1,279 research outputs found
Low, Milky-Way like, Molecular Gas Excitation of Massive Disk Galaxies at z~1.5
We present evidence for Milky-Way-like, low-excitation molecular gas
reservoirs in near-IR selected massive galaxies at z~1.5, based on IRAM Plateau
de Bure Interferometer CO[3-2] and NRAO Very Large Array CO[1-0] line
observations for two galaxies that had been previously detected in CO[2-1]
emission. The CO[3-2] flux of BzK-21000 at z=1.522 is comparable within the
errors to its CO[2-1] flux, implying that the CO[3-2] transition is
significantly sub-thermally excited. The combined CO[1-0] observations of the
two sources result in a detection at the 3 sigma level that is consistent with
a higher CO[1-0] luminosity than that of CO[2-1]. Contrary to what is observed
in submillimeter galaxies and QSOs, in which the CO transitions are thermally
excited up to J>=3, these galaxies have low-excitation molecular gas, similar
to that in the Milky Way and local spirals. This is the first time that such
conditions have been observed at high redshift. A Large Velocity Gradient
analysis suggests that molecular clouds with density and kinetic temperature
comparable to local spirals can reproduce our observations. The similarity in
the CO excitation properties suggests that a high, Milky-Way-like, CO to H_2
conversion factor could be appropriate for these systems. If such
low-excitation properties are representative of ordinary galaxies at high
redshift, centimeter telescopes such as the Expanded Very Large Array and the
longest wavelength Atacama Large Millimeter Array bands will be the best tools
for studying the molecular gas content in these systems through the
observations of CO emission lines.Comment: 5 pages, 4 figures. ApJ Letters in pres
The contribution of starbursts and normal galaxies to infrared luminosity functions at z < 2
We present a parameter-less approach to predict the shape of the infrared
(IR) luminosity function (LF) at redshifts z < 2. It requires no tuning and
relies on only three observables: (1) the redshift evolution of the stellar
mass function for star-forming galaxies, (2) the evolution of the specific star
formation rate (sSFR) of main-sequence galaxies, and (3) the double-Gaussian
decomposition of the sSFR-distribution at fixed stellar mass into a
contribution (assumed redshift- and mass-invariant) from main-sequence and
starburst activity. This self-consistent and simple framework provides a
powerful tool for predicting cosmological observables: observed IR LFs are
successfully matched at all z < 2, suggesting a constant or only weakly
redshift-dependent contribution (8-14%) of starbursts to the star formation
rate density. We separate the contributions of main-sequence and starburst
activity to the global IR LF at all redshifts. The luminosity threshold above
which the starburst component dominates the IR LF rises from log(LIR/Lsun) =
11.4 to 12.8 over 0 < z < 2, reflecting our assumed (1+z)^2.8-evolution of sSFR
in main-sequence galaxies.Comment: 7 pages, 4 figures & 1 table. Accepted for publication in ApJL. Minor
typos corrected in v2 following receipt of proof
Very High Gas Fractions and Extended Gas Reservoirs in z=1.5 Disk Galaxies
We present evidence for very high gas fractions and extended molecular gas
reservoirs in normal, near-infrared selected (BzK) galaxies at z~1.5, based on
multi-configuration CO[2-1] observations obtained at the IRAM PdBI. Six of the
six galaxies observed were securely detected. High resolution observations
resolve the CO emission in four of them, implying sizes of order of 6-11 kpc
and suggesting the presence of rotation. The UV morphologies are consistent
with clumpy, unstable disks, and the UV sizes are consistent with the CO sizes.
The star formation efficiencies are homogeneously low and similar to local
spirals - the resulting gas depletion times are ~0.5 Gyr, much higher than what
is seen in high-z submm galaxies and quasars. The CO luminosities can be
predicted to within 0.15 dex from the star formation rates and stellar masses,
implying a tight correlation of the gas mass with these quantities. We use
dynamical models of clumpy disk galaxies to derive dynamical masses. These
models are able to reproduce the peculiar spectral line shapes of the CO
emission. After accounting for the stellar and dark matter masses we derive gas
masses of 0.4-1.2x10^11 Msun. The conversion factor is very high:
alpha_CO=3.6+-0.8, consistent with the Galaxy but four times higher than that
of local ultra-luminous IR galaxies. The gas accounts for an impressive 50-65%
of the baryons within the galaxies' half light radii. We are witnessing truly
gas-dominated galaxies at z~1.5, a finding that explains the high specific SFRs
observed for z>1 galaxies. The BzK galaxies can be viewed as scaled-up versions
of local disk galaxies, with low efficiency star formation taking place inside
extended, low excitation gas disks. They are markedly different than local
ULIRGs and high-z submm galaxies, which have more excited and compact gas.Comment: Accepted for publication in Astrophysical Journal, 22 pages, 18
figures, minor revision
A molecular line scan in the Hubble Deep Field North
We present a molecular line scan in the Hubble Deep Field North (HDF-N) that covers the entire 3mm window (79-115 GHz) using the IRAM Plateau de Bure Interferometer. Our CO redshift coverage spans z2. We reach a CO detection limit that is deep enough to detect essentially all z>1 CO lines reported in the literature so far. We have developed and applied different line searching algorithms, resulting in the discovery of 17 line candidates. We estimate that the rate of false positive line detections is ~2/17. We identify optical/NIR counterparts from the deep ancillary database of the HDF-N for seven of these candidates and investigate their available SEDs. Two secure CO detections in our scan are identified with star-forming galaxies at z=1.784 and at z=2.047. These galaxies have colors consistent with the `BzK' color selection and they show relatively bright CO emission compared with galaxies of similar dust continuum luminosity. We also detect two spectral lines in the submillimeter galaxy HDF850.1 at z=5.183. We consider an additional 9 line candidates as high quality. Our observations also provide a deep 3mm continuum map (1-sigma noise level = 8.6 μJy/beam). Via a stacking approach, we find that optical/MIR bright galaxies contribute only to <50% of the SFR density at 1<z<3, unless high dust temperatures are invoked. The present study represents a first, fundamental step towards an unbiased census of molecular gas in `normal' galaxies at high-z, a crucial goal of extragalactic astronomy in the ALMA era
A CO emission line from the optical and near-IR undetected submillimeter galaxy GN10
We report the detection of a CO emission line from the submillimiter galaxy
(SMG) GN10 in the GOODS-N field. GN10 lacks any counterpart in extremely deep
optical and near-IR imaging obtained with the Hubble Space Telescope and
ground-based facilities. This is a prototypical case of a source that is
extremely obscured by dust, for which it is practically impossible to derive a
spectroscopic redshift in the optical/near-IR. Under the hypothesis that GN10
is part of a proto-cluster structure previously identified at z~4.05 in the
same field, we searched for CO[4-3] at 91.4 GHz with the IRAM Plateau de Bure
Interferometer, and successfully detected a line. We find that the most likely
redshift identification is z=4.0424+-0.0013, based on: 1) the very low chance
that the CO line is actually serendipitous from a different redshift; 2) a
radio-IR photometric redshift analysis; 3) the identical radio-IR SED, within a
scaling factor, of two other SMGs at the same redshift. The faintness at
optical/near-IR wavelengths requires an attenuation of A_V~5-7.5 mag. This
result supports the case that a substantial population of very high-z SMGs
exists that had been missed by previous spectroscopic surveys. This is the
first time that a CO emission line has been detected for a galaxy that is
invisible in the optical and near-IR. Our work demonstrates the power of
existing and planned facilities for completing the census of star formation and
stellar mass in the distant Universe by measuring redshifts of the most
obscured galaxies through millimeter spectroscopy.Comment: 5 pages, 4 figures. ApJ Letters in pres
Expanded Very Large Arrays Observations of a Proto-Cluster of Molecular Gas-Rich Galaxies at z = 4.05
We present observations of the molecular gas in the GN20 proto-cluster of galaxies at z = 4.05 using the Expanded Very Large Array (EVLA). This group of galaxies is the ideal laboratory for studying the formation of massive galaxies via luminous, gas-rich starbursts within 1.6 Gyr of the big bang. We detect three galaxies in the proto-cluster in CO 2-1 emission, with gas masses (H_2) between 10^(10) and 10^(11) × (α/0.8) M_⊙. The emission from the brightest source, GN20, is resolved with a size ~2'' and has a clear north-south velocity gradient, possibly indicating ordered rotation. The gas mass in GN20 is comparable to the stellar mass (1.3 × 10^(11) × (α/0.8) M_⊙ and 2.3 × 10^(11) M_⊙, respectively), and the sum of gas plus stellar mass is comparable to the dynamical mass of the system (~3.4 × 10^(11)[sin (i)/sin (45°)]^(–2) M_⊙), within a 5 kpc radius. There is also evidence for a tidal tail extending another 2'' north of the galaxy with a narrow velocity dispersion. GN20 may be a massive, gas-rich disk that is gravitationally disturbed, but not completely disrupted. There is one Lyman-break galaxy (BD29079) in the GN20 proto-cluster with an optical spectroscopic redshift within our search volume, and we set a 3σ limit to the molecular gas mass of this galaxy of 1.1 × 10^(10) × (α/0.8) M_⊙
The kiloparsec-scale star formation law at redshift 4: wide-spread, highly efficient star formation in the dust-obscured starburst galaxy GN20
We present high-resolution observations of the 880 m (rest-frame FIR)
continuum emission in the z4.05 submillimeter galaxy GN20 from the IRAM
Plateau de Bure Interferometer (PdBI). These data resolve the obscured star
formation in this unlensed galaxy on scales of
0.30.2 (2.11.3 kpc).
The observations reveal a bright (161 mJy) dusty starburst centered on the
cold molecular gas reservoir and showing a bar-like extension along the major
axis. The striking anti-correlation with the HST/WFC3 imaging suggests that the
copious dust surrounding the starburst heavily obscures the rest-frame
UV/optical emission. A comparison with 1.2 mm PdBI continuum data reveals no
evidence for variations in the dust properties across the source within the
uncertainties, consistent with extended star formation, and the peak star
formation rate surface density (1198 M yr kpc)
implies that the star formation in GN20 remains sub-Eddington on scales down to
3 kpc. We find that the star formation efficiency is highest in the central
regions of GN20, leading to a resolved star formation law with a power law
slope of , and that
GN20 lies above the sequence of normal star-forming disks, implying that the
dispersion in the star formation law is not due solely to morphology or choice
of conversion factor. These data extend previous evidence for a fixed star
formation efficiency per free-fall time to include the star-forming medium on
kpc-scales in a galaxy 12 Gyr ago.Comment: 6 pages, 5 figures, accepted to ApJ
The [CII] 158 m emission line as a gas mass tracer in high redshift quiescent galaxies
Many efforts have been done in recent years to probe the gas fraction
evolution of massive quiescent galaxies (QGs); however, a clear picture has not
yet been established. Recent spectroscopic confirmations at z>3 offer the
chance to measure the residual gas reservoirs of massive galaxies a few
hundreds of Myr after their death and to study how fast quenching proceeds in a
highly star-forming Universe. Even so, stringent constraints at z2 remain
hardly accessible with ALMA when adopting molecular gas tracers commonly used
for the quenched population. In this letter, we propose overcoming this impasse
by using the carbon [CII] 158 m emission line to systematically probe the
gaseous budget of unlensed QGs at z>2.8, when these galaxies could still host
non-negligible star formation on an absolute scale and when the line becomes
best observable with ALMA (Bands 8 and 7). So far predominantly used for
star-forming galaxies, this emission line is the best choice to probe the gas
budget of spectroscopically confirmed QGs at , reaching 2-4 and 13-30
times deeper than dust continuum (ALMA band 7) and CO(2-1)/(1-0) (VLA
K-K bands), respectively, at fixed integration time. Exploiting
archival ALMA observations, we place conservative 3 upper limits on the
molecular gas fraction (f) of ADF22-QG1
(f<21%), ZF-COS-20115 (f<3.2%), two of the
best-studied high-z QGs in the literature, and GS-9209 (f<72%),
the most distant massive QG discovered to date. The deep upper limit found for
ZF-COS-20115 is 3 times lower than previously anticipated for high-z QGs
suggesting, at best, the existence of a large scatter in the f
distribution of the first QGs. Lastly, we discuss the current limitations of
the method and propose ways to mitigate some of them by exploiting ALMA bands 9
and 10.Comment: 7 pages, 2 figures. A&A Letters in pres
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