Detecting the Most Distant (z>7) Objects with ALMA
Noname manuscript No.
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Detecting the Most Distant (z>7) Objects with ALMA
Fabian Walter and Chris Carilli
Received: date / Accepted: date
Abstract Detecting and studying objects at the high-
est redshifts, out to the end of Cosmic Reionization at
z>7, is clearly a key science goal of ALMA. ALMA
will in principle be able to detect objects in this red-
shift range both from high-J (J>7) CO transitions and
emission from ionized carbon, [CII], which is one of the
main cooling lines of the ISM. ALMA will even be able
to resolve this emission for individual targets, which will
be one of the few ways to determine dynamical masses
for systems in the Epoch of Reionization. We discuss
some of the current problems regarding the detection
and characterization of objects at high redshifts and
how ALMA will eliminate most (but not all) of them.
1 Introduction: The highest redshift galaxies
In recent years, deep narrow band surveys have revealed
a major population of Lyman Alpha Emitters (LAE)
out to very high redshifts (e.g. Hu et al. 2002, Kurk
et al. 2004, Stern et al. 2005, Murayama et al. 2007).
In particular, Taniguchi et al. (2005) report the detec-
tion of 9 spectroscopically confirmed LAE at redshifts
of z∼6.6 in the Subaru Deep Field (currently, the pub-
lished LAE redshift record holder is at z=6.98, Iye et
al. 2006). The mere presence of Lyman alpha emission
in these sources provides strong evidence that they are
undergoing bursts of star formation: the star formation
It is our pleasure to thank our collaborators on this project:
Frank Bertoldi, Dominik Riechers, Pierre Cox, Roberto Maiolino
and Axel Weiß.
F. Walter
Max Planck Insitut für Astronomie Heidelberg, E-mail: wal-
ter@mpia.de
C. Carilli
National Radio Astronomy Observatory, E-mail: ccar-
illi@nrao.edu
rates of individual objects are ∼10M⊙ yr
−1 (based on
their FUV luminosities) and their redshifts place them
well within the end of cosmic reionization. They also
appear to be very numerous: Tanaguchi et al. 2005 find
∼30 LAEs in only a quarter degree field (e.g., compared
to ∼10 QSOs at z>6 which are distributed over a quar-
ter of the sky! Fan et al. 2004). This implies that LAEs
may play an important role in reionizing the universe at
z>6 (for a review see Fan, Carilli & Keating, 2006). In-
vestigating the physical properties of these sources are
thus of great interest and ALMA will play a critical role
in studying these objects, as discussed in the following.
2 Interstellar Medium: CO vs. [CII] emission
2.1 Carbon Monoxide (CO)
Constraining the properties of the molecular gas in ob-
jects at the end of cosmic reionization is clearly of key
importance as such observations 1) will measure the
available ’fuel’ for star formation, 2) will help to con-
strain the dynamical mass of the system and will thus
3) allow to put these objects in an evolutionary con-
text for early galaxy formation. Typically, at low and
high redshifts, CO emission is used as a tracer for the
molecular gas phase (e.g. review by Solomon & Vanden
Bout 2004). It is important to keep in mind though,
that, at the highest z, only the very high rotational
lines of CO will be observable with ALMA. E.g. even in
the lowest (currently funded) frequency band of ALMA
(band 3, 84–119GHz), only CO transitions with J>7
(i.e., CO(7–6), CO(8–7), etc.) will be observable at z>7.
This is graphically illustrated in Fig. 1 where we plot
ALMA’s ‘CO discovery space’ (i.e., which line can be
observed at which redshift using which ALMA band).
The high–J transitions correspond to highly excited gas
http://arxiv.org/abs/0704.1858v1
0 5 10 15 20
Redshift
Band  4 (125-163 GHz)
Band  3 ( 84-119 GHz)
Band  4 (125-163 GHz)
Band  6 (211-275 GHz)
Band  7 (275-370 GHz)
Band  8 (385-500 GHz)
Band  9 (602-720 GHz)
Band 10 (787-950 GHz)
<−  [CII]
Fig. 1 ALMA CO ‘discocery space’: The horizontal lines indicate
which CO transition (plotted on the y–axis) can be observed with
which ALMA band as a function of redshift (plotted on the x–
axis). For objects with z>7, only the higher–J CO transitions
can be observed with ALMA. The [CII] ‘discovery space’ is also
indicated.
(either due to high kinetic temperatures, high densities,
or both) which may not be excited in normal starform-
ing environments. This is shown in Fig. 2 (taken from
Weiss et al. 2005) where measured CO line strengths (as
a function of J, this is sometimes referred to as CO line
ladders/SEDs) are plotted for a number of key sources.
What is immediately obvious from this plot is that most
objects have sharply decreasing CO line strengths be-
yond J> 8, in particular starforming systems such as
NGC253, or the sub–millimeter galaxy plotted in this
diagram (the quasars appear to be more excited, but
their CO line SED still turns over at J∼7, for an excep-
tional object see APM07279, Weiss et al. 2007). This
comparison immediately implies that emission from the
CO molecule will typically be very difficult to observe
with ALMA at z>7 as the observable lines will simply
not be excited.
2.2 Ionized Carbon ([CII]) to the Rescue!
An alternative tracer of the interstellar medium is one
of the main cooling line of the ISM, the 2P3/2 →
2P1/2
fine–structure line of C+ (or [CII]). In brief, the [CII]
line is expected to be much stronger than any of the
CO lines. Given its high frequency (157.74µm, corre-
sponding to 1900.54GHz) [CII] studies in the local uni-
verse are limited to airborne or satellite missions (e.g.
Stacey et al. 1991, Malhotra et al. 1997, Madden et
Fig. 2 Comparison of various normalized (by their CO(1–0) flux
density) CO line SEDs at low and high redshift (figure taken from
Weiss et al. 2005). The CO line SEDs decline rapidly beyond
J=6–8.
al. 1997). These studies have demonstrated that this
single line can indeed carry a good fraction of the total
infrared luminosity (LFIR) of an entire galaxy. In the lo-
cal universe, the ratio LCII/LFIR has been found to be
2–5×10−4 in the case of ULIRGS (e.g. Gerin & Phillips
2000), but is more like 5–10×10−3 in more typical star-
forming galaxies (for a discussion on possible reasons
for the supressed ratio in ULIRGs see, e.g., Luhman et
al. 1998). Notably, the ratio has been found to be 1% or
even higher in low metallicity environments. E.g., in the
low- metallicity galaxy IC10, LCII/LFIR reaches values
as high as 4%, with an average value of 2% (Madden
et al. 1997, see Israel et al. 1996 for a similar result for
the LMC). This is the reason why it has long been ar-
gued (e.g., Stark 1997) that observation of the [CII] line
of prestine systems at the highest redshifts will likely
be the key to study molecular gas in the earliest star-
forming systems, in particular in the era of ALMA. The
ALMA [CII] ‘discovery space’ is also indicated in Fig. 1.
3 Expected [CII] line strengths
At the redshifts of the LAEs, the [CII] line is shifted
to the 1mm band of ALMA (band 6, 211–275GHz).
[CII] emission has recently been successfully detected
using the IRAM 30m in the highest redshift quasar
J1148+5251 at z=6.42 (Maiolino et al. 2005, see Fig. 3).
The noteable difference between J1148+5251 and the
z>6 LAEs is that the ratio LCII/LFIR has been found to
be very low (∼5×10−4) in J1148+5251, i.e. in perfect
Fig. 3 Top: First detection of [CII] at high redshift in the z=6.42
QSO J1148+5251 (Maiolino et al. 2005). Bottom: brightest CO
transition (J=6) in the same source (Bertoldi et al. 2003, Walter
et al. 2003). Note that the [CII] line is brighter by a factor of ∼5.
agreement with studies of low redshift ULIRGs that
show a central AGN. On the contrary, the LAE are
presumably pure starbursts (no evidence for an AGN
is found, Taniguchi et al. 2005) and they likely have
lower metallicities compared to the highly overdense
regions in which the luminous quasars are supposedly
present. All these arguments point towards a LCII/LFIR
ratio in LAE that is close to what is found for nearby
normal galaxies, or perhaps even for the metal–poor
dwarf galaxies (i.e., around 1% or even higher). In other
words, the [CII] luminosity of the LAE may well be an
order of magnitude stronger (for a given IR luminos-
ity) than what has been found in the z=6.4 QSO. In
the following we present a quick back–of–the envelope
calculation based on the detected [CII] line strength in
J 1148+5251 (∼10 mJy) which has a SFR of a few 1000
M⊙ yr
−1. This SFR is more than two order of magni-
tudes higher than the SFR found in a typical LAE, but
as the LCII/LFIR may be higher by an order of magni-
tude in the LAEs, the expected [CII] line strength of
the LAE may be as high as 1mJy. Such a line should
be easily detectable with ALMA at high significance in
a few hours.
4 Resolving the ISM
Detecting the [CII] (or CO) emission is critical to esti-
mate the reservoir of the (molecular) gas in these early
systems. A second step is then to spatially resolve the
molecular gas distribution. In particular, given the typ-
RIGHT ASCENSION (J2000)
11 48 16.70 16.68 16.66 16.64 16.62 16.60
52 51 50.8
Fig. 4 High–resolution CO image of the z=6.42 QSO
J 1148+5251 obtained at the VLA (Walter et al. 2004). The res-
olution achieved in these observations (0.15′′, corresponding to
∼1 kpc) will be routinely reached with ALMA.
ical diameters of galaxies of many kpc, a linear resolu-
tion of ∼ 1 kpc is needed to resolve the structure of the
underlying galaxy. Such measurements are needed 1) to
get an estimate for the size of the galaxy (and thus a
better estimate for the dynamical mass), 2) to resolve
potentially merging systems and 3) to better constrain
the physical properties of the gas (e.g., by measuring
the brightness temperature of the hosts). A linear res-
olution of 1 kpc corresponds to a resolution of 0.15′′
at the redshifts under consideration (1′′ ∼ 5.8 kpc at
z = 6). Such observations can then in turn be used to
constrain the predictions by CDM simulations of early
galaxy formation, and, if a large sample was available,
put limits on the frequency of mergers at high redshift.
In addition, such studies can be used to constrain the
possible redshift-evolution of the MBH–σv relation in
high–z quasars. Such observations will clearly be feasi-
ble with ALMA in the extended arrays. High–resolution
CO imaging is already possible with the current genera-
tion of telescopes: we have used the VLA to resolve the
molecular gas in the host galaxy of the z=6.42 QSO
J1148+5251 (see Fig. 4, Walter et al. 2004).
5 The case for ALMA band 5
As a technical note: the ALMA redshift coverage for the
[CII] line is not ideal as its frequency lies between the
CO(17–16) and CO(16–15) transition (see Fig. 1). One
concern is that the critical redshift range (8<z<10.5) is
currently not fully covered: This frequency range corre-
sponds to the ALMA band 5 which is only partly funded
by the European Union as part of the Sixth Frame-
work Programme (FP6) for up to 8 antennas. Clearly,
it would be highly desireable to equip as many ALMA
antennas with band 5 receivers as possible.
6 Concluding remarks
ALMA observations of the [CII] line will play a fun-
damental role in studying the youngest galaxies in the
Epoch of Cosmic Reionization at z>7. Given the ex-
pected line strengths it should be possible to resolve
these galaxies in the [CII] line emission on kpc scales.
Such measurements would not only constrain the sizes
but would also help to derive the dynamical masses in
these early starforming systems. Given the typical CO
excitation in starforming galaxies (i.e. drop in excita-
tion around the J∼6 transitions), ALMA will likely act
as a [CII]– rather than a CO–machine for objects at
these extreme redshifts.
References
1. Bertoldi, F., Carilli, C. L., Cox, P., Fan, X., et al. 2003, A&A,
406, L55
2. Fan, X., Hennawi, J. F., Richards, G. T., et al. 2004, AJ, 128,
3. Fan, X., Carilli, C.L., Keating, B., 2006, ARA&A, 44, 415
4. Gerin, M., & Phillips, T. G. 2000, ApJ, 537, 644
5. Hu, E. M., Cowie, L. L., McMahon, R. G., et al. 2002, ApJ,
568, L75
6. Israel, F. P., Maloney, P. R., Geis, N., Herrmann, F., Madden,
S. C., Poglitsch, A., Stacey, G.J., 1996, ApJ, 465, 738
7. Iye, M., Ota, K., Kashikawa, N., Furusawa, H., Hashimoto,
T., Hattori, T., Matsuda, Y., Morokuma, T., Ouchi, M., Shi-
masaku, K., 2006, Nature, 443, 186
8. Kurk, J. D., Cimatti, A., di Serego Alighieri, et al. 2004, A&A,
422, L13
9. Luhman, M. L., et al. 1998, ApJL, 504, L11
10. Madden, S. C., Poglitsch, A., Geis, N., Stacey, G. J., &
Townes, C. H. 1997, ApJ, 483, 200
11. Maiolino, R., Cox, P., Caelli, P., et al. 2005, astro-ph/0508064
12. Malhotra, S., et al. 1997, ApJL, 491, L27
13. Murayama, T., Taniguchi, Y., Scoville, N.Z., et al. 2007,
ApJS, astro-ph/0702458
14. Stacey, G. J., Geis, N., Genzel, R., Lugten, J. B., Poglitsch,
A., Sternberg, A., & Townes, C. H. 1991, ApJ, 373, 423
15. Stark, A. A. 1997, ApJ, 481, 587
16. Stern, D., Yost, S. A., Eckart, M. E., Harrison, F. A., Helfand,
D. J., Djorgovski, S. G., Malhotra, S., & Rhoads, J. E. 2005 ,
ApJ, 619, 12
17. Taniguchi, Y., Ajiki, M., Nagao, T. et al. 2005, PASJ, 57,
18. Walter, F., Bertoldi, F., Carilli, C. L., et al. 2003, Nature,
424, 406
19. Walter, F., Carilli, C., Bertoldi, F., et al. 2004, ApJ, 615, L17
20. Weiss, A., Downes, D., Neri, R., Walter, F., Henkel, C.,
Wilner, D.J., Wagg, J., Wiklind, T., 2007, A&A, in press
(astro–ph/0702669)
http://arxiv.org/abs/astro-ph/0508064
http://arxiv.org/abs/astro-ph/0702458
http://arxiv.org/abs/astro--ph/0702669
	Introduction: The highest redshift galaxies
	Interstellar Medium: CO vs. [CII] emission
	Expected [CII] line strengths
	Resolving the ISM
	The case for ALMA band 5
	Concluding remarks
