion and guiding camera (A&G camera) which is equipped with
a 1k×1k CCD. Seeing values listed in Table 2 refer to the see-
ing FWHM measured in the A&G camera images. Determining
actual spectrophotometric conditions requires monitoring of the
extinction coefficients which cannot be determined from the
http://www.phys.unsw.edu.au/~sjc/dla/
L. Christensen et al.: An IFS survey for high-z DLA galaxies 3
Coordinate name Alt. name zem zabs logN(H i) [Fe/H] [Si/H] Ref.
Q0151+048A PHL 1222 1.93 1.934 20.36±0.10 (1)
Q0953+4749 PC 0953+4749 4.457 3.404 21.15±0.15 >–2.178 >–2.09 (2,3)
3.891 21.20±0.10 >–1.712 >–1.60
4.244 20.90±0.15 –2.50±0.17 –2.23±0.15
Q1347+112 2.679 2.471 20.3 (4,5,6)
2.05 20.3† (7)
Q1425+606 SBS 1425+606 3.163 2.827 20.30±0.04 –1.33±0.04 >–1.03 (8,9,10,21)
Q1451+1223 B1451+123 3.246 2.469 20.39±0.10 –2.54±0.12 –1.95±0.16 (11,24)
3.171 19.70±0.15 –1.87±0.16 –1.62±0.15 (19)
2.254 20.30±0.15 –1.47±0.17 >–0.40 (6,12,19)
Q1759+7539 GB2 1759+756 3.05 2.625 20.76±0.10 –1.21±0.10 –0.82±0.10 (13,18,20)
2.91 19.8 –1.65±0.01 –1.26±0.01 (18)
Q1802+5616 PSS J1802+5616 4.158 3.391 20.30±0.10 –1.54±0.11 >–1.55 (23)
3.554 20.50±0.10 –1.93±0.12 >–1.82
3.762 20.55±0.15 –1.82±0.26 >–1.74
3.811 20.35±0.20 –2.19±0.23 –2.04±0.22
Q2155+1358 PSS J2155+1358 4.256 3.316 20.55±0.15 >–1.68 –1.26±0.17 (3)
3.142 19.94±0.10 –2.21±0.21 –1.85±0.12 (14,19)
3.565 19.37±0.15 <–2.40 –1.27±0.16 (14,19)
4.212 19.61±0.15 –2.18±0.25 –1.92±0.11 (14,19)
Q2233+131 3.295 3.153 20.0 >–1.4±0.1 >–1.04 (15,16,17)
2.551 20.0 (12,16)
Table 1. List of the observed DLA and sub-DLA systems with column densities and metallicities taken from the litera-
ture. † denotes a system where the reported N(H i) needs to be confirmed through high resolution spectroscopy. References
for either DLA redshifts or metallicities: (1) Møller et al. (1998), (2) Schneider et al. (1991), (3) Prochaska et al. (2003d), (4)
Smith et al. (1986), (5) Wolfe et al. (1986), (6) Turnshek et al. (1989), (7) Wolfe et al. (1995), (8) Chaffee et al. (1994), (9)
Stepanian et al. (1996), (10) Prochaska et al. (2002a), (11) Bechtold (1994), (12) Lanzetta et al. (1991), (13) Prochaska et al. (2001),
(14) Péroux et al. (2003), (15) Steidel et al. (1995), (16) Lu et al. (1997), (17) Prochaska et al. (2003a), (18) Outram et al. (1999),
(19) Dessauges-Zavadsky et al. (2003), (20) Prochaska et al. (2002b), (21) Lu et al. (1996), (22) Prochaska et al. (2003c), (23)
Prochaska et al. (2003b), (24) Petitjean et al. (2000).
A&G camera images. In Table 2 ‘stable’ means that the pho-
tometry of the guiding star was constant within 1% during the
observations.
The data were obtained using 2 gratings; one with 300 lines
mm−1 and one with 600 lines mm−1, set at a chosen tilt to cover
a selected wavelength range. The FWHM of the sky lines were
measured to be 6.4 and 3.2 Å, respectively. Observations of spec-
trophotometric standard stars were carried out at the beginning
and end of each night at the grating position used for the obser-
vations.
Data reduction was done by first subtracting an average bias
frame. Before extracting the 256 spectra most cosmic ray hits
were removed by the routine described in Pych (2004). A high
threshold was chosen such that not all cosmic rays were re-
moved, because a low threshold would also remove bright sky
emission lines from some spectra. Remaining cosmic rays were
removed from the extracted spectra using the program L.A.
Cosmic (van Dokkum 2001).
The locations of the spectra on the CCD were found from
exposures of a continuum lamp, taken either before or after the
science exposures, using a tracing algorithm developed for the
IDL based PMAS data reduction package P3D (Becker 2002).
The spectral extraction was done in two ways; a ‘simple extrac-
tion’ that added all flux from each spectrum on the CCD (i.e. an
extraction width of 7 pixels), and another method that took into
account the profile of the spectrum on the CCD. This second
method assumed that the spectral profiles were represented by
Gaussian functions (Gaussian extraction) where the widths were
allowed to vary with wavelength. Widths were determined by fits
to each of the 256 spectra as a function of the wavelength, and
the extraction used these width in combination with the centre
found from the tracing algorithm. The Gaussian profile is an ap-
proximation because the profiles are not strictly Gaussian. The
second method increased the signal-to-noise ratio by >10% for
faint objects and therefore unless otherwise noted, the results
from the ‘Gaussian extraction’ data cubes will be reported (see
also Sánchez 2006).
After extraction, the spectra were wavelength calibrated us-
ing exposures of emission line lamps taken just before or after
the observations. The wavelength calibration was done using the
P3D reduction tool. Comparisons with sky emission lines indi-
cated an accuracy of the wavelength calibration of about 10% of
the spectral resolution.
The spectra show a wavelength dependent fibre to fibre trans-
mission. To correct for this effect, we extracted sky spectra ob-
tained from twilight sky observations in the same way as the sci-
ence observations. A one dimensional average sky spectrum was
calculated. Each of the 256 spectra were divided by this average
spectrum, and the fraction was fit by a polynomial function to re-
duce noise. These polynomials were used to flat field the science
spectra.
Before combining individual frames, the extracted spectra
were arranged into data cubes. Each data cube was corrected
for extinction using an average extinction curve for Calar Alto
(Hopp & Fernandez 2002). The data cube combination took into
account a correction for the differential atmospheric refraction
4 L. Christensen et al.: An IFS survey for high-z DLA galaxies
QSO date exp.time grating λ coverage seeing conditions
(s) (Å)
Q0151+048A 2003-08-27 5×1800 V600 3500–5080 0.8–1.2 stable
Q0953+4749 2004-04-16 4×1800 V300 3630–6980 0.9 stable
2004-04-21 5×1800 V300 3630–6980 1.0 non-phot.
Q1347+112 2004-04-20 7×1800 V300 3630–6980 0.6 non-phot.
Q1425+606 2004-04-19 6×1800 V300 3630–6750 1.0 stable
Q1451+1223 2004-04-17 7×1800 V300 3630–6980 0.8 non-phot.
Q1759+7539 2004-04-21 7×1800 V300 3630–6980 1.0–1.5 non-phot.
Q1802+5616 2003-06-18 2×1800 V600 5100–6650 1.0 non-phot.
2003-06-20 3×1800 V600 5100–6650 1.0 non-phot.
2003-06-21 4×1800 V600 5100–6650 1.8 non-phot.
2003-06-22 6×1800 V600 5100–6650 0.9 stable
Q2155+1358 2003-08-26 7×1800 V600 4015–5610 0.7 stable
2003-08-27 4×1800 V600 4015–5610 0.8 non-phot.
Q2233+131 2002-09-03† 4×1800 V300 3930–7250 1.0–1.3
2003-08-24 6×1800 V600 4000–5600 0.6 stable
2003-08-25 4×1800 V600 4000–5600 0.7 non-phot.
Table 2. Log of the observations. The last two columns show the average seeing during the integrations and the photometric
conditions derived from the A&G camera images. † Results from these observations are published in Christensen et al. (2004).
using a theoretical prediction (Filippenko 1982). Relative spa-
tial shifts between individual data cubes were determined from
a two-dimensional Gaussian fit to the QSO PSF at a wavelength
close to the strong DLA absorption lines.
Subtraction of the sky background was an iterative process
because the locations of the emission line objects of interest were
not known beforehand. PMAS, in the configuration used, does
not have specifically allocated sky fibres. Instead, we selected
10–20 fibres uncontaminated by the QSO emission and the av-
erage spectrum was subtracted from all 256 spectra. Different
spaxel selections were examined visually to select an appropri-
ate sky spectrum which had no emission line or noisy pixels in
the spectral region of interest.
Flux calibration was done in the standard way using obser-
vations of spectrophotometric standard stars. A one-dimensional
spectrum of the standard star was constructed by co-adding flux
from all 256 spaxels. This was used to create a sensitivity func-
tion that could be applied to each of the 256 spectra in the science
exposures. For non-photometric nights the flux calibrated spec-
tra were compared with QSO spectra from the literature to esti-
mate photometric errors. However, no correction factor was ap-
plied to our spectra, because an intrinsic variability of the QSOs
would make such scaling uncertain. For some cases we note in
Sect. 5.5 that there are differences which could be caused by ei-
ther non-photometric conditions or intrinsic variability.
For reference we show spectra of the target QSOs in Fig. 1.
Where present, metal absorption lines corresponding to the
highest column density DLAs are indicated. For QSOs with
multiple DLAs lines only the DLA lines and their redshifts
are indicated since the wavelength coverage does not include
lines redwards of the QSO Lyα line. A detailed analysis of
metal absorption lines requires higher resolution spectroscopy as
presented elsewhere (e.g. Prochaska et al. 2003c; Péroux et al.
2003; Dessauges-Zavadsky et al. 2003). DLA redshifts derived
from the metal absorption lines were consistent with those re-
ported in the literature within the accuracy of the wavelength
calibration of the data cubes.
4. Search for DLA optical counterparts
The observations covered the wavelengths of Lyα for all but one
of the strong absorption systems listed in Table 1. Only the high-
est redshift sub-DLA system towards Q2155+1358 was not cov-
ered, i.e. the total number of systems included in this analysis is
21 DLA and sub-DLA systems.
4.1. Expected sizes
For this project we are only interested in small wavelength re-
gions corresponding to Lyα at the DLA redshifts, and thus the
search for candidate galaxies could be carried out using cus-
tomised narrow-band filters. IFS, on the other hand, has the ad-
vantage that the widths of the narrow-band filters can be ad-
justed to match those of the emission lines. Typically customised
narrow-band filters have a larger transmission FWHM than the
spectral resolution of IFS data. Hence, IFS allows detection of
emission line objects with a higher signal-to-noise ratio than
that possible with narrow-band filters. A disadvantage is the rela-
tively small field of view of current IFUs, but this is not a serious
concern. One can estimate the expected sizes of DLA galaxies
(see Wolfe et al. 1986). Using a Schechter luminosity function
and a power-law relation between the disk luminosity and gas
radius given by the Holmberg relation R/R∗ = (L/L∗)β, one can
calculate the expected impact parameter. Combining β = 0.26
found for DLA galaxies at z < 1 (Chen et al. 2005) with the
luminosity function in for z = 3 galaxies (Poli et al. 2003) one
finds R∗ ≈ 30 kpc. If DLA galaxies are similar to or fainter than
L∗ galaxies this implies that DLA galaxies at z > 2 are expected
to lie closer than ∼4′′ from the QSO line of sight. The small
field of view of IFUs is therefore well suited to search for Lyα
emission from DLA galaxies.
The estimated galaxy sizes are highly dependent on the
parameters of the DLA galaxy luminosity and slope β. Most
probably, high redshift DLA galaxies are not regular disks like
those in the local universe. Numerical models of DLAs predict
that the galaxies are mostly smaller than 10 kpc, while obser-
vations that give limits on the star-formation rates associated
with DLAs suggest that DLAs are located in neutral gas around
Lyman break galaxies (Wolfe & Chen 2006). As DLA galaxies
hristensen
survey
high-z
galaxies
fλ  [10
−16 erg s−1cm−2 Å−1]
Lyα(1215.67)
SiII(1260.42)
OI(1302.17)
CII(1334.53)
SiIV(1393.76)
SiIV(1402.77)
SII(1526.71)
SII(1533.43)
CIV(1549)
Si IV
fλ  [10
−16 erg s−1cm−2 Å−1]
Lyα(z=3.404)
Lyα(z=3.891)
Lyα(z=4.244)
Lyα(QSO)
fλ  [10
−16 erg s−1cm−2 Å−1]
Lyα(1215.67)
SiII(1260.42)
OI(1302.17)
CII(1334.53)
SiIV(1393.76)
SiIV(1402.77)
SII(1526.71)
CIV(1549)
FeII(1608.45)
AlII(1670.79)
AlIII(1854.7)
AlIII(1862.8)
Si IV
fλ  [10
−16 erg s−1cm−2 Å−1]
Lyβ(1026)
Lyα(1215.67)
SiII(1260.42)
OI(1302.17)
CII(1334.53)
SiIV(1393.76)
SiIV(1402.77)
SII(1526.71)
SII(1533.43)
CIV(1549)
FeII(1608.45)
AlII(1670.79)
Si IV
0 2 4 6 8
fλ  [10
−16 erg s−1cm−2 Å−1]
Lyα(1215.67)
SiII(1260.42)
OI(1302.17)
CII(1334.53)
SiIV(1393.76)
SiIV(1402.77)
SII(1526.71)
CIV(1549)
Si IV
fλ  [10
−16 erg s−1cm−2 Å−1]
Lyβ(1026)
Lyα(1215.67)
SiII(1260.42)
OI(1302.17)
CII(1334.53)
SiIV(1393.76)
SiIV(1402.77)
SII(1526.71)
CIV(1549)
FeII(1608.45)
AlII(1670.79)
AlIII(1854.7)
AlIII(1862.8)
Si IV
fλ  [10
−16 erg s−1cm−2 Å−1]
Lyα(z= 3.391)
Lyα(z= 3.554)
Lyα(z= 3.762)
Lyα(z= 3.811)
Lyα(QSO)
fλ  [10
−16 erg s−1cm−2 Å−1]
| Lyα(1215.67)
0 2 4 6 8
fλ  [10
−16 erg s−1cm−2 Å−1]
Lyβ(1026)
Lyα(1215.67)
SiII(1260.42)
OI(1302.17)
CII(1334.53)
ig.1.
ectra
tracted
rrectio
alactic
ciated
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icated
catio
ritten
ectra
issio
6 L. Christensen et al.: An IFS survey for high-z DLA galaxies
at z > 2 are generally found to be fainter than an L∗ galaxy
(Colbert & Malkan 2002), we choose to consider only objects
with impact parameters smaller than 30 kpc for a more detailed
analysis.
The impact parameters that we measure in the data corre-
spond to the radially projected distances so the real distances to
the absorber can be larger. Two candidates are found at impact
parameters larger than 30 kpc, and they are likely not associated
directly with the absorbers themselves.
4.2. Candidate selection
Some Lyα emission lines from DLA galaxies are offset from the
QSO- DLA line by ∼ 200 km s−1 (Møller et al. 2002), whereas
Lyα emission from high redshift galaxies can have even larger
offsets from the galaxy systemic redshift (Shapley et al. 2003).
We therefore chose to focus on regions in the data cubes with
velocities ranging from approximately –1000 to +1000 km s−1
from the DLA lines.
First, the reduced data cubes were stacked in a two-
dimensional frame and inspected visually around the DLA lines
for emission line objects. When the spatial offset from the QSO
is larger than the seeing, or alternatively when the QSO is very
faint, emission line objects can be identified directly because
of the ordering of the spectra in the stacked spectrum. Where
no objects could be detected visually further sampling of the
data cubes was necessary to increase the signal-to-noise ratio
to detect candidate emission line objects. Inspections of the data
cubes was done using the Euro3D visualisation tool (Sánchez
2004).
From the reduced, sky-subtracted and combined data cubes,
narrow-band images were created with an initial width of 10–
15 Å depending on the spectral resolution of the observations. A
set of images was created offset by –10 to +10 Å from the DLA
line to allow for possible velocity shifts of the Lyα emission line,
and inspected visually for objects brighter than the background.
If detected, spectra from these brighter regions were co-added
and inspected for emission lines at the wavelength chosen in the
narrow-band image. This step was necessary to discriminate be-
tween emission lines and individual noisy spectra. It is known
that three blocks of 16 fibres, i.e. 48 fibres in an area of 1.′′5 to the
west in the field of view, have lower than average transmission.
The effect of correcting for the total throughput was that these
spectra had lower signal-to-noise ratios. When narrow-band im-
ages were created from the cubes, the higher variance in these
spaxels could result in extreme values, seemingly inconsistent
with the neighboring spaxels. Only by looking at the spectrum
associated with a bright spaxel could it be determined if an emis-
sion line was present, or if the spectrum was just noisy. If an
emission line was seen, a second pass narrow-band image was
created using the value of the emission line width to increase the
signal of the detection. A second pass one-dimensional spectrum
was created after inspecting the narrow-band image for more
bright spaxels surrounding the emission line candidate. This pro-
cedure was iterated until the signal in either narrow-band images
or spectra did not increase. We found that an interactive visual
identification of faint emission lines was more effective than an
automatic routine.
To allow a better visual detection of emission line objects,
the narrow-band images were interpolated to pixel scales 0.′′2
pixel−1 as shown in Fig. 2. In all panels the images are 8′′ by
8′′, with orientation north up and east left. The left panels show
interpolated images of the QSO at wavelengths near to the DLA
line. Contours correspond to an image centered on the visually
detected emission feature close to the DLA redshift. In the mid-
dle panels in Fig. 2 the plots are reversed, such that the image
shows the emission line object and the contours correspond to
the QSO narrow-band image. Here, the innermost contour cor-
responds to the seeing FWHM. To enhance the visibility of the
candidates the QSO emission was subtracted from the data cubes
before creating the images. This subtraction of the QSO emis-
sion was done using a simple approach (see Christensen et al.
2006). A scale factor was determined for each spaxel by dividing
each spectrum by the extracted one-dimensional QSO spectrum.
Using this scale factor, the QSO emission was subtracted, a pro-
cess which retains objects with spectral characteristics different
from the QSO in the data cube.
The spectra of the candidates are shown in the right hand
column in Fig. 2. These are created by co-adding spectra from
between 4 and 10 spaxels. The dotted line corresponds to the
1σ noise level determined from a statistical analysis of the pixel
values in the data cube, while the lower sub-panels show the
background noise spectra in the data cubes, obtained from 4-10
background spaxels.
Properties of the candidate objects corresponding to those
with spectra in Fig. 2 are listed in Table 3. Offsets in RA, DEC
from the QSO and the corresponding projected distance at the
DLA redshift are listed in columns 2, 3, and 4. Emission lines
were fit using ngaussfit in IRAF, redshifts listed in column 5
are derived. Fluxes in column 6 are derived from the Gaussian
fits, and errors in the peak intensity, line width, and contin-
uum placement are propagated to calculate the uncertainties. The
fluxes have not been corrected for the Galactic extinction. The
flux measurements and the associated errors indicate that most
of the candidates are detected with a signal-to-noise ratio < 4σ.
Column 7 gives the velocity difference between the systemic
DLA redshift and the candidate Lyα emission lines. We integrate
the signal-to-noise estimate over the emission line (Column 8),
S/Nint = f /(
N × σ), where f is the line flux, N is the number
of pixels the emission line covers and σ is the noise in adjacent
wavelength intervals. Column 9 gives the observed emission line
FWHM after correcting for the instrumental resolution. Finally
column 10 gives the significance classes of the candidate detec-
tion, which is explained in Section 5.1.
Columns 3 and 4 in Table 4 list the values of Galactic redden-
ing towards each QSO (Schlegel et al. 1998), and the correction
factors to be applied to the candidate fluxes for a Milky Way
extinction curve (Fitzpatrick 1999).
5. Results
This section describes the classification of the emission line can-
didates. We estimate the contamination from spurious detections
and from interlopers. Notes on each observed object are pre-
sented as well.
5.1. Candidate significance class
To estimate how reliable the candidate detection was, various
tests were applied to the data cubes. The candidates were as-
signed a significance class: 1, 2, 3, and 4 according to how many
of the following tests were passed.
1. Instead of co-adding all data-cubes, two independent subsets
of exposures were created, and the emission line candidate
was visible in both sub-set combinations. In Sect. 5.5 these
will be referred to as subcombinations.
L. Christensen et al.: An IFS survey for high-z DLA galaxies 7
DLA Q0151+048A
3500 3550 3600 3650
Wavelength [Å]
Sky noise
DLA (z=3.404) Q0953+4749
5200 5300 5400 5500
Wavelength [Å]
Sky noise
DLA (z=3.891) Q0953+4749
5800 5900 6000 6100
Wavelength [Å]
Sky noise
DLA (z=4.244) Q0953+4749       no candidate
6200 6300 6400 6500
Wavelength [Å]
Sky noise
       
DLA Q1347+112 (z=2.484)
4100 4200 4300 4400
Wavelength [Å]
Sky noise
Fig. 2. Left panels: narrow-band images of the QSOs with overlayed contours of narrow-band images centered on the Lyα wave-
lengths of the DLAs. The images are 8′′ square. Contour levels correspond to 2, 3, 4σ levels above the background noise. Middle
panels: the reverse, where the contours are arbitrary apart from the central one that shows the QSO seeing FWHM. Right hand
panels: Spectra of candidate emission line objects created from co-adding spectra from spaxels associated with the emission line
candidates. The width of the grey bars over the emission lines correspond to the wavelength ranges of the narrow band images. The
lines below the spectra show the background sky noise spectra determined from background spaxels.
8 L. Christensen et al.: An IFS survey for high-z DLA galaxies
      
DLA Q1347+112 (z=2.057)
3650 3700 3750 3800 3850 3900
Wavelength [Å]
Sky noise
10 DLA  Q1425+606
4500 4600 4700 4800
Wavelength [Å]
Sky noise
DLA Q1451+1223
4100 4200 4300 4400
Wavelength [Å]
Sky noise
sub−DLA Q1451+1223
4900 5000 5100 5200
Wavelength [Å]
Sky noise
Fig. 2. Plots of candidates– continued. No candidate is found for the z = 2.254 DLA towards Q1451+1223.
L. Christensen et al.: An IFS survey for high-z DLA galaxies 9
DLA  Q1759+7539
4300 4400 4500 4600
Wavelength [Å]
Sky noise
10 sub−DLA  Q1759+7539
4600 4700 4800 4900
Wavelength [Å]
Sky noise
DLA Q1802+5616 (z=3.391)
5200 5300 5400 5500
Wavelength [Å]
Sky noise
10 DLA Q1802+5616 (z=3.762)
5600 5700 5800 5900
Wavelength [Å]
Sky noise
Fig. 2. Plots of candidates– continued. No candidate is found for the z = 3.554 DLA towards Q1802+5616.
10 L. Christensen et al.: An IFS survey for high-z DLA galaxies
10 sub−DLA Q2155+1358 (z=3.146)
4900 5000 5100 5200
Wavelength [Å]
Sky noise
DLA Q2155+1358
5100 5200 5300 5400
Wavelength [Å]
Sky noise
subDLA Q2233+131 (z=3.153)
4900 5000 5100 5200
Wavelength [Å]
Sky noise
Fig. 2. Plots of candidates– continued. No candidates are found for the z = 3.811 DLA towards Q1802+5616, or the z = 3.565
sub-DLA towards Q2155+1358.
L. Christensen et al.: An IFS survey for high-z DLA galaxies 11
Fig. 2. Plots of candidates– continued. No candidate is found for the z = 2.51 sub-DLA towards Q2233+131.
QSO ∆RA ∆DEC b z fλ ∆ v S/Nint FWHM significance
(′′) (′′) (kpc) (km s−1) (km s−1) class
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
Q0151+048A 2.5:0.4 –2.5: 1.7 3.3:25.4 1.9363 (150±70) +225 7 280±220 conf.
Q0953+4749 –1.2 –0.2 9.0 3.4041 (6.6±2.9) 0 15 290±260 2
–0.5 1.8 11.1 3.9029 (4.9±2.1) +730 16 570±230 3
Q1347+112 –2.0 1.5 20.2 2.4835 (3.5±1.9) +1080 8 190±370 3
–4.1 0.4 34.3 2.0568 (4.2±3.0) +670 7 610±210 1
Q1425+606 –4.1 0.4 32.3 2.8280 (8.5±3.1) +80 14 590±220 3
Q1451+1223 –3.0 3.8 39.2 2.4764 (5.8±2.6) +640 10 320±260 3
–1.0 2.8 22.5 3.1739 (3.1±2.0) +210 7 <100 3
Q1759+7539 –1.4 3.5 30.0 2.6377 (5.8±3.0) +1050 10 290±220 2
0.1 –1.2 9.4 2.9090 (6.0±2.9) –80 21 240±260 1
Q1802+5616 –0.2 –2.0 14.9 3.3820 (3.5±0.9) –610 10 180±90 4
0.2 1.8 12.9 3.7652 (4.6±1.9) +200 7 <100 2
Q2155+1358 0.7 1.2 10.2 3.3174 (9.4±3.0) +100 9 780±210 3
1.7 1.9 19.4 3.1461 (4.1±2.4) +290 7 260±220 3
Q2233+131 0.6 2.3 18.0 3.1543 (9.6±2.5) +90 9 230±110 conf.
Table 3. Properties of candidate Lyα emission lines. Column 2, 3, and 4 list the offsets of the candidate in RA and DEC and in pro-
jected kpc at the Lyα emission redshifts (in col. 5), respectively. Column 6 lists the integrated Lyα flux in units of 10−17 erg cm−2 s−1,
and column 7 the velocity offset from the DLA redshift. Column 8 lists the integrated signal-to-noise ratio of the Lyα emission line,
and column 9 gives the line width of the emission lines. Fluxes have not been corrected for Galactic extinction. Column 10 lists the
significance class of the detections as described in Sect. 5.5. ‘Conf.’ implies candidates that were confirmed previously (Møller et al.
1998; Djorgovski et al. 1996).
2. The emission line candidate was visible in the ‘simple ex-
tractions’, i.e. where a Gaussian profile was not assumed.
3. Emission line candidates were visible in the narrow-band im-
ages when the QSO spectrum was subtracted from the data
cube.
4. Emission line objects that were directly visible in individual
or combined data cubes, or in the stacked spectra.
In all cases, for a candidate to be considered further it was re-
quired to be detected above 3σ in both narrow-band images and
the associated spectra. The significance classes of the candidates
are listed in column 10 in Table 3, and comments for each object
are described in Section 5.5. Since the candidates were found
from visual inspections of the data cubes, this classification was
done to describe the candidates in a more qualitative manner. As
the classes involve various tests on the data sets, this classifica-
tion goes beyond the simple statistical significance in terms of to
what σ level the object is detected.
5.2. Non-detections
In the data cubes where no candidates were found, we esti-
mated the upper limits for the emission line fluxes. Spectra from
spaxels within one seeing element (e.g. 4 spaxels corresponding
to a seeing of 1′′) were co-added to create a one-dimensional
spectrum. Artificial emission lines with varying line fluxes were
added to this spectrum at the DLA wavelength, and Gaussian
profile fits to these lines were used to estimate the detection level.
The results are listed in Table 5. The varying limits are due to the
wavelength dependent noise in the data cubes and in particular
the presence of residuals from nearby sky emission lines.
12 L. Christensen et al.: An IFS survey for high-z DLA galaxies
QSO EB−V ffrac
Q0151+048A 0.044 1.216
Q0953+4749 (z =3.4041) 0.011 1.032
Q0953+4749 (z =3.9028) 1.028
Q1347+112 (z =2.4835) 0.035 1.145
Q1347+112 (z =2.0568) 1.163
Q1425+606 (z =2.827) 0.012 1.043
Q1451+1223 (z =2.4764) 0.031 1.128
Q1451+1223 (z =3.1739) 1.102
Q1759+7539 (z =2.6377) 0.053 1.220
Q1759+7539 (z =2.91) 1.199
Q1802+5616 (z =3.3820) 0.052 1.164
Q1802+5616 (z =3.7652) 1.145
Q2155+1358 (z =3.3174) 0.067 1.222
Q2155+1358 (z =3.1461) 0.067 1.237
Q2233+131 0.068 1.240
Table 4. Column 2 and 3 give values of the Galactic redden-
ing and the corresponding correction factor to be applied to the
emission line candidates.
QSO zabs flim(3σ)
Q0953+4749 4.244 2.5
Q1451+1223 2.256 4.0
Q1802+5616 3.554 4.0
Q1802+5616 3.811 2.2
Q2155+1358 3.565 3.5
Q2233+131 2.551 4.8
Table 5. DLA and sub-DLA systems where no candidate emis-
sion lines are found and 3σ upper limits for the line fluxes.
Fluxes are in units of 10−17 erg cm−2 s−1.
5.3. Experiments with artificial objects
To investigate how the efficiency of the visual inspection de-
pended on object properties, several experiments with artificial
data cubes were made. Similar to artificial experiments for one-
and two-dimensional data sets, artificial emission line objects
were added to the data cubes. These objects were described by
the location in RA and DEC, central wavelength, peak emission
intensity, and the widths in RA, DEC and wavelength. For sim-
plicity we assumed that an emission line object seen as a point
source in the data cube could be represented by a Gaussian pro-
file in each direction, i.e. described by a Gaussian ellipsoid in
the data cube.
We first tested completely simulated data cubes with statisti-
cal noise levels corresponding to the typical noise level in the
combined data cubes. An emission line object with a flux of
5 × 10−17 erg cm−2 s−1, a width of 800 km s−1, and spatial
FWHM of 1′′ was placed at a previously known wavelength. In
the stacked spectra no objects could be seen immediately. The
emission line was only identified after inspecting the data cube
in the visualisation tool, and it was extracted and analysed in the
same way as the real data. Similar tests were made by adding an
emission line to a real data cube, where the background noise in-
cluded the systematic noise as well as the pure Poissonian noise.
These tests produced similar results for the faint emission lines
with Lyα flux f ∼ 5 × 10−17 erg cm−2 s−1, i.e. 1) the emission
line flux could be recovered within uncertainties, 2) even at very
small impact parameters the object could be found 3) the recon-
structed PSF of the emission line object was irregular as in any
of the images in Fig. 2.
We also tested an automatic routine where the re-detection
of the artificial objects was done with no visual intervention. A
set of narrow-band images were created in wavelength ranges
around the artificial line. For the detection of an emission line
the location was constrained to be within ±10 Å of the input
central wavelengths. These images were smoothed and a two-
dimensional Gaussian profile was fit to the images. When an ob-
ject was detected above a certain threshold, spaxels around the
centre within the seeing FWHM were co-added. A series of tests
showed that the recovered flux was consistent within 1σ errors
for fluxes down to f = 5 × 10−17 erg cm−2 s−1. In a typical data
cube this was also the detection limit where 50% of the objects
were re-identified, while the fraction of re-identified emission
lines at this flux level from a visual inspection was larger.
Tests on the frequency of false detections in data cubes where
no objects were present showed that simultaneous detections
of objects in narrow-band images and associated spectra with
S/N > 3 occurred at a rate of less than 5% in a series of ex-
periments. Therefore false detections cannot explain the large
number of candidate objects.
5.4. Field Lyα emitters
We estimate here whether the detected candidates are likely to
be field Lyα emitters having no association with the DLAs.
Observations of high redshift objects have partly focused on
detecting Lyα emission from galaxies to determine the global
comoving star-formation rates (e.g. Hu et al. 1998, 2004). The
density of Lyα emitters at z ∼ 3 is estimated to be 15000 deg−2
∆z−1 with line fluxes brighter than a mean of f = 1.5 × 10−17
erg cm−2 s−1 (Hu et al. 1998; Kudritzki et al. 2000). From the
luminosity function at z ≈ 3 (van Breukelen et al. 2005), the ex-
pected number of field Lyα emitters at a flux limit of 5 × 10−17
erg cm−2 s−1 is 1.7×10−4 arcsec−2 ∆z−1. In our survey, the 9 data
cubes sample a total redshift interval of ∆z = 21.55 around z ≈ 3.
Statistically, it is expected that there are 0.2 field Lyα emitters in
the whole sample presented here. Because these very faint lines
are difficult to locate when the approximate wavelength is not
known in advance, we did not look for field emission objects.
The negligible number of expected field emitters furthermore
shows that the emission candidates, if proved to be real, are un-
likely to be interloping field Lyα emitters. They are much more
likely to be associated with the DLA galaxies.
5.5. Notes on individual objects
This section explains the significance of the candidates for each
individual QSO.
Q0151+045A. – This is a zem ≈ zabs system at z ≈ 1.93. After
flux calibration, the QSO spectrum is 2 magnitudes brighter than
that presented in Møller et al. (1998). The low instrument sensi-
tivity at 3560 Å combined with a variable extinction coefficient
at Calar Alto makes the calibration uncertain.
Extended Lyα emission was observed in a region of 3′′×6′′
around the QSO mostly to the east of the QSO (Fynbo et al.
1999). Long slit spectroscopy along the long axis revealed ve-
locity structures of 400 km s−1 that could be interpreted as a
rotation curve (Møller 1999). In the IFS data extended emission
is detected to some degree in Fig. 2, but not with the same detail
as in the higher spatial resolution and larger field of view data
in Fynbo et al. (1999). This is the only case in the sample where
extended emission is found, but the signal is not strong enough
to determine the velocity structure over the extended region. The
L. Christensen et al.: An IFS survey for high-z DLA galaxies 13
spectrum shown in Fig. 2 is the total spectrum co-added from the
whole nebula.
Q0953+4749. – This zem = 4.457 QSO has three DLAs at
zabs = 3.407, 3.891, and 4.244 (Bunker et al. 2003). A candidate
associated with the lowest redshift DLA is visible in the narrow-
band image in Fig. 2. Independent subcombinations, the simple
extraction, and the corresponding spectra show a faint emission
line. This emission line coincides with a sky emission line 1.6 Å
away and could be due to sky subtraction errors, so we only as-
sign this candidate a significance class of 2. A Lyα emission
line from the DLA galaxy has been reported (A. Bunker, pri-
vate comm.) but its line flux is below our detection limit. For the
second DLA system at z = 3.891 the object is present in sub-
combinations, the simple-extracted images, and in the extracted
spectra. This candidate is assigned significance class 3. No can-
didate is found for the highest redshift DLA to the detection limit
reported in Table 5.
The locations of the candidates are compared to WFPC2 im-
ages obtained from the HST archive, but no continuum counter-
part could be identified.
Q1347+112. – This zem = 2.679 QSO has a DLA at zabs =
2.471 and another possible one at zabs = 2.05, which needs con-
firmation from spectroscopy at higher spectral resolution. An
emission candidate for the z = 2.471 DLA is visible in the sub-
combinations and the extracted one-dimensional spectra. In the
simple extraction, the spectrum has a low signal-to-noise ratio
and the emission feature in the spectrum is faint. We assign a
significance class of 3 to this candidate. For the z = 2.0568 DLA
system we detect a candidate emission line object, but note an
increase in the background noise shortwards of 3750 Å. The ob-
ject is not seen in one of the subcombinations, nor the extracted
spectra, and therefore the candidate is assigned a significance
class of 1.
A snapshot WFPC F555W image (Bahcall et al. 1992) ob-
tained from the HST archive has a 5σ limiting magnitude of
24.4 mag arcsec−2, but no continuum counterpart can be seen at
the location of the candidate.
Q1425+606. – This zem = 3.163 QSO has a DLA at zabs =
2.827. Because this QSO is very bright, strong residuals within
1′′ from the QSO centre are present in the narrow-band image
where the QSO emission is subtracted. A faint object offset by
∼4′′ to the west is visible in the narrow-band image in Fig. 2. The
candidate is present in subcombinations and in the constructed
spectra and is assigned a significance class of 3. In PMAS data
cubes, spaxels in the west region are more noisy than the average
due to an overall lower transmission. Note that the tests suggest
a good candidate, but the impact parameter (> 30 kpc) is large.
Q1451+1223. – This zem = 3.246 QSO has two DLAs at
zabs = 2.469 and zabs = 2.254 and a sub-DLA at zabs = 3.171.
For the DLA system at z = 2.469 an object appears after the QSO
subtraction close to the centre. It is caused by residuals, since the
spectrum has no emission lines at the expected wavelength. For
the same DLA, a region ∼4′′ to the north west appears in both
narrow-band imaging, subcombinations, simple extractions, and
the constructed spectra. We therefore assign a significance class
of 3 to it, but note that is has a large impact parameter (39 kpc).
For the z = 3.171 sub-DLA an object is detected to the north.
Narrow-band images from subcombinations, and simple extrac-
tions show the emission line candidate, but the corresponding
spectra have emission lines with very low signals. The candidate
is assigned a significance class of 3. No candidate is found for
the z = 2.254 DLA.
A deep optical broad-band image of the field surrounding
this QSO was obtained by Steidel et al. (1995), who found no
obvious candidates to the absorbers. Warren et al. (2001) found
one candidate offset by 3.′′9 to the south-west of the QSO in a
NICMOS image, but this object is outside the field of view of
the IFS data. An HST/STIS archive image shows that the emis-
sion line candidates lie in regions where no continuum emitting
counterpart is found.
Q1759+7539. – This zem = 3.05 QSO has a DLA at zabs =
2.625 and a sub-DLA at zabs = 2.91. The candidate detected
for the DLA system in Fig. 2 lies near the northern edge of the
field of view and can be affected by flat field errors. Although it
is bright, the candidate is not visible in both subcombinations,
and is therefore assigned a significance class of 2. A bright area
1.′′8 south west of the QSO appears after the QSO emission is
subtracted but it is likely due to residuals. It has no emission lines
at the expected wavelength and is not considered further. The
higher redshift sub-DLA system has an emission line candidate
which is visible only after the QSO PSF has been subtracted
from the final cube. However, the candidate is only visible in
one out of two subcombinations and we assign this candidate a
low significance class of 1.
A NICMOS snapshot image showed no bright galaxies
near the QSO to a limit corresponding to an L∗ galaxy
(Colbert & Malkan 2002).
Q1802+5616. – This zem = 4.158 QSO has four DLAs at
zabs = 3.391, 3.554, 3.762, and 3.811. The candidate for the low-
est redshift DLA system is directly visible in the reduced and
combined data cube when looking at the stacked spectra. The
candidate can also be identified in individual subcombinations
and in the simple extracted spectrum. Therefore this candidate is
assigned the class 4. In a narrow-band image at the wavelength
of Lyα at z = 3.7652 there is an emission region to the south (see
Fig. 2) and the corresponding spectrum shows an emission fea-
ture. However, this line is coincident with a faint sky emission
line, so this candidate is assigned the class 2. No candidates are
found for the other two DLA systems.
Q2155+1358. – This zem = 4.256 QSO has a DLA at zabs =
3.316 and three sub-DLAs at zabs = 3.142, 3.565, and 4.212. The
observations only cover Lyα for the three lower redshift systems.
IFS covering the highest redshift system has revealed a possi-
ble faint candidate emission line object (Francis & McDonnell
2006). The candidate Lyα emission line associated with the DLA
system is visible in independent subcombinations and in the
simple extraction and is therefore assigned a high value of 3.
Because of the partial spatial overlap with the QSO, the emis-
sion from the QSO is subtracted to give a cleaned emission line
object and the associated spectrum shown in Fig. 2. A candidate
is found to the south for the z = 3.142 sub-DLA system. This
object is visible in the simple extraction, and subcombinations,
but only one associated spectrum shows a clearly detected emis-
sion line. We assign a significance class of 3 to this candidate.
No candidate is found for the z = 3.565 sub-DLA.
Q2233+131. – This zem = 3.295 QSO has two sub-DLAs at
zabs = 3.153 and zabs = 2.551. The galaxy responsible for the
z = 3.153 DLA was found by Steidel et al. (1995), and follow-
up spectroscopy confirmed this by the detection of Lyα emission
(Djorgovski et al. 1996). Previous IFS of this object suggested
that the Lyα emission was extended (Christensen et al. 2004).
This is not confirmed by the higher spectral resolution data in-
cluded in this paper, although there appears to be some faint
emission to the east of the object in Fig. 2. The new data and im-
proved data reduction which optimises the signal-to-noise ratio,
confirm the line flux Lyα line flux reported in Djorgovski et al.
14 L. Christensen et al.: An IFS survey for high-z DLA galaxies
(1996). No candidate was found for the z = 2.551 sub-DLA sys-
tem, consistent with the upper limit from a deeper Fabry-Perot
imaging analysis (Kulkarni et al. 2006).
6. Properties of candidate DLA counterparts
We proceed with a more detailed analysis of the properties of
the detected candidate Lyα emission lines. Only those candi-
dates assigned values 3 and 4 are included. Of the eight good
candidates, we reject two due to their large impact parameters
(> 30 kpc). However, since they fulfill the criteria for good can-
didates, they could instead belong to a brighter component in a
group. The average redshift of all the DLAs in the whole sam-
ple is z̄sample = 3.13 while that of the six remaining candidates
is z̄cand = 3.23, hence we find no preference for detection of ei-
ther lower or higher redshift candidates. We emphasise that the
candidates emission lines have fluxes that are detected at the 3σ
level, but with this in mind we compare their properties with
those of confirmed Lyα emission lines from DLA galaxies.
6.1. Line fluxes
Fig. 3 shows the inferred line fluxes of the candidates as a func-
tion of redshift. The triangles denote our candidates and square
symbols indicate already confirmed objects from the literature
(Møller & Warren 1993; Djorgovski et al. 1996; Møller et al.
1998; Leibundgut & Robertson 1999; Møller et al. 2002, 2004).
This figure shows that the line fluxes for the candidates are simi-
lar to those for the previously confirmed ones, which have deeper
observations and detection levels of 5–10σ.
Fabry-Perot imaging studies of QSOs with DLAs have man-
aged to reach similar or lower flux limits than our IFS survey
(Lowenthal et al. 1995; Kulkarni et al. 2006). With their detec-
tion limit some objects should have been detected if the Lyα
fluxes of DLA galaxies are around the level we find for the
candidates and the confirmed objects. IFS is useful to look for
emission lines as it allows us to adjust a posteriori the cen-
tral wavelength, whereas in Fabry-Perot images, the emission
line could fall at the wings of the filter where the transmis-
sion is lower. Another advantage of IFS observations is the
knowledge of the spatial QSO PSF as a function of wave-
length which allows a modeling and subtraction of the QSO
emission (Wisotzki et al. 2003; Sánchez et al. 2004). This al-
lows detection of emission lines even when they are superim-
posed on the QSO. Nevertheless we do not detect emission line
candidates closer than about 1′′ from the QSO possibly due to
subtraction residuals. The fact that the confirmed objects are
found at smaller impact parameters compared to the candidates
(Sect. 6.3) could indicate a bias.
6.2. Velocity differences
An anticorrelation is expected between the Lyα luminosity and
the velocity difference between the Lyα emission line and optical
emission lines (Weatherley et al. 2005). The resonant nature of
Lyα causes a shift of the emission line towards slightly longer
wavelengths where the photons can escape absorption. When a
larger fraction of the blue part of the line profile is absorbed,
the remaining emission line of lower luminosity will be more
shifted in velocity compared to brighter ones. This explanation
is supported by the study of Lyα emission lines from Lyman
break galaxies (LBGs) (Shapley et al. 2003).
1.5 2.0 2.5 3.0 3.5 4.0
redshift
10−18
10−17
10−16
10−15
10−14
L= 1043 erg s−1
L= 1042 erg s−1
confirmed
candidates
Fig. 3. Line fluxes of Lyα emission objects as function of red-
shift, where square symbols represent already confirmed objects
and triangles the candidates. The solid and dashed lines corre-
spond to Lyα luminosities of 1042 and 1043 erg s−1, respectively.
Fig. 4 shows the velocity differences between Lyα emission
lines and the DLA redshifts for the candidates as a function of
the Lyα luminosity. There is no evidence for a correlation for the
candidates. We note that the only candidate that shows a negative
velocity offset is the best candidate in the sample; the z = 3.391
DLA towards Q1802+5616. For the candidates we find an aver-
age velocity difference of 300±580 km s−1, which is similar to
the velocity differences measured for LBGs; Pettini et al. (2001)
find 560±410 km s−1 while a larger sample has ∆v = 650 km s−1
between the Lyα emission line and low-ionisation absorption
lines (Shapley et al. 2003). In the case that DLAs are associated
with bright galaxies we would expect to see large velocity off-
sets too. Furthermore, as the line of sight towards the emission
line object and the QSOs differ by 10–30 kpc, a larger velocity
offset can be expected due to differences in kinematics within
the host and its environment. Instead, if the DLA galaxy resides
in a group, the velocity offset will reflect the velocity disper-
sion in the group instead of being related to the host. In support
of this idea, it has been shown that bright Lyman break galax-
ies at z > 2 are surrounded by gas extending to large distances
(Adelberger et al. 2005). Correlation studies have revealed that
DLAs cluster on almost the same scale as LBGs (Cooke et al.
2006), indicating that a similar amount of gas is present in their
environments.
Like flux-limited surveys, this IFS study selects the brightest
emission component, and it is possible that the real absorbing
galaxy is a fainter component in a group.
In the case that DLA galaxies are related to rotating large
disks, it can be expected that the velocity difference increases
with impact parameter but Fig. 5 shows no clear correlation. In
three of the confirmed DLA galaxies optical emission lines have
velocity differences between –200 and 30 km s−1 relative to Lyα
(Weatherley et al. 2005). Some candidates have larger offsets,
possibly affected more strongly by resonant scattering.
6.3. H i extension
The average impact parameter of 16 kpc derived for the can-
didates is larger than that expected by numerical simulations
which favor impact parameters of b = 3 kpc for DLA galax-
L. Christensen et al.: An IFS survey for high-z DLA galaxies 15
42.2 42.4 42.6 42.8 43.0
log (L
confirmed
candidates
Fig. 4. Velocity differences between the Lyα emission lines and
DLA redshifts as a function of the Lyα luminosity. An average
error bar for the Lyα emission candidates is shown in the lower
right corner.
0 10 20 30
b (kpc)
confirmed
candidates
Fig. 5. Velocity differences between the Lyα emission lines and
DLA redshifts as a function of the impact parameter.
ies, and have fewer than 25% with b > 10 kpc at all redshifts
(Haehnelt et al. 2000; Okoshi & Nagashima 2005; Hou et al.
2005). Larger DLA galaxy sizes of 10–15 kpc at 2 < z < 4
are inferred by other simulations (Gardner et al. 2001). A pos-
sibility for the difference between observations and simulations
is that the simulations assume a single disk scenario, while DLA
galaxies could exist in groups (Hou et al. 2005). The real absorb-
ing galaxy could be fainter and lie closer to the QSO line of sight
than the detected candidate galaxy.
An anticorrelation between N(H i) and the distance to the
nearest galaxy is found in simulations (Gardner et al. 2001), but
no analysis of this effect for observed DLA galaxies has been at-
tempted. A trend for larger column density absorbers at smaller
impact parameters was observed in a sample of DLA galax-
ies at z < 1 (Rao et al. 2003). At lower column densities in
the Lyα forest such an anticorrelation has been shown to ex-
ist (Chen et al. 1998, 2001). Observations of the galaxies giving
rise to Mg ii absorption lines showed an anticorrelation between
the impact parameters and column densities of both Mg ii and
2.0 2.5 3.0 3.5 4.0
redshift
confirmed
candidates
Fig. 6. Angular impact parameter vs. redshift. The solid line
corresponds to the size-luminosity relation for an L∗B galaxy
(Chen & Lanzetta 2003), while the dotted and dashed lines cor-
respond to galaxies with B band luminosities LB = 10%L
B and
LB = 1%L
B, respectively. Squares are objects from the literature
and triangles the candidates from this survey.
H i (Churchill et al. 2000), but recent observations of a larger
sample have indicated that the correlation is not always present
(Churchill et al. 2005).
We here investigate whether the candidates show a similar
anticorrelation using the impact parameters for the candidates as
a proxy for the sizes of neutral gas envelopes around proto galax-
ies. Assuming such a correlation is necessarily a rough approx-
imation because large morphological differences between indi-
vidual systems are expected (Rao et al. 2003; Chen & Lanzetta
2003). Specifically, the possible presence of sub-clumps is ne-
glected.
6.3.1. DLA galaxy sizes and luminosities
To analyse the sizes of DLA galaxies at z < 1, Chen & Lanzetta
(2003) describe the extension of the neutral gas cloud associated
with DLA galaxies as
, (1)
assuming that DLA galaxies follow a Holmberg relation be-
tween galaxy sizes and luminosities. Their fit to the observed
DLA galaxies gives R∗ = 30h−1 kpc, t = 0.26+0.24−0.06, where L
B cor-
responds to a galaxy with M∗B = −19.6. Based on the morpholo-
gies and impact parameters Chen et al. (2005) argued that dwarf
galaxies alone cannot represent the DLA galaxy population at
z < 1. In the case that the DLA galaxy population evolves from
low luminosity objects at high redshifts to higher luminosity at
lower redshifts, this will affect the expected impact parameters.
In Fig. 6 the impact parameters of candidates and confirmed ob-
jects are shown as a function of their redshifts. Overlayed on this
figure are curves for the size-luminosity relation derived for low
redshift DLA galaxies. If DLA galaxies at high redshift follow
the low redshift scaling relation, they comprise a mix of galaxy
luminosities.
16 L. Christensen et al.: An IFS survey for high-z DLA galaxies
0 10 20 30
b (kpc)
confirmed
candidates
Fig. 7. Column density of neutral hydrogen as a function of
impact parameters for the candidates and previously confirmed
objects. The symbols are similar to the previous figures. The
solid and dashed lines are fits to the power-law relation b/b∗ =
(N/N∗)β for the candidates and confirmed objects, respectively.
6.3.2. Powerlaw profiles
Using similar arguments as above one could expect that there is a
relation between the impact parameter b, and the column density
measured for the DLA. Fig. 7 shows the N(H i) measured for the
DLA as a function of the impact parameters in kpc. We assume
a similar scaling relation as in Eq. (1) for the impact parameter
and N(H i), i.e.
( N(H i)
N(H i)∗
We set log N(H i)∗ = 20.3 and the error on the measured
impact parameter is given by the fibre size of 0.′′5, which corre-
sponds to ∼4 kpc. A fit of the observed impact parameters for
the candidates gives b∗ = 15.9 ± 1.4 kpc, and β = −0.23 ± 0.08
as shown by the solid line, while for the confirmed objects, we
find b∗ = 12.0± 3.7 kpc and β = −0.36± 0.14 as represented by
the dashed line. The fits to the candidates and confirmed objects
are consistent within 1σ uncertainties.
6.3.3. Exponential profiles
Radio observations of the 21 cm emission from H i disks in the
local Universe have shown that an exponential profile is a poor
representation in the central part of disk galaxies, where the
21 cm flux density either stays constant or decreases towards
the centre (e.g. Verheijen & Sancisi 2001). However, at optical
wavelengths disk galaxies are well represented by exponential
profiles. Here we fit the impact parameter distribution by the re-
lation
N(H i) = N(H i)0 exp(−b/h) (3)
where h is the scale length, N(H i)0 the central column density
of a simple exponential disk, and N(H i) is the column density
measured for the DLAs. The resulting fit to all the candidates
is shown by the solid line in Fig. 8, which has log N(H i)0 =
21.7± 1.1 cm−2 and h = 5.1+2.5−1.3 kpc. This result is similar within
the uncertainties to a fit to the confirmed objects only (log N0 =
21.7 ± 1.1 cm−2 and h = 4.5+3.6−1.4 kpc).
0 5 10 15 20 25 30
b (kpc)
confirmed
candidates
Fig. 8. Column density of neutral hydrogen as a function of im-
pact parameter for the candidates and previously confirmed ob-
jects. The solid and dashed lines show the fit to the exponential
relation N(H i) = N(H i)0 exp(−b/h) for the candidates and con-
firmed objects, respectively.
Local disk galaxies have optical scale lengths ranging from
∼2 kpc to ∼6 kpc, and observations of the H i profile in low sur-
face brightness galaxies have indicated scale lengths > 10 kpc
(Matthews et al. 2001). In contrast, higher redshift spiral galax-
ies in the HST Ultra Deep field have smaller optical scale lengths
of 1.5–3 kpc (Elmegreen et al. 2005), possibly biased towards
smaller values due to the easier detection of high surface bright-
ness, high star formation rate regions. The question is how ex-
tended the gaseous envelopes are around these young galaxies.
The impact parameters of the candidates suggest that high red-
shift DLAs reside far from the host galaxy, if not in a regular
proto-galactic disk (Wolfe et al. 1986), then in a region of the
same physical scale, possibly in merging clumps of gas sur-
rounding the actual proto galaxy. In this picture, DLAs can be
found far from the center of the parent galaxy.
The two objects that were originally discarded as candidates
due to their large impact parameters (> 30 kpc) do not follow
the relations for either the exponential or power-law profiles.
Including them would make the scatter around the fit substan-
tial.
6.4. Metallicity effects on Lyα emission
Using a space based imaging survey and follow up long-slit
spectroscopic observations, Møller et al. (2004) found indica-
tions for a positive metallicity–Lyα luminosity relation, such
that Lyα emission was preferentially observed in higher metal-
licity systems. They argued that this positive correlation could
over-power the negative dust–Lyα luminosity effects which
are expected to be strong in high metallicity environments
(Charlot & Fall 1993). Studies of Lyα emission from nearby
star-forming galaxies have not revealed any correlations between
metallicity and Lyα emission strength (Keel 2005). Differences
in the velocity-metallicity relation between high and low redshift
DLAs could be explained by higher redshift DLAs residing in
lower mass galaxies (Ledoux et al. 2006), that have fainter Lyα
emission.
In this context we investigate the distribution of metallici-
ties for the emission line candidates in comparison to the total
L. Christensen et al.: An IFS survey for high-z DLA galaxies 17
-2.5 -2.0 -1.5 -1.0 -0.5
[Si/H]
good candidates
no candidates
Fig. 9. Cumulative distribution of Si metallicities of the six good
emission line candidates compared to the remaining part of the
sample. The probability that the two distributions are similar is
38% estimated from a Kolmogorov-Smirnov test.
sample. We compare the cumulative distributions of metallici-
ties ([Si/H]) for the DLAs that have candidate Lyα detections
with the metallicities of the remaining objects that have either
no Lyα candidates or rejected candidates. The distributions in
Fig. 9 show the fraction of DLAs with metallicities larger than a
given value. Table 1 has several lower limits on [Si/H] and Fig. 9
treats the limits as actual detections.
A two sided Kolmogorov Smirnov (KS) test gives a prob-
ability of 38% that the two samples have the same underlying
distributions. A similar analysis for [Fe/H] gives the same re-
sult. Hence, none of the tests give clear statistical evidence for a
difference between the two populations. Only a small number of
DLAs are included in this survey. For the two samples with N1
and N2 being the number of objects in each sample respectively
we find N1N2/(N1 + N2) = 4. For the KS test to be statistically
valid a value larger than 4 is required (Press et al. 1992), hence
a few more objects are needed to make the test more statistically
significant.
6.5. Metallicity gradients
In local galaxies the metallicities of H ii regions are shown to de-
crease with increasing radial distance in the disk (Zaritsky et al.
1994). If DLAs arise in disks lower metallicities are expected
at larger linear separations between the QSO line of sight and
the galaxy centers. A comparison of absorption metallicities for
3 DLAs at z < 0.6 with abundances derived from strong emis-
sion line diagnostics from the galaxy spectra revealed that gra-
dients are likely present at at level –0.041±0.012 dex kpc−1
(Chen et al. 2005). Uncertainties in the gradients arise due to
an unknown correction for dust depletion and also the inclina-
tion of the galaxy plays an important role due to projection ef-
fects. Metallicity differences due to differential depletion within
a singe DLA galaxy can be strong as demonstrated by observa-
tions of a lensed QSO (Lopez et al. 2005).
Observations of nebular emission lines from seven galax-
ies at 2.0 < z < 2.5 indicate an average solar metallicity
(Shapley et al. 2004), with evidence for the presence of metal-
licity gradients (Förster Schreiber et al. 2006). If metallicity gra-
dients exist for high redshift DLA galaxies, we would expect
0 10 20 30
b (kpc)
confirmed
candidates
Fig. 10. DLA metallicities as a function of the impact pa-
rameters, where symbols shapes have similar meanings as
in the previous figures. The dotted line shows a fit to all
the objects excluding the limits. This line has a gradient of
−0.024 ± 0.015 dex kpc−1. Data for the confirmed objects are
either [Si/H] of [Zn/H] taken from Møller et al. (2004).
to see a tendency for higher metallicities for the DLA galax-
ies detected at smaller impact parameters. DLA galaxies are
on the average fainter than LBGs detected in flux limited sur-
veys (Møller et al. 2002). Combining this with a high redshift
luminosity-metallicity relation can imply low DLA metallicities
without involving metallicity gradients.
Fig. 10 shows the DLA metallicities as a function of the im-
pact parameters. The line shows a fit to all objects ignoring those
with lower limits on [Si/H]. This gradient is –0.024±0.015, i.e.
is consistent with zero at the 2σ level. The large scatter in the
plot could be real and unrelated to gradients. Different DLAs
exhibit a large range of star formation histories (Erni et al. 2006;
Herbert-Fort et al. 2006), which makes it unreasonable to expect
a smooth relation between metallicities and impact parameters
for a sample of DLAs. Clearly more data are needed to deter-
mine the reality of any relation.
7. Conclusions
We have presented an integral field spectroscopic survey of 9
high redshift QSOs, which have a total of 14 DLA systems and
8 sub-DLA systems. We detect eight good candidates for Lyα
emission lines from DLA and sub-DLA galaxies. Two of these
are found at impact parameters larger than 30 kpc, and are not
likely associated directly with the absorbing galaxy, but could
be associated with galaxy groups in which the real absorbing
galaxy resides. All candidates are detected at a statistically sig-
nificant level in reconstructed narrow-band images as well as
in the co-added one-dimensional spectra. Further observations
will be useful to independently confirm the candidates at an even
higher signal to noise ratio. We compare the properties inferred
from the IFS data with those for previously spectroscopically
confirmed Lyα emission lines from DLA galaxies reported in
the literature. We find that line luminosities are similar to those
of previously confirmed objects, that the average impact param-
eters are larger by a factor of ∼2, and that some candidates have
larger velocity offsets between the Lyα emission line and the
systemic redshift of the DLA system.
18 L. Christensen et al.: An IFS survey for high-z DLA galaxies
We analyse the distribution of DLA column densities as a
function of impact parameters. Assuming that the average DLA
galaxy is similar to a disk galaxy with an exponential profile, we
show that it has a scale length of 5 kpc. Such a scale length is
similar to disk scale lengths found for local spiral galaxies. This
could imply that DLAs belong to large disks even at high red-
shifts as originally suggested by Wolfe et al. (1986). However,
it is probably too simplistic to expect that high redshift DLAs
reside in regular disks with similar structure to large local disks.
DLA systems are generally not associated with luminous galax-
ies (e.g. Colbert & Malkan 2002; Møller et al. 2002). The large
impact parameters found for the candidates indicate that the dis-
tribution of H i clouds in DLA galaxies extends significantly be-
yond the optical sizes of fainter dwarf galaxies.
Furthermore, Wolfe & Chen (2006) showed that high red-
shift DLAs do not reside in extended disks that follow the lo-
cal Schmidt-Kennicutt law for star formation. The IFS results
presented here suggest that the Lyα emission is generally not ex-
tended, and that star formation takes place at a distance of several
kpc from the DLA. Hence, we may speculate that DLAs arise in
the outskirts of proto-galaxies, for example in clouds of neutral
gas around LBGs. In this case one would expect a significant
scatter in the relation between the impact parameter and column
density of the DLA since neutral clouds could be distributed ir-
regularly around the galaxy. Contrary to expectations, the ob-
jects show a small scatter around the relations in Figs. 7 and 8.
Regardless of the distribution of neutral gas in DLA galaxies we
conclude that there is a tendency to find a lower column density
DLA with increasing impact parameter. Extending the investiga-
tion to include DLAs and sub-DLAs with N(H i)> 1019.6 cm−2
this tendency emerges for both the candidates and the confirmed
objects.
The velocity offsets between the Lyα emission lines and the
systemic redshifts of the DLAs are larger for half of the can-
didates compared to the confirmed objects. This could indicate
an origin in groups of galaxies, where the DLA resides in a less
luminous component than the galaxy detected in Lyα. To deter-
mine whether resonant scattering affects the candidate Lyα lines
more strongly and gives rise to larger velocity offsets than for
the confirmed objects, observations of the corresponding opti-
cal emission lines which are shifted to the near-IR are required
(e.g. Weatherley et al. 2005). Optical emission lines furthermore
have the advantage of being less affected by dust absorption
and therefore are better for estimating the star-formation rates.
Alternatively non-resonance UV lines such as C iv could be stud-
This survey was carried out with IFS on a 4-m class tele-
scope, and the signals were generally near the detection limit. To
verify this IFS method, it is necessary to get independent, higher
signal-to-noise ratio spectra with a larger aperture telescope to
confirm the existence of the emission lines.
Acknowledgements. This study was supported by the German
Verbundforschung associated with the ULTROS project, grant no.
05AE2BAA/4. S.F. Sánchez acknowledges the support from the Euro3D
Research Training Network, grant no. HPRN-CT2002-00305. K. Jahnke
acknowledges support from DLR project No. 50 OR 0404. We thank the referee
for useful suggestions that clarified the paper.
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	Introduction
	Sample selection
	Observations and data reduction
	Search for DLA optical counterparts
	Expected sizes
	Candidate selection
	Results
	Candidate significance class
	Non-detections
	Experiments with artificial objects
	Field Ly emitters
	Notes on individual objects
	Properties of candidate DLA counterparts
	Line fluxes
	Velocity differences
	Hi extension
	DLA galaxy sizes and luminosities
	Powerlaw profiles
	Exponential profiles
	Metallicity effects on Ly emission
	Metallicity gradients
	Conclusions
