3,403 research outputs found
GOODS-Herschel~: Gas-to-dust mass ratios and CO-to-H_2 conversion factors in normal and starbursting galaxies at high-z
We explore the gas-to-dust mass ratio (G/D) and the CO luminosity-to-Mgas
conversion factor (a_co) of two well studied galaxies in the GOODS-N field,
that are expected to have different star forming modes, the starburst GN20 at
z=4.05 and the normal star-forming galaxy BzK-21000 at z=1.52. Detailed
sampling is available for their Rayleigh-Jeans emission via ground based mm
interferometry (1.1-6.6mm) along with Herschel, PACS and SPIRE data that probe
the peak of their infrared emission. Using the physically motivated Draine & Li
(2007) models, as well as a modified black body function, we measure the dust
mass (Md) of the sources and find 2.0^{+0.7}_{-0.6} x 10^{9} Msun for GN20 and
8.6^{+0.6}_{-0.9} x 10^{8} Msun for BzK-21000. The addition of mm data reduces
the uncertainties of the derived Md by a factor of ~2, allowing the use of the
local G\D vs metallicity relation to place constraints on the a_co values of
the two sources. For GN20 we derive a conversion factor of a_co < 1.0 Msun
pc^{-2}(K km s^{-1})^{-1}, consistent with that of local ULIRGs, while for
BzK-21000 we find a considerably higher value, a_co ~4.0 Msun pc^{-2}(K km
s^{-1})^{-1}, in agreement with an independent kinematic derivation reported
previously. The implied star formation efficiency is ~25 Lsun/Msun for
BzK-21000, a factor of ~5-10 lower than that of GN20. The findings for these
two sources support the existence of different disk-like and starburst
star-formation modes in distant galaxies, although a larger sample is required
to draw statistically robust resultsComment: Accepted for publication in Ap
Different star formation laws for disks versus starbursts at low and high redshifts
We present evidence that 'bona fide' disks and starburst systems occupy
distinct regions in the gas mass versus star formation (SF) rate plane, both
for the integrated quantities and for the respective surface densities. This
result is based on CO observations of galaxy populations at low and high
redshifts, and on the current consensus for the CO luminosity to gas mass
conversion factors. The data suggest the existence of two different star
formation regimes: a long-lasting mode for disks and a more rapid mode for
starbursts, the latter probably occurring during major mergers or in dense
nuclear SF regions. Both modes are observable over a large range of SF rates.
The detection of CO emission from distant near-IR selected galaxies reveals
such bimodal behavior for the first time, as they allow us to probe gas in disk
galaxies with much higher SF rates than are seen locally. The different regimes
can potentially be interpreted as the effect of a top-heavy IMF in starbursts.
However, we favor a different physical origin related to the fraction of
molecular gas in dense clouds. The IR luminosity to gas mass ratio (i.e., the
SF efficiency) appears to be inversely proportional to the dynamical (rotation)
timescale. Only when accounting for the dynamical timescale, a universal SF law
is obtained, suggesting a direct link between global galaxy properties and the
local SF rate.Comment: 5 pages, 4 figures. ApJ Letters in pres
Tracing the formation of massive spheroids from high-z galaxy clustering
The high-z progenitors of local massive early-type galaxies should be
characterized by a strong level of clustering, larger than that measured for
z=3 Lyman Break Galaxies and comparable to that of z~1 EROs. First possible
evidences for such strongly clustered objects at z~2 were found by the FIRES
and K20 surveys, that have identified new classes of faint high-z K-selected
galaxies. Some details are given here for the new population of massive
star-forming galaxies at z~2, found by the K20 survey in the GOODS-South area.
Because of their much redder UV continuum, most of these galaxies would not be
selected by the Lyman Break criterion. Such objects are good candidates for the
precursors of local ellipticals caught in their formation phase. We have
calibrated a two color criterion to allow the identification of these highest
redshift galaxies in bright K-limited samples.Comment: 6 pages, 3 figures, in "Multiwavelength Mapping of Galaxy Formation
and Evolution" Venice, October 2003, conference proceeding
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
EVLA 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 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)
between and M. The emission
from the brightest source, GN20, is resolved with a size , and has a
clear north-south velocity gradient, possibly indicating ordered rotation. The
gas mass in GN20 is comparable to the stellar mass ( M and M, respectively), and
the sum of gas plus stellar mass is comparable to the dynamical mass of the
system ( M), within a
5kpc radius. There is also evidence for a tidal tail extending another
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 3
limit to the molecular gas mass of this galaxy of M.Comment: AAStex format, 4 figures; prepared for the ApJ Letters EVLA special
issu
A deep search for molecular gas in two massive Lyman break galaxies at z=3 and 4: vanishing CO-emission due to low metallicity
We present deep IRAM Plateau de Bure Interferometer (PdBI) observations, searching for CO-emission toward two massive, non-lensed Lyman break galaxies (LBGs) at z=3.216 and 4.058. With one low significance CO detection (3.5 sigma) and one sensitive upper limit, we find that the CO lines are >~ 3-4 times weaker than expected based on the relation between IR and CO luminosities followed by similarly, massive galaxies at z=0-2.5. This is consistent with a scenario in which these galaxies have low metallicity, causing an increased CO-to-H_2 conversion factor, i.e., weaker CO-emission for a given molecular (H_2) mass. The required metallicities at z>3 are lower than predicted by the fundamental metallicity relation (FMR) at these redshifts, consistent with independent evidence. Unless our galaxies are atypical in this respect, detecting molecular gas in normal galaxies at z>3 may thus remain challenging even with ALMA
Optical Spectroscopy of K-selected Extremely Red Galaxies
We have obtained spectroscopic redshifts for 24 red galaxies from a sample
with median Ks=18.7 and F814W - Ks > 4, using the Keck telescope. These
EROshave high resolution morphologies from HST (Yan & Thompson 2003). Among the
24 redshifts, the majority (92%) are at . We derived the
rest-frame J-band luminosity function at . Our result
suggests that the luminosity evolution between bright EROs at and the
present-day L massive galaxies is at most about 0.7 magnitude. Combining
the morphologies and deep spectroscopy revealed the following properties: (1)
86% of the spectra have absorption features from old stars, suggesting that the
dominant stellar populations seen in the rest-frame UV are old stars. 50% of
the sources have pure absorption lines, while the remaining 50% have emission
lines, indicating recent star formation. We conclude that the color criterion
for EROs is very effective in selecting old stellar populations at ,
and a large fraction of these systems with prominent old stellar populations
also have recent star formation. (2) The 12 emission line systems have the same
number of disk and bulge galaxies as in the remaining 12 pure absorption line
systems. We conclude that spectral classes do not have a simple, direct
correspondence with morphological types. (3) Three EROs could be isolated, pure
passively evolving early-type galaxies at . This implies that only a
small fraction (10%--15%) of early-type galaxies are formed in a rapid burst of
star formation at high redshifts and evolved passively since then. (Abridged).Comment: 27 pages, 8 figures. Accepted for publication in Astronomical
Journal, issue March 200
Star formation rates and masses of z ~ 2 galaxies from multicolour photometry
Fitting synthetic spectral energy distributions (SED) to the multi-band
photometry of galaxies to derive their star formation rates (SFR), stellar
masses, ages, etc. requires making a priori assumptions about their star
formation histories (SFH). A widely adopted parameterization of the SFH, the
so-called tau-models where SFR goes as e^{-t/tau) is shown to lead to
unrealistically low ages when applied to star forming galaxies at z ~ 2, a
problem shared by other SFHs when the age is left as a free parameter in the
fitting. This happens because the SED of such galaxies, at all wavelengths, is
dominated by their youngest stellar populations, which outshine the older ones.
Thus, the SED of such galaxies conveys little information on the beginning of
star formation. To cope with this problem, we explore a variety of SFHs, such
as constant SFR and inverted-tau models - with SFR as e^{+t/tau) - along with
various priors on age, including assuming that star formation started at high
redshift in all the galaxies. We find that inverted-tau models with such latter
assumption give SFRs and extinctions in excellent agreement with the values
derived using only the UV part of the SED. These models are also shown to
accurately recover the SFRs and masses of mock galaxies at z ~ 2 constructed
from semi-analytic models. All other explored SFH templates do not fulfil these
two test. In particular, direct-tau models with unconstrained age in the
fitting procedure overstimate SFRs and underestimate stellar mass, and would
exacerbate an apparent mismatch between the cosmic evolution of the volume
densities of SFR and stellar mass. We conclude that for high-redshift star
forming galaxies an exponentially increasing SFR with a high formation redshift
is preferable to other forms of the SFH so far adopted in the literature.Comment: 19 pages, 28 figures, Monthly Notices of the Royal Astronomical
Society in pres
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