9.7 micrometer Silicate Absorption in a Damped Lyman-alpha Absorber at
  z=0.52
Submitted to ApJ Letters (Revised Version including Referee’s Comments)
Preprint typeset using LATEX style emulateapj v. 08/22/09
9.7 µ M SILICATE ABSORPTION IN A DAMPED LYMAN-α ABSORBER AT Z = 0.52
Varsha P. Kulkarni1, Donald G. York2,3, Giovanni Vladilo4, Daniel E. Welty2
Submitted to ApJ Letters (Revised Version including Referee’s Comments)
ABSTRACT
We report a detection of the 9.7 µm silicate absorption feature in a damped Lyman-α (DLA) system
at zabs = 0.524 toward AO0235+164, using the Infrared Spectrograph (IRS) onboard the Spitzer Space
Telescope. The feature shows a broad shallow profile over ≈ 8-12 µm in the absorber rest frame and
appears to be > 15 σ significant in equivalent width. The feature is fit reasonably well by the silicate
absorption profiles for laboratory amorphous olivine or diffuse Galactic interstellar clouds. To our
knowledge, this is the first indication of 9.7 µm silicate absorption in a DLA. We discuss potential
implications of this finding for the nature of the dust in quasar absorbers. Although the feature is
relatively shallow (τ9.7 ≈ 0.08− 0.09), it is ≈ 2 times deeper than expected from extrapolation of the
τ9.7 vs. E(B − V ) relation known for diffuse Galactic interstellar clouds. Further studies of the 9.7
µm silicate feature in quasar absorbers will open a new window on the dust in distant galaxies.
Subject headings: Quasars: absorption lines–ISM:dust
1. INTRODUCTION
Damped Lyman-alpha (DLA) absorption systems in
quasar spectra dominate the neutral gas content in galax-
ies and offer venues for studying the evolution of metals
and dust in galaxies. Recent observations, however, sug-
gest that the majority of DLAs have low metallcities at
all redshifts studied (0 . z . 4), with the mean metal-
licity reaching at most ≈ 10 − 20% solar at the lowest
redshifts (see, e.g., Prochaska et al. 2003; Kulkarni et al.
2005, 2007; Péroux et al. 2006; and references therein).
These results appear to contradict the predictions of a
near-solar global mean interstellar metallicity of galaxies
at z ∼ 0 in most chemical evolution models based on the
cosmic star formation history inferred from galaxy imag-
ing surveys such as the Hubble Deep Field (HDF) (e.g.,
Madau et al. 1996). Furthermore, for a large fraction
of the DLAs, the SFRs inferred from emission-line imag-
ing searches fall far below the global predictions (e.g.,
Kulkarni et al. 2006, and references therein).
A possible explanation of these puzzles is that the cur-
rent DLA samples are biased due to dust selection ef-
fects, i.e. that the more dusty and more metal-rich ab-
sorbers obscure the background quasars more, making
them harder to observe (e.g., Fall & Pei 1993; Boissé et
al. 1998; Vladilo & Péroux 2005). DLAs are known to
have some dust, based on both the (generally mild) deple-
tions of refractory elements and the (typically slight) red-
dening of the background quasars (e.g., Pei et al. 1991;
Pettini et al. 1997; Kulkarni et al. 1997). Combining
∼ 800 quasar spectra from the Sloan Digital Sky Survey
(SDSS), York et al. (2006b) found a small but significant
amount of dust in absorbers at 1 < z < 2, with E(B−V )
of 0.02-0.09 for 9 of their 27 sub-samples (see also Khare
et al. 2007). York et al. (2006b) also showed that the
extinction in the composite spectra is best fitted by a
1 Department of Physics and Astronomy, University of South
Carolina, Columbia, SC 29208; E-mail: kulkarni@sc.edu
2 Department of Astronomy and Astrophysics, University of
Chicago, Chicago, IL 60637
3 Also, Enrico Fermi Institute
4 INAF, Osservatorio Astronomico di Trieste, Trieste, Italy
Small Magellanic Cloud (SMC) curve (with no 2175 Å
bump). Some recent studies suggest that dusty DLAs
could hide as much as 17% of the total metal content at
z ∼ 2, and more at lower z (Bouché et al. 2005). To un-
derstand whether this is the case, and to understand the
role of dust in quasar absorbers in general, it is essential
to directly probe the basic properties of the dust.
Recently, a small number of very dusty quasar ab-
sorbers have been discovered, via various signatures of
the dust in optical and UV observations: substantial red-
dening of the background quasars, large element deple-
tions (e.g., for Cr, Fe), and/or a detectable 2175 Å bump
(e.g., Junkkarinen et al. 2004; Wang et al. 2004). It is
not yet clear, however, whether the dust in these systems
is similar to that in the Milky Way or SMC or LMC.
The 2175 Å bump is generally, though not conclusively,
attributed to carbonaceous grains. The silicate compo-
nent of the dust, believed to comprise ≈ 70% of the core
mass of interstellar dust grains in the Milky Way (see,
e.g., Draine 2003) has not yet been probed in quasar ab-
sorbers. A unique opportunity to study this important
dust component is provided by the Spitzer IRS (Werner
et al. 2004; Houck et al. 2004), which provides the spec-
tral coverage, sensitivity, and resolution needed for the
detection of the strongest of the silicate spectral features
near 9.7 µm. The 9.7 µm feature, thought to arise in Si-O
stretching vibrations, is seen in a wide range of Galac-
tic and extragalactic environments (e.g., Whittet 1987
and references therein; Spoon et al. 2006; Imanishi et al.
2007). We have been carrying out an exploratory study
of the silicate dust in quasar absorbers by searching for
the 9.7 µm absorption feature with the Spitzer IRS. Here
we report on the detection of the 9.7 µm feature in one of
the systems studied, while the remaining three systems
observed recently will be reported in a separate paper
(Kulkarni et al. 2007b, in preparation).
2. OBSERVATIONS AND DATA ANALYSIS
The DLA at zabs = 0.524 (Junkkarinen et al. 2004)
toward the blazar AO 0235+164 (zem = 0.94) offers an
excellent venue for comparing dust in a distant galaxy
http://arxiv.org/abs/0704.0826v2
2 Kulkarni et al.
with that in near-by galaxies. It has one of the largest
H I column densities seen in DLAs (log NHI = 21.70)
and shows 21-cm absorption (Roberts et al. 1976). It
also shows X-ray absorption, consistent with a metallic-
ity of 0.7 solar (Junkkarinen et al. 2004). Candidate
absorber galaxies (much fainter than the blazar) within
a few arcseconds from the blazar sightline have been de-
tected (e.g., Smith et al. 1977; Yanny et al. 1989; Chun
et al. 2006). This absorber is one of a very few DLAs
producing appreciable reddening [E(B−V ) = 0.23 in the
absorber rest frame] and detection of a strong broad 2175
Å extinction bump (Junkkarinen et al. 2004). Finally,
this absorber is the only DLA with detections of several
diffuse interstellar bands (Junkkarinen et al. 2004; York
et al. 2006a). All of these data suggest that this absorber
is very dusty and may contain molecular gas.
The observations were obtained with the Spitzer IRS
on January 30, 2006 (UT) as GO program 20757 (PI
V. P. Kulkarni). IRS modules Short-Low 1 (SL1) and
Long-Low 2 (LL2) were used to cover 7.5-21.4 µm in the
observed frame (4.9-14.1 µm in the DLA rest frame). The
target was acquired with high-accuracy peakup using a
near-by bright star. The IRS standard staring mode was
used, with 2-pixel slit widths of 3.6” for SL1 and 10.5” for
LL2. Integration times were 60 s ×8 cycles for SL1 and
120 s ×11 cycles for LL2. For each cycle, observations
were performed at both nod positions A and B (offset by
1/3 the slit length), so the total integration times were
960 s and 2640 s, respectively, for SL1 and LL2.
The data were processed using the IRS S15.0 calibra-
tion pipeline (the latest version available at present), Im-
age Reduction and Analysis Facility (IRAF5), and In-
teractive Data Language (IDL). As detailed below, the
S15.0 pipeline yielded significant improvements for the
reliable detection and measurement of weak, broad fea-
tures in our spectra. The pipeline performs a number of
standard processing steps to produce the basic calibrated
data (BCD) files (see, e.g., the IRS Data Handbook
at http://ssc.spitzer.caltech.edu/irs/dh). Subtraction of
the sky (mostly zodiacal light) was performed by sub-
tracting the coadded frames at nod position B from those
at nod position A, and vice versa. The 1-dimentional
spectra were extracted from the 2-dimensional images us-
ing the Spitzer IRS Custom Extraction (SPICE) software
using the default extraction windows, and flux calibrated
using the standard S15.0 flux calibration files. The spec-
tra from the two nod positions were averaged together,
and the corresponding flux uncertainties calculated us-
ing both measurement uncertainties and “sampling un-
certainties” between the two nod positions.
The absolute flux levels in the different IRS mod-
ules were scaled to match the continuum levels in the
overlapping regions, using the bonus segment available
in the LL2 images. There was no mismatch between
the SL1 and LL2 flux levels; we used the SL1 data for
λ < 14.23µm and LL2 data for λ > 14.23µm. The data
at λ > 20µm bonus segment level had to be scaled up by
5.5% to match with the LL2 data at λ < 20µm. Fig. 1(a)
shows the final merged spectrum of AO0235+164. The
5 IRAF is distributed by the National Optical Astronomy Ob-
servatories, which are operated by the Association of Universities
for Research in Astronomy, Inc. (AURA), under cooperative agree-
ment with the National Science Foundation
S/N achieved per unbinned pixel in the final spectrum,
determined from rms fluctuations in the continuum re-
gions, is ≈ 100. The error bars denote 1 σ uncertainties.
The dashed line in Fig. 1(a) shows an estimate of the
power-law continuum of the quasar. This line joins the
observed continuum fluxes at 5.6 and 7.1 µm in the ab-
sorber rest frame and is extrapolated to the remaining
wavelength region. These wavelengths are chosen to be
in regions free of any other potential emission or absorp-
tion features (e.g., Imanishi et al. 2007). In principle,
significant 9.7 µm emission at the quasar redshift could
affect continuum determination redward of the suspected
silicate absorption feature from the DLA. However, (a)
our spectrum does not extend that far to the red, (b)
the power law provides a good fit to the continuum in
our data, and (c) the 9.7 µm emission is not particularly
strong in most quasars (e.g., Hao et al. 2007).
3. RESULTS
The spectrum shown in Fig. 1(a) exhibits a broad
absorption feature between about 12.4 and 18.3 µm rel-
ative to the power law continuum. The flux decline from
the continuum begins near the long wavelength end of
SL1 and continues smoothly into the LL2 data. The
broad feature is centered at 15.41 µm (10.11 µm in the
DLA rest frame). The observed frame equivalent width
is 0.31µm, with a 1 σ uncertainty of 0.014-0.020 µm, in-
cluding contributions from photon noise and continuum
fitting uncertainties (Sembach & Savage 1992).
We have performed several checks of our data analy-
sis to see whether the observed broad feature could be
an artifact. Since the possible silicate feature is broad
and shallow, extending from the long wavelength end of
SL1 through most of LL2, flux calibration and contin-
uum fitting are critical issues. In the S14 pipeline ver-
sion of these data, the possible silicate feature was some-
what stronger than in the S15 version. These differences
are due to a low-level non-linearity problem in the S14
pipeline, which produces a 4% tilt in LL2 spectra and a
5% mismatch at the SL1/LL2 boundary. This problem
has been eliminated in the S15 pipeline, and we find no
mismatch at the SL1/LL2 boundary in the S15 data.
The possible silicate feature does not show any visi-
ble signature of the “teardrop” feature known to exist
near 14.1 µm in some SL1 data (see, e.g., the IRS data
handbook). The beginning of decline in flux at the long-
wavelength end of SL1 matches smoothly with the flux at
the short-wavelength end of LL2 (which does not suffer
from the teardrop problem). Our results do not change
much even if the SL1 data are truncated at 14 µm to
avoid the region potentially affected by the teardrop (the
region 14-14.23 µm is a small fraction of the whole fea-
ture stretching out to 18.3 µm in the observed frame).
Inaccuracies in pointing (which can affect SL1 and LL2
fluxes at the ±1% level) also do not appear to be sig-
nificant for our data. Based on an examination of the
spectral images and the pointing difference keywords in
the data file headers, the telescope pointing was accu-
rate to within 0.09-0.11” for LL2 and within 0.22-0.29”
for SL1. Integrating a Gaussian intensity distribution
from a point source with the Spitzer point spread func-
tion over the known slit dimensions (57′′ × 3.6′′ for SL1,
168′′×10.5′′ for LL2), we estimate that the effect of such
an offset would be about 0.26% for SL1 and 0.05% for
http://ssc.spitzer.caltech.edu/irs/dh
Silicate Feature in A DLA 3
LL2, far too small to account for the observed feature.
We also compared our results with IRS spectra from
the literature for quasars without strong absorption sys-
tems (e.g., Sturm et al. 2006; Hao et al. 2007), and did
not find the broad absorption feature from our data in
those other quasars. In fact, quasar spectra in general
show no silicate absorption, but rather (generally rela-
tively weak) silicate emission at the quasar emission red-
shift. We also compared our IRS data for AO0235+164
with those for other targets in our study. The feature
seen in AO0235+164 is not seen at the same observed
wavelength in the other objects, suggesting that it is
not an instrumental artifact. [In fact, in Kulkarni et al.
2007b (in prep.), we will report the possible detection of
redshifted broad 9.7 µm silicate absorption in other parts
of the Spitzer spectral coverage toward other quasars.]
Given the results of the above tests and the fact that
the DLA toward AO0235+164 is already known to be
dusty (from detection of 2175 Å bump and diffuse inter-
stellar bands and reddening of the background quasar),
it seems very likely that the feature detected is the broad
9.7 µm silicate feature arising in the absorber galaxy.
4. DISCUSSION
The suggested silicate feature in the DLA toward
AO0235+164 is relatively shallow/weak compared to the
silicate features typically observed in Galactic interstel-
lar material (ISM) because of the modest reddening and
lower amounts of dust in quasar absorbers than in the
Milky Way. Indeed, the dust-to-gas ratio in the DLA
toward AO0235+164 is estimated to be 0.19 times the
Galactic value (Junkkarinen et al. 2004). On the other
hand, the observed feature is stronger than expected
from E(B−V ) = 0.23±0.01 for this absorber (Junkkari-
nen et al. 2004). In Galactic diffuse interstellar clouds,
the peak optical depth in the 9.7 µm silicate feature (τ9.7)
is observed to correlate with the reddening along the line
of sight, with τ9.7 = AV /18.5 (e.g., Whittet 1987). Ex-
trapolating this relation, and assuming RV = 3.1, one
would expect τ9.7 ≈ 0.039 for the DLA in AO0235+164.
Our observations, however, indicate τ9.7 ≈ 0.08 for this
DLA, ∼ 2 times higher than expected from the relation
for Galactic diffuse ISM. The dust in this absorber may
thus be somewhat richer in silicates than typical Galac-
tic dust. We note, however, that the silicate feature is
also known to be stronger in the Galactic Center region,
perhaps due to fewer carbon stars (and thus less carbona-
ceous dust) there (e.g., Roche & Aitken 1985). If future
observations of other DLAs also reveal material richer in
silicates, it might indicate that those DLAs probe denser
regions near the centers of the respective galaxies.
The Galactic interstellar 9.7 µm feature is generally
broad and relatively featureless, which is taken as an in-
dication that interstellar silicates are largely amorphous.
(Crystalline silicates would produce structure within the
broad feature.) In principle, silicate grains may be com-
posed of a mixture of pyroxene-like [(MgxFe1−x)SiO3]
and olivine-like [(MgxFe1−x)2SiO4] silicates, with the
shape and central wavelength of the 9.7 µm absorp-
tion somewhat dependent on the exact composition (e.g.,
Kemper et al. 2004; Chiar & Tielens 2006). Fig. 1(b)
shows a closer view of the data, normalized by the power
law continuum shown in Fig. 1(a), and binned by a fac-
tor of 3. The dotted and short-dashed curves are fits
based on silicate emissivities derived from observations
of the M supergiant µ Cep and of the Orion Trapezium
region (e.g., Roche & Aitken 1984; Hanner et al. 1995),
which are taken to be representative of diffuse Galactic
ISM and denser molecular material, respectively . The
long-dashed and dot-dashed curves are fits based on the
silicate absorption profile observed toward the Galactic
Center Source GCS3, and on laboratory measurements
for amorphous olivine (Spoon et al. 2006). The shape
of the silicate profile observed toward AO 0235+164 is
most similar to that of laboratory amorphous olivine,
but the µ-Cep and GCS3 templates also yield reasonable
fits. The DLA silicate profile does not exhibit the red-
ward extension seen for the Trapezium profile, suggest-
ing that the DLA dust resembles dust in diffuse Galactic
clouds more than that in molecular clouds. Using χ2
minimization for 8.0-13.3 µm in the DLA rest frame, the
peak optical depth values τ9.7 for the laboratory olivine,
GCS3, µ Cep, and Trapezium templates are 0.081±0.018,
0.088 ± 0.020, 0.083 ± 0.018, and 0.071 ± 0.016, respec-
tively for the binned data (0.081± 0.020, 0.091± 0.023,
0.084± 0.021, and 0.069± 0.017, respectively, for the un-
binned data). The error bars on τ9.7 correspond to op-
tical depths that give reduced χ2 larger by 1.0 than the
minimum values. The respective reduced χ2 values are
1.22, 1.32, 1.51, and 1.92 for the binned data (1.82, 2.08,
2.10, and 2.65 for the unbinned data). It is interesting
to note that the best-fit astronomical template is GCS3,
consistent with the enhanced τ9.7/E(B−V ) ratio seen in
the DLA as toward the Galactic center. While the min-
imum reduced χ2 values are greater than 1.0, they are
similar to those found in other studies of the silicate ab-
sorption toward both Galactic and extragalactic sources
(e.g., Hanner et al. 1995; Bowey et al. 1998; Roche et
al. 2006, 2007). Indeed, we do not expect a perfect fit,
since possible differences in dust grain size and chemical
composition can alter the shape of the silicate feature, in-
cluding the peak wavelength and the FWHM (Bowey et
al. 1998 and references therein). Higher S/N and higher
resolution data would be needed to shed further light on
the specific types of silicates present in DLAs.
With a larger absorber sample, it would be possible to
explore correlations between the strengths of the 9.7 µm
silicate feature and the 2175 Å extinction bump (which
is thought to be produced by a carbonaceous component
of the dust). For example, it would be interesting to
understand whether the relative amounts of silicate and
carbonaceous dust vary with redshift or with the gas-
phase abundances of C or Si. High-S/N observations of
other possible features (e.g., the 18.5 µm silicate feature
or the 3.0 µm H2O ice feature) would provide additional
constraints on dust composition. (While those features
are generally weaker than the 9.7 µm feature in the Milky
Way, the 3.0 µm feature can be stronger than the 9.7 µm
feature in highly reddened molecular sightlines.)
Our exploratory study has demonstrated the potential
of the Spitzer IRS to study dust in quasar absorbers.
It would be very interesting to obtain similar spectra
for other dusty quasar absorbers. The E(B − V ) val-
ues for dusty absorbers such as that reported here (0.23)
are much larger than those for typical Mg II absorbers
[E(B − V ) of 0.002; York et al. 2006b]. These relatively
large reddening values are comparable to some of those
4 Kulkarni et al.
for Ly-break galaxies (LBGs), which show E(B − V ) up
to 0.4 and a median E(B − V ) of ≈ 0.15 at z ∼ 2
and z ∼ 3 (Shapley et al. 2001, 2005; Papovich et al.
2001). Such dusty absorbers appear to be chemically
more evolved (Wild et al. 2006) than typical DLAs, and
may possibly provide a link in terms of SFRs, masses,
metallicities, and dust content between the primarily
metal-poor and dust-poor general DLA population with
low SFRs and the actively star-forming, metal-rich, and
dust-rich LBGs. Further Spitzer IRS observations of
more dusty quasar absorbers thus will help to open a
new window on this interesting class of distant galaxies.
This work is based on observations made with the
Spitzer Space Telescope, which is operated by the Jet
Propulsion Laboratory, California Institute of Technol-
ogy under a contract with NASA. Support for this work
was provided by NASA through an award issued by
JPL/Caltech. VPK acknowledges support from NSF
grant AST-0607739 to University of South Carolina.
DEW acknowledges support from NASA LTSA grant
NAG5-11413 to the University of Chicago. We are grate-
ful to the Spitzer Science Center staff for helpful advice
on data analysis and to an anonymous referee for helpful
comments.
Facilities: SST (IRS).
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http://arxiv.org/abs/astro-ph/0608126
Silicate Feature in A DLA 5
0.85 0.95 1.05 1.15 1.25 1.35
−1.65
−1.55
−1.45
−1.35
−1.25
−1.15
−1.05
SL order 1
LL order 2
LL bonus order
log λobserved
5.0 10.06.0 8.0 12.0
Rest Wavelength(
Q0235+164
z abs = 0.524
6.5 7.5 8.5 9.5 10.5 11.5 12.5 13.5
Rest Wavelength
tau_9.7=0.083, mu Cep
tau_9.7=0.071, Trapezium
tau_9.7=0.088, GCS3
tau_9.7=0.081, Lab Olivine
( m)µ
Fig. 1.— (a) Left: Spitzer IRS spectrum of AO0235+164. The lower scale for the abscissa denotes the logarithm of the observed
wavelength in µm; rest frame wavelengths at the absorber redshift are shown at the top. The errorbars denote 1 σ flux uncertainties. The
dashed line shows a power law estimate of the continuum. (b) Right: A closer look at the suggested silicate feature. The abscissa denotes
the rest frame wavelength at the DLA redshift. The data points show the spectrum, normalized by the power law continuum and binned
by a factor of 3. The errorbars denote 1 σ uncertainties. The smooth curves show profiles for four templates of silicate optical depth, based
on observations for three Galactic sightlines and laboratory measurements for amorphous olivine.
