The Redshift and Nature of AzTEC/COSMOS 1: A Starburst Galaxy at z = 4.6

Abstract

Based on broadband/narrowband photometry and Keck DEIMOS spectroscopy, we report a redshift of z = 4.64+0.06 –0.08 for AzTEC/COSMOS 1, the brightest submillimeter galaxy (SMG) in the AzTEC/COSMOS field. In addition to the COSMOS-survey X-ray to radio data, we report observations of the source with Herschel/PACS (100, 160 μm), CSO/SHARC II (350 μm), and CARMA and PdBI (3 mm). We do not detect CO(5 → 4) line emission in the covered redshift ranges, 4.56-4.76 (PdBI/CARMA) and 4.94-5.02 (CARMA). If the line is within this bandwidth, this sets 3σ upper limits on the gas mass to 8 × 109 M ☉ and 5 × 1010 M ☉, respectively (assuming similar conditions as observed in z ~ 2 SMGs). This could be explained by a low CO-excitation in the source. Our analysis of the UV-IR spectral energy distribution of AzTEC 1 shows that it is an extremely young (50 Myr), massive (M * ~ 1011 M ☉), but compact (2 kpc) galaxy, forming stars at a rate of ~1300 M ☉ yr–1. Our results imply that AzTEC 1 is forming stars in a "gravitationally bound" regime in which gravity prohibits the formation of a superwind, leading to matter accumulation within the galaxy and further generations of star formation.

Full text

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arXiv:1102.4329v1 [astro-ph.CO] 21 Feb 2011
Preprint typeset using LATEX style emulateapj v. 11/10/09
THE REDSHIFT AND NATURE OF AZTEC/COSMOS 1: A STARBURST GALAXY AT Z = 4.6
V. Smolˇ ci´ c1,2, P. Capak3,4, O. Ilbert5, A.W. Blain3, M. Salvato3,9, I. Aretxaga14, E. Schinnerer23, D. Masters3,6,
I. Mori´ c3,7, D. A. Riechers3,8, K. Sheth4, M. Aravena10, H. Aussel11, J. Aguirre12,13, S. Berta15, C. L. Carilli16,
F. Civano17, G. Fazio17, J. Huang17, D. Hughes14, J. Kartaltepe18, A. M. Koekemoer19, J.-P. Kneib5,
E. LeFloc’h18,20, D. Lutz15, H. McCracken5, B. Mobasher6, E. Murphy4, F. Pozzi21, L. Riguccini22,
D. B. Sanders18, M. Sargent23, K. S. Scott24, N.Z. Scoville3, Y. Taniguchi25, D. Thompson26, C. Willott27,
G. Wilson28, M. Yun28
ABSTRACT
Based on broad/narrow-band photometry and Keck DEIMOS spectroscopy we report a redshift of
z = 4.64+0.06
−0.08for AzTEC/COSMOS 1, the brightest sub-mm galaxy in the AzTEC/COSMOS field.
In addition to the COSMOS-survey X-ray to radio data, we report observations of the source with
Herschel/PACS (100, 160 µm), CSO/SHARC II (350 µm), CARMA and PdBI (3 mm). We do not
detect CO(5 → 4) line emission in the covered redshift ranges, 4.56-4.76 (PdBI/CARMA) and 4.94-
5.02 (CARMA). If the line is within this bandwidth, this sets 3σ upper limits on the gas mass to
? 8 × 109M⊙ and ? 5 × 1010M⊙, respectively (assuming similar conditions as observed in z ∼ 2
SMGs). This could be explained by a low CO-excitation in the source. Our analysis of the UV-IR
spectral energy distribution of AzTEC 1 shows that it is an extremely young (? 50 Myr), massive
(M∗ ∼ 1011M⊙), but compact (? 2 kpc) galaxy forming stars at a rate of ∼ 1300 M⊙yr−1. Our
results imply that AzTEC 1 is forming stars in a ’gravitationally bound’ regime in which gravity
prohibits the formation of a superwind, leading to matter accumulation within the galaxy and further
generations of star formation.
Subject headings: galaxies: distances and redshifts – galaxies: high-redshift – galaxies: active – galax-
ies: starburst – galaxies: fundamental parameters
1ESO ALMA COFUND Fellow, European Southern Ob-
servatory, Karl-Schwarzschild-Strasse 2, 85748 Garching b.
Muenchen, Germany
2Argelander Institut for Astronomy, Auf dem H¨ ugel 71, Bonn,
53121, Germany
3California Institute of Technology, MC 249-17, 1200 East
California Boulevard, Pasadena, CA 91125
4Spitzer Science Center, 314-6 Caltech, 1201 E. California
Blvd. Pasadena, CA, 91125
5Laboratoire d’Astrophysique de Marseille, Universit´ e de
Provence, CNRS, BP 8, Traverse du Siphon, 13376 Marseille
Cedex 12, France
6Department of Physics and Astronomy, University of Cali-
fornia, Riverside, CA, 92521, USA
7University of Zagreb, Physics Department, Bijeniˇ cka cesta
32, 10000 Zagreb, Croatia
8Hubble Fellow
9Max-Planck-Institut fr Plasmaphysik, Boltzmanstrasse 2,
Garching 85748, Germany
10National Radio Astronomy Observatory, 520 Edgemont
Road, Charlottesville, VA 22903, USA
11UMR AIM (CEA-UP7-CNRS), CEA-Saclay, Orme des
Merisiers, bt. 709, F-91191 Gif-sur-Yvette Cedex, France
12Jansky Fellow, National Radio Astronomy Observatory
13University of Pennsylvania, Department of Physics and As-
tronomy, 209 South 33rd Street, Philadelphia, PA 19104
14Instituto Nacional de Astrof´ ısica,
(INAOE), Aptdo. Postal 51 y 216, 72000 Puebla, Pue., Mex-
ico
15Max-Planck-Institut f¨ ur extraterrestrische Physik, Postfach
1312, 85741 Garching, Germany
16National Radio Astronomy Observatory, P.O. Box 0, So-
corro, NM 87801-0387
17Harvard-Smithsonian Centre for Astrophysics, 60 Garden
Street, Cambridge, MA 02138, USA
18Institute for Astronomy, University of Hawaii, 2680 Wood-
lawn Drive, Honolulu, HI, 96822, USA
19Space Telescope Science Institute, 3700 San Martin Drive,
Baltimore, MD 21218
20Spitzer Fellow
21INAF Osservatorio Astronomico di Roma, via di Franscati
´Optica y Electr´ onica
33, 00040 Monte Porzio Catone, Italy
22Laboratoire AIM-Paris-Saclay, CEA/DSM/Irfu
Universite Paris Diderot, CE-Saclay, pt courrier 131, F-91191
Gif-sur-Yvette, France
23Max Planck Institut f¨ ur Astronomie, K¨ onigstuhl 17, Hei-
delberg, D-69117, Germany
24Department of Physics and Astronomy, University of Penn-
sylvania,Philadelphia PA, 19104
25Research Center for Space and Cosmic Evolution, Ehime
University, Bunkyo-cho 2-5, Matsuyama 790-8577, Japan
26Large Binocular Telescope Observatory, University of Ari-
zona, 933 N. Cherry Ave., Tucson, AZ, 85721, USA
27Herzberg Institute of Astrophysics, National Research
Council, 5071 West Saanich Rd., Victoria, BC V9E 2E7, Canada
28Department of Astronomy, University of Massachusetts,
Amherst, MA 01003, USA
⋆Based on observations with: the W.M. Keck Observatory,
the Canada-France-Hawaii Telescope; the United Kingdom In-
frared Telescope; the Subaru Telescope; the NASA/ESA Hubble
Space Telescope; the NASA Spitzer Telescope; the Caltech Sub-
mm Observatory; the Smithsonian Millimeter Array; and the Na-
tional Radio Astronomy Observatory. Herschel is an ESA space
observatory with science instruments provided by European-led
Principal Investigator consortia and with im- portant participa-
tion from NASA.
CNRS

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2V. Smolˇ ci´ c et al.
1. INTRODUCTION
Submillimeter galaxies (SMGs; S850µm > 5 mJy)
are ultra-luminous, dusty starbursting systems with ex-
treme star formation rates (SFR ∼ 100−1000 M⊙yr−1;
e.g. Blain et al. 2002). The bulk of this population has
been shown to lie at 2 < z < 3 (e.g. Chapman et al.
2005). However, only recently have blank-field sub-mm
surveys started to discover the high-redshift (z > 4) tail
of the SMG distribution. To date seven z > 4 SMGs have
been spectroscopically confirmed (and published: three
in GOODS-N, Daddi et al. 2009a,b; two in COSMOS,
Capak et al. 2008; Schinnerer et al. 2008; Riechers et al.
2010; Capak et al. 2010, one in ECDFS, Coppin et al.
2009, 2010; and one in Abell 2218, Knudsen et al.
2010).These high-redshift SMGs, presenting a chal-
lenge to cosmological models of structure growth (see
e.g. Coppin et al. 2009), may alter our understanding of
the role of SMGs in galaxy evolution.
Galaxies are thought to evolve in time from an ini-
tial stage with irregular/spiral morphology towards pas-
sive, very massive elliptical systems (M∗ > 1011M⊙;
Faber et al. e.g. 2007).The morphology and spec-
tral properties of passive galaxies indicate that they
have formed in a single intense burst at z > 4 (e.g.
Cimatti et al. 2008). SMGs represent short-lasting (<<
100 Myr) starburst episodes of the highest known in-
tensity. Thus, they would be the perfect candidates for
z ∼ 2 passive galaxy progenitors. In this Letter we report
on a new z > 4 SMG – AzTEC/COSMOS 1 (AzTEC 1
hereafter), the brightest SMG detected in the AzTEC-
COSMOS field (Scott et al. 2008).
We adopt H0= 70, ΩM= 0.3,ΩΛ= 0.7, use a Salpeter
initial mass function, and AB magnitudes.
2. DATA
The available photometric (X-ray–radio) data for
AzTEC 1 (α = 09 : 59 : 42.863, δ = +02 : 29 : 38.19)
are summarized in⁀tab:phot . Its optical/IR counterpart
– identified by Younger et al. (2007) in follow-up SMA
observations of the original JCMT/AzTEC 1.1 mm de-
tection (Scott et al. 2008) – has been targeted by the
COSMOS project (Scoville et al. 2007) in more than
30 filters:.ground-based optical/NIR imaging in 22
bands (Capak et al. 2007)30, Chandra (Elvis et al. 2009),
GALEX (Zamojski et al. 2007), HST (Scoville et al.
2007; Koekemoer et al. 2009; Leauthaud et al. 2007),
Spitzer (Sanders et al. 2007), and VLA (Schinnerer et al.
2007, 2010, Smolˇ ci´ c et al., in prep.) (see⁀tab:phot ).
Herschel (100 and 160 µm) data are drawn from
the PACS Evolutionary Probe observations (Lutz et
al., in prep; Berta et al. 2010).
Observations at 350 µm with CSO/SHARC II were ob-
tained during two nights in March/2009 with an average
225 GHz opacity of τ225< 0.05. The data were reduced
using the standard CRUSH tool. A total of ∼ 6 hrs of
integration time reached an rms of 10 mJy. Combined
with previous data (Aguirre et al., in prep) we detect no
flux at 1σ = 7 mJy.
Observations at 3 mm were obtained with CARMA in
30An updated version of the UV-NIR catalog, available at
http://irsa.ipac.caltech.edu/data/COSMOS/tables/photometry,
has been used.
E-array configuration in July/2009. The target was ob-
served for 8.5 hrs on-source. The 3 mm receivers were
tuned to 98.95 GHz (3.03 mm), with lower (upper) side-
bands centered at 96.43 (101.46) GHz, respectively. Each
sideband was observed with 45 31.25MHz wide channels,
leading to a total bandwidth of 2.56GHz. The data re-
duction was performed with the MIRIAD package. No
line emission (the CO(5→4) transition is expected at
the source’s redshift) was detected across the observed
bands covering 4.64 < z < 4.72 and 4.94 < z < 5.02.
The uv-data were imaged merging both sidebands to-
gether and using natural weighting. We infer an rms of
0.36 mJy/beam in the continuum map, but no detection
of the source.
Using the new WideX correlator on PdBI, AzTEC-1
was observed with 6 antennas in Apr/May/2010 for ∼
5.5 hrs on-source. The WideX correlatorcovered 3.6 GHz
bandwidth using polarizations centered at 101.866394
GHz. 1005+066 and 3C273 were used as phase and gain
calibrators, respectively. The flux calibration error is es-
timated to be < 10%. The naturally weighted beam
is 6.38”×5.01” (PA 32o). The 3 mm continuum emis-
sion, shown in Fig. 1, is detected at 7.5σ with S3mm=
0.3 ± 0.04mJy and unresolved. No line emission is de-
tected across the band covering 4.56 < z < 4.76. The rms
per 180km/s wide channel (61.2 MHz) is 0.35mJy/beam.
Fig. 1.— PdBI 3 mm continuum image of AzTEC 1. Contours
are at ±3σ, ±5σ, ±7σ (1σ = 0.04 mJy/beam). The inset shows
the clean beam.
AzTEC
DEIMOS on Keck-II in Nov/2008 with clear conditions
and ∼ 1′′seeing and a 4 hr integration time split
into 30 min exposures. The data were collected with
the 830l/mm grating tilted to 7900˚ A and the OG550
blocker. The objects were dithered ±3′′along the slit to
remove ghosting.
The data were reduced via the modified DEEP2
DEIMOS pipeline (see Capak et al. 2008).
1was spectroscopicallytargeted with
The over-

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The redshift and nature of AzTEC/COSMOS 13
all instrumental throughput was determined using the
standard stars HZ-44 and GD-71. Bright stars in the
mask were used to determine the amount of atmospheric
extinction, wavelength dependent slit losses from atmo-
spheric dispersion, and to correct for the A, B, and wa-
ter absorption bands.The 2D- and 1D- spectra are
shown in Fig. 2. No strong emission lines are present
in the spectrum.The continuum is clearly detected
(see 2D-spectrum in Fig. 2), however at low signal-to-
noise, consistent with the faint magnitude of the source
(i+= 25.2).
3. THE REDSHIFT OF AZTEC 1
From features in the DEIMOS spectrum we determine
a redshift for AzTEC 1 of 4.650 ± 0.005 based on the
blue cut-off of Lyα. Note that in this redshift range,
Lyα, the most prominent emission line that may be ex-
pected, would be attenuated by the atmospheric B-band
(6860-6890˚ A). The 1216˚ A Lyα forest break is however
clearly seen in the 2D-spectrum, as well as in the heavily
smoothed 1D-spectrum (see Fig. 2, Fig. 3). If this were
the 4000˚ A break at z = 0.71 we would expect strong
160 µm and 350 µm detections for any known galaxy
type. As these do not exist for AzTEC 1, low redshifts
(z < 1) can be ruled out. Note that the inferred high
redshift is consistent with both, the source being a B-
band drop-out, and its FIR/radio ratio (Younger et al.
2008; Yun & Carilli 2002).
Due to a) the low S/N, b) the general absence of strong
emission lines, and c) the atmospheric B-band bias at
the expected position of Lyα, we utilize the photometric
data available for AzTEC 1 along with the spectrum to
refine our redshift estimate. Using 31 NUV-NIR photo-
metric measurements (Tab. 1) and the binned spectrum
we constrain the redshift via a χ2minimization SED fit-
ting technique described in detail by Ilbert et al. (2009).
Our best fit results, as well as the redshift probability
[exp(−χ2/2)] distribution, are shown in Fig. 3. We find
a redshift of z = 4.64+0.06
−0.08, where the errors are drawn
from the 68% confidence interval. Note that this anal-
ysis yields also a secondary redshift peak at z = 4.44,
albeit with a significantly lower probability than that at
z = 4.64
As it is possible that heavy extinction in the UV biases
UV-NIR-derived photometric redshifts towards higher
values, we estimate the photometric redshift using FIR-
radio data via a Monte-Carlo approach, described in de-
tail in Aretxaga et al. (2003). We find that the upper
limits at λ < 450 µm strongly suggest z > 4.0 (at
∼ 90% confidence). The redshift probability distribu-
tion reaches a plateau with equally plausible solutions
between z = 4.5 and z = 6.0, supporting the optical-IR
redshift solution.
The inferred most probable redshift z = 4.64 (based
on UV-NIR data) is close to the spectroscopically deter-
mined redshift of z = 4.65, and supported by the FIR-
radio data. Thus, hereafter we take z = 4.64+0.06
best estimate for the redshift of AzTEC 1.
−0.08as the
4. SPECTRAL ENERGY DISTRIBUTION OF AZTEC 1
In Fig. 4 we show the SED of AzTEC 1. Fixing the
redshift to z = 4.64 (Sec. 3) we fit the UV-NIR SED us-
ing various model spectrum libraries. For each model
we compute the total χ2and define the most proba-
ble parameter values and their errors from the prob-
ability distribution function.
(2003) library (see Smolˇ ci´ c et al. 2008 for details) the
UV-NIR SED is best described by a 740+200
starburst with SFR = 410 ± 50 M⊙/yr, an extinction
of AV = 2 ± 0.2 mag, and a stellar mass of M∗ =
(1.5 ± 0.2) × 1011M⊙ (see top panel in Fig. 4).
find consistent results when using the Maraston (2003) li-
brary. However, as pointed out by Maraston et al. (2010),
using exponentially decaying star formation histories as
above some of the free parameters may be poorly param-
eterized in young starburst galaxies whose SED is domi-
nated by the youngest stellar populations that outshine
the old ones. Thus, we additionally fit to the optical-
NIR SED of AzTEC 1 the model library presented in Ef-
stathiou et al. (2000), specifically developed for starburst
galaxies. These (UV-mm) models are treated as an en-
semble of optically thick giant molecular clouds (GMCs)
centrally illuminated by recently formed stars. The evo-
lution of the stellar population within the GMC is mod-
eled using the Bruzual et al. (2003) stellar population
synthesis models. The Efstathiou et al. (2000) models
yield a 37±4 Myr old starburst with AV= 100±20 and
SFR = 1300± 150 M⊙/yr.
We fit the IR portion of the SED of AzTEC 1 (fixing
z = 4.64) using the Chary & Elbaz (2001; CE hereafter),
Dale & Helou (2002), and Lagache et al. (2003) models.
The best fit IR model, shown in Fig. 4 (bottom panel),
is a Lagache et al. (2003) template with a total IR (8 −
1000 µm) luminosity of 2.9 × 1013L⊙, and a FIR (60 −
1000 µm) luminosity of 9×1012L⊙. For comparison, the
CE SED models yield the second best fit with integrated
luminosities a factor of 3-4 higher. Converting the (8 −
1000 µm) IR luminosity to a SFR, using the Kennicutt
(1998) conversion, we find a SFR of ∼ 1600 M⊙/yr. To
obtain the dust temperature and dust mass in AzTEC 1
we perform a gray-body dust model fit to the data as
described in detail in Aravena et al. (2008). Using β =
1.5 and β = 2 we consistently find a dust temperature of
TD∼ 50 K and dust mass of MD∼ 1.5× 109M⊙(while
the IR luminosity is within a factor of two compared to
that given above).
Using the Bruzual et al.
−60
Myr old
We
5. DISCUSSION
5.1. Lack of molecular gas?
Based on observations of AzTEC 1 from radio to X-
rays, and a Keck II/DEIMOS spectrum, we have shown
that AzTEC 1 is a LFIR= 9 × 1012L⊙starburst galaxy
at z = 4.64+0.06
−0.08(the given errors are 1σ uncertain-
ties).However, contrary to expectations our searches
for the CO(5→4) transition line (νRF = 576.268 GHz)
in this galaxy with the PdBI/CARMA interferometers
have yielded no detection.
500 km s−1the 3σ limits in the line luminosity based
on PdBI and CARMA observations are estimated to
be L′
4.76) and L′
z < 4.72 and 4.94 < z < 5.02), respectively.
ing Mgas/L′
& Solomon 1998) implies 3σ gas mass upper limits of
Mgas ? 8 × 109M⊙ (4.56 < z < 4.76) and Mgas ?
Assuming a line width of
CO? 9.8 × 109K
CO? 6.5 × 1010K
km s−1pc2(4.56 < z <
km s−1pc2(4.64 <
Tak-
CO= 0.8 M⊙ (K km s−1pc2)−1(Downes

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4V. Smolˇ ci´ c et al.
Fig. 2.— The top panel shows the Keck II/DEIMOS 2D-spectrum of AzTEC 1. Note the increase in continuum flux beyond Lyα (see
also Fig. 3). In the bottom panel the extracted 1D-spectrum is shown. Note that the atmospheric B-band (6860-6890˚ A) is coincident with
the expected Lyα emission line.
5 × 1010M⊙(4.94 < z < 5.02). Turning the arguments
around, i) assuming a typical L′
(Riechers et al. 2006) the FIR luminosity inferred here
for AzTEC 1, LFIR = 9 × 1012L⊙, yields an expected
CO luminosity of L′
assuming a gas-to-dust-ratio of 50-150 (e.g. Calzetti et
al. 2000), and Mgas/L′
the dust mass we inferred here for AzTEC 1 (MD ∼
1.5×109M⊙) translates into a line luminosity of L′
(9 − 30) × 1010K km s−1pc2. Such a gas reservoir
should have been detected (especially with the more sen-
sitive PdBI observations) within our interferometric ob-
servations in the 3 mm band. Below we discuss a few
possibilities why the CO(5→4) line was not detected.
First, it is possible that the systemic redshift of the
CO− LFIR conversion
CO≈ 4×1010K km s−1pc2, and ii)
CO= 0.8 M⊙(K km s−1pc2)−1
CO∼
source is outside the bandwidth range covered with
our interferometric observations (encompassing redshift
ranges of 4.56−4.76and 4.94−5.02). Our UV-NIR analy-
sis of the SED yields a 68% probability that the redshift
of the source is within 4.56 < z < 4.70. However, we
also find a second redshift peak at z ∼ 4.44 in our red-
shift probability distribution (see Fig. 3). Furthermore,
the systemic (CO) redshift of the source is not neces-
sarily expected to coincide with the one inferred from
UV-NIR data (typical velocity offsets are several hun-
dred km s−1for narrow-line objects). Thus, it is possible
that the CO redshift is outside the range covered by our
interferometric observations. Note however, that if this
were the case, it would not significantly alter the results
of our SED analysis (Sec. 4). Alternatively, assuming

Page 5

The redshift and nature of AzTEC/COSMOS 15
44.55
0
0.2
0.4
0.6
0.8
1
redshift
Fig. 3.—
AzTEC 1 (symbols). The spectral template, best fit to the multi-
band photometry (filled symbols) and the binned DEIMOS spec-
trum (open symbols), redshifted to the most probable redshift
(z = 4.64) is also plotted (in red). The redshift probability dis-
tribution p ∝ exp(−0.5χ2) is shown in the inset.
redshift and 1σ uncertainties (z = 4.64+0.06
grees of freedom (dof) and the total χ2of the best fit are indicated
in the top-left.
The UV-IR spectral energy distribution (SED) of
The median
−0.08), as well as the de-
Fig. 4.—
The best fit model spectra from the Maraston et al. (2003, gray)
and Bruzual et al. (2003, black) library to the UV-NIR SED and
Lagache et al. (2003) library to the IR SED are also shown (see
text for details).
UV-NIR (top) and IR (bottom) SED of AzTEC 1.
the systemic redshift is within the covered bandwidth,
the CO(5→4) non-detection could be explained by a low
CO-excitation resulting in a low line brightness of the
CO(5→4) transition. Assuming a CO 5→4 to 1→0 line
brightness temperature ratio of ∼ 1/3, as found for the
z > 4 SMG GN20 (Carilli et al. 2010), the PdBI 3σ limit
in the CO(1→0) line is L′
This is roughly consistent with the CO-FIR relation.
Furthermore, an uncertainty of a factor of a few in the
inferred dust mass (including possible AGN heating) and
the Mgas/L′
roughly consistent with L′
COestimated from AzTEC 1’s
dust mass. Thus, a low CO-excitation in AzTEC 1 may
explain the non-detection of CO(5→4).
CO? 3 × 1010K km s−1pc2.
COconversion factor makes this limit also
5.2. Mode of star formation
Our analysis of the UV-radio SED of AzTEC 1 implies
that AzTEC 1 is an extremely young and massive galaxy,
forming stars at a rate of ∼ 1300 M⊙yr−1at z = 4.6. In
general, vigorous star formation induces strong negative
feedback that can terminate (and then self-regulate) the
starburst by dispersing and expelling gas from the grav-
itational potential well (Elmegreen 1999; Scoville 2003;
Thompson et al. 2005; Riechers et al. 2009). This sets
a number of physical limits on the starburst. Assuming
that a) the maximum intensity of a radiation-pressure
supported starburst is determined by the Eddington limit
for dust, b) a constant gas-to-dust ratio with radius, and
c) that the disk is self-regulated (i.e. Toomre Q ∼ 1) such
an Eddington limited starburst will have a SFR surface
density ΣSFR∼ 1000 M⊙yr−1kpc−2, a FIR luminosity
surface density FFIR∼ 1013L⊙kpc−2, and an effective
temperature of 88 K (see eqs. 33-36 in Thompson et al.
2005).
A SFR of ∼ 1300 M⊙yr−1in AzTEC 1 (based on the
NUV-NIR SED fit) then implies a SFR surface density
of ΣSFR= SFR/(πr2) ? 420 M⊙yr−1kpc−2(assuming
r ? 1 kpc based on SMA imaging; Younger et al. 2008).
The inferred value does not violate the Eddington limited
starburst models. The FIR luminosity surface density in
AzTEC 1, FFIR = LFIR/(πr2) ? 2.8 × 1012L⊙ kpc−1,
and the dust temperature of ∼ 50 K support that the
starburst in AzTEC 1 is consistent, but not in violation
of its Eddington limit.
It is noteworthy that the inferred value of the SFR
surface density for AzTEC 1 is somewhat higher com-
pared to SMGs at z ∼ 2, which typically have ΣSFR∼
80 M⊙yr−1kpc−2(Tacconi et al. 2006), pointing to the
compactness of the star formation region in AzTEC 1.
Tacconi et al. have shown that z ∼ 2 SMGs are well de-
scribed within a starburst picture (Elmegreen 1999) in
which star formation cannot self-regulate and thus a sig-
nificant fraction of gas is converted into stars in only a
few times the dynamical timescale. Continuing this line
of reasoning we make use of a detailed hydrodynamical
study of matter deposition in young assembling galax-
ies performed by Silich et al. (2010). We estimate that
AzTEC 1 is forming stars in a ’gravitationally bound’
regime in which gravity prohibits the formation of a
superwind, leading to matter accumulation within the
galaxy and further generations of star formation. Specif-
ically, Silich et al. show that there are three hydrody-
namic regimes that develop in starbursting galaxies: i)
generation of a superwind, that expels matter from the
star forming region, ii) a ’gravitationally bound’ regime,
in which gravity prohibits the formation of a superwind
and contains the matter within the galaxy, and iii) an

Page 6

6V. Smolˇ ci´ c et al.
intermediate, bimodal regime. The specific regime is de-
pendent on the SFR and the size of the star formation
region in the galaxy (see Fig. 1 in Silich et al. 2010). Tak-
ing the size of the star forming region in AzTEC 1 to be
∼ 1 kpc (Younger et al. 2008), its SFR ∼ 1300 M⊙yr−1
yields that, consistent with SCUBA detected galaxies,
AzTEC 1 is forming stars in the gravitationally bound
regime.
In summary, our analysis of the properties of AzTEC 1
points to an extremely young and massive galaxy, form-
ing stars at a rate of ∼ 1300 M⊙yr−1at z = 4.6.
We find that it has already assembled a stellar mass of
1.5 × 1011M⊙, in a region covering only ∼ 1 − 2 kpc
in total extent (based on HST and SMA imaging; see
Younger et al. 2007, 2008) yielding that AzTEC 1 is a
compact massive galaxy at z = 4.6.
The high stellar mass and compactness of AzTEC 1
resemble that of a recently identified population of qui-
escent, passively evolving, already massive (typically
M∗ = 1.7 × 1011M⊙), but compact galaxies at z ∼ 2
(e.g. van Dokkum et al. 2008) deemed to evolve into the
most massive red-and-dead galaxies at z ∼ 0. The up-
per gas mass limit inferred for AzTEC 1 (although quite
uncertain) is ∼ 1010M⊙. If AzTEC 1 continues to form
stars at the current rate it will deplete the available gas in
Mgas/SFR ∼ 6 Myr (assuming 100% efficiency). Unless
further gas is supplied and high levels of star formation
are induced the galaxy’s stellar body will have time to
age and redden till z ∼ 2 − 3.
The surface density of the (likely still incomplete) sam-
ple of three confirmed z > 4 SMGs in the AzTEC-
COSMOS field (0.3 deg2) is ? 10 deg−2. This is already
higher than ∼ 7 deg−2predicted by semi-analytic mod-
els of structure growth (e.g. Baugh et al. 2005; see also
Coppin et al. 2009, 2010). Thus, further studies of z > 4
SMGs are key to understand the SMG population (e.g.
Wall et al. 2008) and its cosmological role.
6. CONCLUSIONS
Based on UV-FIR observations of AzTEC 1, and
a Keck II/DEIMOS spectrum, we have shown that
AzTEC 1 is a LFIR = 9 × 1012L⊙ starburst at z =
4.64+0.06
−0.08(with a secondary, less likely, redshift probabil-
ity peak at z ∼ 4.44). Based on our revised FIR values
we find that AzTEC 1 fits comfortably within the lim-
its of a maximal starburst, and that it forms stars in a
gravitationally bound regime which traps the gas within
the galaxy leading to formation of new generations of
stars. Our SED analysis yields that AzTEC 1 is an ex-
tremely young (? 50 Myr), massive (M∗ ∼ 1011M⊙),
but compact (? 2 kpc) galaxy forming stars at a rate of
∼ 1300 M⊙yr−1at z = 4.64. These interesting proper-
ties suggest that AzTEC 1 may be a candidate of pro-
genitors of quiescent, already massive, but very compact
galaxies regularly found at z ∼ 2, and thought to evolve
into the most massive, red-and-dead galaxies found in
the local universe.
The authors acknowledge the significant cultural role
that the summit of Mauna Kea has within the in-
digenous Hawaiian community; NASA grants HST-
GO-09822 (contracts 1407, 1278386; SSC); HST-HF-
51235.01 (contract NAS 5-26555; STScI); GO7-8136A;
Blancheflor Boncompagni Ludovisi foundation (F.C.);
French Agene National de la Recheche fund ANR-07-
BLAN-0228; CNES; Programme National Cosmologie
et Galaxies; UKF; DFG; DFG Leibniz Prize (FKZ HA
1850/28-1); European Union’s Seventh Framework pro-
gramme (grant agreement 229517); making use of the
NASA/ IPAC IRSA, by JPL/Caltech, under contract
with the National Aeronautics and Space Administra-
tion; IRAM PdBI supported by INSU/CNRS (France),
MPG (Germany) and IGN (Spain); CARMA supported
by the states of California, Illinois, and Maryland, the
Gordon and Betty Moore Foundation, the Eileen and
Kenneth Norris Foundation, the Caltech Associates, and
NSF.
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Page 7

The redshift and nature of AzTEC/COSMOS 17
TABLE 1
AzTEC 1 photometry
wavelength
0.5-2 keV
1551˚ A
2307˚ A
3911˚ A
4270˚ A
4440˚ A
4640˚ A
4728˚ A
4840˚ A
5050˚ A
5270˚ A
5449˚ A
5740˚ A
6240˚ A
6295˚ A
6790˚ A
7090˚ A
7110˚ A
7380˚ A
7641˚ A
7670˚ A
8040˚ A
8150˚ A
8270˚ A
9037˚ A
12444˚ A
16310˚ A
21537˚ A
3.6 µm
4.5 µm
5.8 µm
8.0 µm
16 µm
24 µm
70 µm
100 µm
160 µm
160 µm
350 µm
450 µm
850 µm
890 µm
1.1 mm
1.3 mm
3 mm
3 mm
20 cm
90 cm
band/telescope
Chandra-soft-band
FUV
NUV
u∗
IA427
BJ
IA464
g+
IA484
IA505
IA527
VJ
IA574
IA624
r+
IA679
IA709
NB711
IA738
i+
IA767
F814W
NB816
IA827
z+
J
H
Ks
IRAC1
IRAC2
IRAC3
IRAC4
IRS-16
MIPS-24
MIPS-70
PACS-100
MIPS-160
PACS-160
CSO
SCUBA-2
SCUBA-2
SMA
JCMT/AzTEC
CARMA
CARMA
PdBI
VLA
VLA
flux density (µJy)
< 0.0003a,b
< 0.20a
< 0.09a
< 0.01a
< 0.03a
< 0.01a
< 0.04a
< 0.03a
< 0.03a
< 0.04a
< 0.03a
< 0.02a
0.09 ± 0.04
0.08 ± 0.04
0.12 ± 0.02
0.20 ± 0.04
0.25 ± 0.04
0.26 ± 0.10
0.24 ± 0.05
0.29 ± 0.02
0.26 ± 0.05
0.31 ± 0.02
0.21 ± 0.05
0.33 ± 0.06
0.35 ± 0.07
< 0.5a
1.03 ± 0.22
1.33 ± 0.23
3.87 ± 0.13
4.53 ± 0.23
7.90 ± 4.50
13.01 ± 2.88
12.80 ± 4.20
46.40 ± 4.90
< 2600a
< 3600a
< 8200a
< 6900a
< 15000a
< 44000a
16000 ± 3500
15600 ± 1100
9300 ± 1300
9400 ± 1600
< 720a
300 ± 40
42.0 ± 10
< 1000a
aThe given limits are 2σ upper limits.bCorresponds to = 10−15erg cm−2s−1
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