982 research outputs found
A wide area survey for high-redshift massive galaxies. I. Number counts and clustering of BzKs and EROs
We have combined deep BRIz' imaging over 2x940 arcmin^2 fields obtained with
the Suprime-Cam on the Subaru telescope with JKs imaging with the SOFI camera
at the New Technology Telescope to search for high-redshift massive galaxies.
K-band selected galaxies have been identified over an area of ~920 arcmin^2 to
K_Vega=19.2, of which 320 arcmin^2 are complete to K_Vega=20. The BzK selection
technique was used to obtain complete samples of ~500 candidate massive
star-forming galaxies (sBzKs) and ~160 candidate massive, passively-evolving
galaxies (pBzKs), both at 1.4 5 criterion we also
identified ~850 extremely red objects (EROs). The surface density of sBzKs and
pBzKs is found to 1.20+/-0.05 arcmin^{-2} and 0.38+/-0.03 arcmin^{-2},
respectively. Both sBzKs and pBzKs are strongly clustered, at a level at least
comparable to that of EROs, with pBzKs appearing more clustered than sBzKs. We
estimate the reddening, star formation rates (SFRs) and stellar masses (M_*) of
the sBzKs, confirming that to K_Vega~20 median values are M_*~10^{11}M_sun, SFR
190M_sun yr^{-1}, and E(B-V)~0.44. The most massive sBzKs are also the most
actively star-forming, an effect which can be seen as a manifestation of
downsizing at early epochs. The space density of massive pBzKs at z~1.4-2 is
20%+/-7% that of similarly massive early-type galaxies at z~0, and similar to
that of sBzKs of the same mass. We argue that star formation quenching in these
sBzKs will result in nearly doubling the space density of massive early-type
galaxies, thus matching their local density.Comment: 19 pages, 13 figures, accepted by ApJ. While checking the proofs we
became aware of a material mistake of non-trivial scientific relevance. In
the original it was reported that the comoving volume density of passive
BzK-selected galaxies with =1.7 and more massive than 10^{11}M_sun was
45%+/-15% of the local number density of similarly massive early-type
galaxies. This fraction actually turns out to be 20%+/-7%. Section 6.4, point
5 in section 7, and the abstract have been modified accordingl
Extremely Red Objects in Two Quasar Fields at z ~ 1.5
We present an investigation of the properties and environments of bright
extremely red objects (EROs) found in the fields of the quasars TXS 0145+386
and 4C 15.55, both at z ~ 1.4. There is marginal evidence from Chandra ACIS
imaging for hot cluster gas with a luminosity of a few 10^44 ergs/s in the
field of 4C 15.55. The TXS 0145+386 field has an upper limit at a similar
value, but it also clearly shows an overdensity of faint galaxies. None of the
EROs are detected as X-ray sources. For two of the EROs that have
spectral-energy distributions and rest-frame near-UV spectra that show that
they are strongly dominated by old stellar populations, we determine
radial-surface-brightness profiles from adaptive-optics images. Both of these
galaxies are best fit by profiles close to exponentials, plus a compact nucleus
comprising ~30% of the total light in one case and 8% in the other. Neither is
well fit by an r^1/4-law profile. This apparent evidence for the formation of
massive ~2 X 10^11 disks of old stars in the early universe indicates that at
least some galaxies formed essentially monolithically, with high star-formation
rates sustained over a few 10^8 years, and without the aid of major mergers.Comment: 25 pages, 13 figures, accepted to Ap
Passively Evolving Early-type Galaxies at 1.4<z<2.5 in the Hubble Ultra Deep Field
We report on a complete sample of 7 luminous early-type galaxies in the
Hubble Ultra Deep Field (UDF) with spectroscopic redshifts between 1.39 and
2.47 and to K<23 AB. Using the BzK selection criterion we have pre-selected a
set of objects over the UDF which fulfill the photometric conditions for being
passively evolving galaxies at z>1.4. Low-resolution spectra of these objects
have been extracted from the HST+ACS grism data taken over the UDF by the
GRAPES project. Redshift for the 7 galaxies have been identified based on the
UV feature at rest frame 2640<lambda<2850 AA. This feature is mainly due to a
combination of FeII, MgI and MgII absorptions which are characteristic of
stellar populations dominated by stars older than about 0.5 Gyr. The redshift
identification and the passively evolving nature of these galaxies is further
supported by the photometric redshifts and by the overall spectral energy
distribution (SED), with the ultradeep HST+ACS/NICMOS imaging revealing compact
morphologies typical of elliptical/early-type galaxies. From the SED we derive
stellar masses of 10^{11}Msun or larger and ages of about 1 Gyr. Their space
density at =1.7 appears to be roughly a factor of 2--3 smaller than that
of their local counterparts, further supporting the notion that such massive
and old galaxies are already ubiquitous at early cosmic times. Much smaller
effective radii are derived for some of the objects compared to local massive
ellipticals, which may be due to morphological K corrections, evolution, or the
presence of a central point-like source. Nuclear activity is indeed present in
a subset of the galaxies, as revealed by them being hard X-ray sources, hinting
to AGN activity having played a role in discontinuing star formation.Comment: 18 pages, 15 figures, ApJ in pres
COLDz: Karl G. Jansky Very Large Array discovery of a gas-rich galaxy in COSMOS
The broad spectral bandwidth at mm and cm-wavelengths provided by the recent upgrades to the Karl G. Jansky Very Large Array (VLA) has made it possible to conduct unbiased searches for molecular CO line emission at redshifts, z > 1.31. We present the discovery of a gas-rich, star-forming galaxy at z = 2.48, through the detection of CO(1-0) line emission in the COLDz survey, through a sensitive, Ka-band (31 to 39 GHz) VLA survey of a 6.5 square arcminute region of the COSMOS field. We argue that the broad line (FWHM ~570 +/- 80 km/s) is most likely to be CO(1-0) at z=2.48, as the integrated emission is spatially coincident with an infrared-detected galaxy with a photometric redshift estimate of z = 3.2 +/- 0.4. The CO(1-0) line luminosity is L'_CO = (2.2 +/- 0.3) x 10^{10} K km/s pc^2, suggesting a cold molecular gas mass of M_gas ~ (2 - 8)x10^{10}M_solar depending on the assumed value of the molecular gas mass to CO luminosity ratio alpha_CO. The estimated infrared luminosity from the (rest-frame) far-infrared spectral energy distribution (SED) is L_IR = 2.5x10^{12} L_solar and the star-formation rate is ~250 M_solar/yr, with the SED shape indicating substantial dust obscuration of the stellar light. The infrared to CO line luminosity ratio is ~114+/-19 L_solar/(K km/s pc^2), similar to galaxies with similar SFRs selected at UV/optical to radio wavelengths. This discovery confirms the potential for molecular emission line surveys as a route to study populations of gas-rich galaxies in the future
GALEX selected Lyman Break Galaxies at z~2: Comparison with other Populations
We present results of a search for bright Lyman break galaxies at 1.5<=z<=2.5
in the GOODS-S field using a NUV-dropout technique in combination with
color-selection. We derived a sample of 73 LBG candidates. We compare our
selection efficiencies to BM/BX- and BzK methods (techniques solely based on
ground-based data sets), and find the NUV data to provide greater efficiency
for selecting star-forming galaxies. We estimate LBG candidate ages, masses,
star formation rates, and extinction from fitting PEGASE synthesis evolution
models. We find about 20% of our LBG candidates are comparable to infrared
luminous LBGs or sub-millimeter galaxies which are thought to be precursors of
massive elliptical galaxies today. Overall, we can show that although BM/BX and
BzK methods do identify star-forming galaxies at z~2, the sample they provide
biases against those star-forming galaxies which are more massive and contain
sizeable red stellar populations. A true Lyman break criterion at z~2 is
therefore more directly comparable to the populations found at z~3, which does
contain a red fraction.Comment: 14 pages, 11 figures, accepted for publication in Ap
The Impact of cold gas accretion above a mass floor on galaxy scaling relations
Using the cosmological baryonic accretion rate and normal star formation
efficiencies, we present a very simple model for star-forming galaxies (SFGs)
that accounts for the mass and redshift dependencies of the SFR-Mass and
Tully-Fisher relations from z=2 to the present. The time evolution follows from
the fact that each modelled galaxy approaches a steady state where the SFR
follows the (net) cold gas accretion rate. The key feature of the model is a
halo mass floor M_{min}~10^{11} below which accretion is quenched in order to
simultaneously account for the observed slopes of the SFR-Mass and
Tully-Fischer relations. The same successes cannot be achieved via a
star-formation threshold (or delay) nor by varying the SF efficiency or the
feedback efficiency. Combined with the mass ceiling for cold accretion due to
virial shock heating, the mass floor M_{min} explains galaxy "downsizing",
where more massive galaxies formed earlier and over a shorter period of time.
It turns out that the model also accounts for the observed galactic baryon and
gas fractions as a function of mass and time, and the cosmic SFR density from
z~6 to z=0, which are all resulting from the mass floor M_{min}. The model
helps to understand that it is the cosmological decline of accretion rate that
drives the decrease of cosmic SFR density between z~2 and z=0 and the rise of
the cosmic SFR density allows us to put a constraint on our main parameter
M_{min}~10^{11} solar masses. Among the physical mechanisms that could be
responsible for the mass floor, we view that photo-ionization feedback (from
first in-situ hot stars) lowering the cooling efficiency is likely to play a
large role.Comment: 19pages, 14 figures, accepted to ApJ, updated reference
A mature cluster with X-ray emission at z=2.07
We report evidence of a fully established galaxy cluster at z=2.07,
consisting of a ~20sigma overdensity of red, compact spheroidal galaxies
spatially coinciding with extended X-ray emission detected with XMM-Newton. We
use VLT VIMOS and FORS2 spectra and deep Subaru, VLT and Spitzer imaging to
estimate the redshift of the structure from a prominent z=2.07 spectroscopic
redshift spike of emission-line galaxies, concordant with the accurate 12-band
photometric redshifts of the red galaxies. Using NICMOS and Keck AO
observations, we find that the red galaxies have elliptical morphologies and
compact cores. While they do not form a tight red sequence, their colours are
consistent with that of a >1.3$~Gyr population observed at z~2.1. From an X-ray
luminosity of .2*10^43 erg s^-1 and the stellar mass content of the red galaxy
population, we estimate a halo mass of 5.3-8*10^13 Msun, comparable to the
nearby Virgo cluster. These properties imply that this structure could be the
most distant, mature cluster known to date and that X-ray luminous,
elliptical-dominated clusters are already forming at substantially earlier
epochs than previously known.Comment: 14 pages, 12 figures; accepted for publication in Astronomy &
Astrophysic
The K20 survey. VI. The Distribution of the Stellar Masses in Galaxies up to z~2
We present a detailed analysis of the stellar mass content of galaxies up to
z=2.5 in the K20 galaxy sample, that has a 92% spectroscopic completeness and a
complete multicolor coverage. We find that the M/L ratio decreases
with redshift: in particular, the average M/L ratio of early type galaxies
decreases with , with a scatter that is indicative of a range of
star--formation time-scales and redshift of formation. More important, the
typical M/L of massive early type galaxies is larger than that of less massive
ones, suggesting that their stellar population formed at higher z. The final
K20 galaxy sample spans a range of stellar masses from M*=10^9Msun to
M*=10^12Msun, with massive galaxies ($M*>10^11Msun) detected up to z~2. We
compute the Galaxy Stellar Mass Function at various z, of which we observe only
a mild evolution (i.e. by 20-30%) up to z~1. At z>1, the evolution of the GSMF
appears to be much faster: at z~2, about 35% of the present day stellar mass in
objects with M*~10^11Msun appear to have assembled. We also detect a change in
the physical nature of the most massive galaxies, since at z>1 a population of
massive star--forming galaxies progressively appears. We finally analyze our
results in the framework of Lambda-CDM hierarchical models. First, we show that
the large number of massive galaxies detected at high z does not violate any
fundamental Lambda-CDM constraint based on the number of massive DM halos.
Then, we compare our results with the predictions of renditions of both
semianalytic and hydro-dynamical models, that range from severe underestimates
to slight overestimates of the observed mass density at z<~2. We discuss how
the differences among these models are due to the different implementation of
the main physical processes. (Abridged)Comment: Accepted for publication on Astronomy & Astrophysic
Molecular Gas, Dust and Star Formation in Galaxies: II. Dust properties and scalings in \sim\ 1600 nearby galaxies
We aim to characterize the relationship between dust properties. We also aim
to provide equations to estimate accurate dust properties from limited
observational datasets.
We assemble a sample of 1,630 nearby (z<0.1) galaxies-over a large range of
Mstar, SFR - with multi-wavelength observations available from wise, iras,
planck and/or SCUBA. The characterization of dust emission comes from SED
fitting using Draine & Li dust models, which we parametrize using two
components (warm and cold ). The subsample of these galaxies with global
measurements of CO and/or HI are used to explore the molecular and/or atomic
gas content of the galaxies.
The total Lir, Mdust and dust temperature of the cold component (Tc) form a
plane that we refer to as the dust plane. A galaxy's sSFR drives its position
on the dust plane: starburst galaxies show higher Lir, Mdust and Tc compared to
Main Sequence and passive galaxies. Starburst galaxies also show higher
specific Mdust (Mdust/Mstar) and specific Mgas (Mgas/Mstar). The Mdust is more
closely correlated with the total Mgas (atomic plus molecular) than with the
individual components. Our multi wavelength data allows us to define several
equations to estimate Lir, Mdust and Tc from one or two monochromatic
luminosities in the infrared and/or sub-millimeter.
We estimate the dust mass and infrared luminosity from a single monochromatic
luminosity within the R-J tail of the dust emission, with errors of 0.12 and
0.20dex, respectively. These errors are reduced to 0.05 and 0.10 dex,
respectively, if the Tc is used. The Mdust is correlated with the total Mism
(Mism \propto Mdust^0.7). For galaxies with Mstar 8.5<log(Mstar/Msun) < 11.9,
the conversion factor \alpha_850mum shows a large scatter (rms=0.29dex). The SF
mode of a galaxy shows a correlation with both the Mgass and Mdust: high
Mdust/Mstar galaxies are gas-rich and show the highest SFRs.Comment: 24 pages, 28 figures, 6 tables, Accepted for publication in A&
The COSMOS-WIRCam near-infrared imaging survey: I: BzK selected passive and star forming galaxy candidates at z>1.4
(abridged) We present a new near-infrared survey covering the 2 deg sq COSMOS
field. Combining our survey with Subaru B and z images we construct a deep,
wide-field optical-infrared catalogue. At Ks<23 (AB magnitudes) our survey
completeness is greater than 90% and 70% for stars and galaxies respectively
and contains 143,466 galaxies and 13,254 stars. At z~2 our catalogues contain
3931 quiescent and 25,757 star-forming BzK-selected galaxies representing the
largest and most secure sample of these objects to date. Our counts of
quiescent galaxies turns over at Ks~22 an effect which we demonstrate cannot be
due to sample incompleteness. In our survey both the number of faint and bright
quiescent objects exceeds the predictions of a semi-analytic galaxy formation
model, indicating potentially the need for further refinements in the amount of
merging and AGN feedback at z~2 in these models. We measure the angular
correlation function for each sample and find that at small scales the
correlation function for passive BzK galaxies exceeds the clustering of dark
matter. We use 30-band photometric redshifts to derive the spatial correlation
length and the redshift distributions for each object class. At Ks<22 we find
r_0^{\gamma/1.8}=7.0 +/-0.5h^{-1} Mpc for the passive BzK candidates and
4.7+/-0.8h^{-1} Mpc for the star-forming BzK galaxies. Our pBzK galaxies have
an average photometric redshift of z_p~1.4, in approximate agreement with the
limited spectroscopic information currently available. The stacked Ks image
will be made publicly available from IRSA.Comment: Accepted for publication in Astrophysical Journal. 17 pages, 17
figures, minor revisions to match published version available at
http://adsabs.harvard.edu/abs/2010ApJ...708..202
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