142 research outputs found
New constraints on dust emission and UV attenuation of z=6.5-7.5 galaxies from millimeter observations
We have targeted two recently discovered Lyman break galaxies (LBGs) to
search for dust continuum and [CII] 158 micron line emission. The strongly
lensed z~6.8 LBG A1703-zD1 behind the galaxy cluster Abell 1703, and the
spectroscopically confirmed z=7.508 LBG z8-GND-5296 in the GOODS-N field have
been observed with the Plateau de Bure interferometer (PdBI) at 1.2mm. These
observations have been combined with those of three z>6.5 Lya emitters (named
HCM6A, Himiko, and IOK-1), for which deep measurements were recently obtained
with the PdBI and ALMA. [CII] is undetected in both galaxies, providing a deep
upper limit for Abell1703-zD1, comparable to recent ALMA non-detections. Dust
continuum emission from Abell1703-zD1 and z8-GND-5296 is not detected with an
rms of 0.12 and 0.16 mJy/beam. From these non-detections we derive upper limits
on their IR luminosity and star formation rate, dust mass, and UV attenuation.
Thanks to strong gravitational lensing the limit for Abell1703-zD1 is probing
the sub-LIRG regime ( Lsun) and very low dust
masses ( Msun). We find that all five galaxies are
compatible with the Calzetti IRX- relation, their UV attenuation is
compatible with several indirect estimates from other methods (the UV slope,
extrapolation of the attenuation measured from the IR/UV ratio at lower
redshift, and SED fits), and the dust-to-stellar mass ratio is not incompatible
with that of galaxies from z=0 to 3. For their stellar mass the high-z galaxies
studied here have an attenuation below the one expected from the mean relation
of low redshift (z<1.5) galaxies. More and deeper (sub)-mm data are clearly
needed to directly determine the UV attenuation and dust content of the
dominant population of high-z star-forming galaxies and to establish more
firmly their dependence on stellar mass, redshift, and other properties.Comment: 10 pages, 7 figures. Minor revisions. Accepted for publication in A&
Probing Very Bright End of Galaxy Luminosity Function at z >~ 7 Using Hubble Space Telescope Pure Parallel Observations
We report the first results from the Hubble Infrared Pure Parallel Imaging
Extragalactic Survey, which utilizes the pure parallel orbits of the Hubble
Space Telescope to do deep imaging along a large number of random sightlines.
To date, our analysis includes 26 widely separated fields observed by the Wide
Field Camera 3, which amounts to 122.8 sq.arcmin in total area. We have found
three bright Y098-dropouts, which are candidate galaxies at z >~ 7.4. One of
these objects shows an indication of peculiar variability and its nature is
uncertain. The other two objects are among the brightest candidate galaxies at
these redshifts known to date L>2L*. Such very luminous objects could be the
progenitors of the high-mass Lyman break galaxis (LBGs) observed at lower
redshifts (up to z~5). While our sample is still limited in size, it is much
less subject to the uncertainty caused by "cosmic variance" than other samples
because it is derived using fields along many random sightlines. We find that
the existence of the brightest candidate at z~7.4 is not well explained by the
current luminosity function (LF) estimates at z~8. However, its inferred
surface density could be explained by the prediction from the LFs at z~7 if it
belongs to the high-redshift tail of the galaxy population at z~7.Comment: ApJL in press (accepted Dec. 27, 2010); minor corrections and one
figure added to address referee's comment
ALMA detection of [CII] 158 micron emission from a strongly lensed z=2 star-forming galaxy
Our objectives are to determine the properties of the interstellar medium
(ISM) and of star-formation in typical star-forming galaxies at high redshift.
Following up on our previous multi-wavelength observations with HST, Spitzer,
Herschel, and the Plateau de Bure Interferometer (PdBI), we have studied a
strongly lensed z=2.013 galaxy, the arc behind the galaxy cluster MACS
J0451+0006, with ALMA to measure the [CII] 158 micron emission line, one of the
main coolants of the ISM. [CII] emission from the southern part of this galaxy
is detected at 10 . Taking into account strong gravitational lensing,
which provides a magnification of , the intrinsic lensing-corrected
[CII]158 micron luminosity is . The observed
ratio of [CII]-to-IR emission, , is found to be similar to that in nearby galaxies. The same also
holds for the observed ratio , which is
comparable to that of star-forming galaxies and active galaxy nuclei (AGN) at
low redshift. We utilize strong gravitational lensing to extend diagnostic
studies of the cold ISM to an order of magnitude lower luminosity () and SFR than previous work at high redshift.
While larger samples are needed, our results provide evidence that the cold ISM
of typical high redshift galaxies has physical characteristics similar to
normal star forming galaxies in the local Universe.Comment: 5 pages, 4 figures. Accepted for publication in Astronomy &
Astrophysics, Letter
Spectral Energy Distributions of Type 1 AGN in the COSMOS Survey I - The XMM-COSMOS Sample
The "Cosmic Evolution Survey" (COSMOS) enables the study of the Spectral
Energy Distributions (SEDs) of Active Galactic Nuclei (AGN) because of the deep
coverage and rich sampling of frequencies from X-ray to radio. Here we present
a SED catalog of 413 X-ray (\xmm) selected type 1 (emission line FWHM km
s) AGN with Magellan, SDSS or VLT spectrum. The SEDs are corrected for
the Galactic extinction, for broad emission line contributions, constrained
variability, and for host galaxy contribution. We present the mean SED and the
dispersion SEDs after the above corrections in the rest frame 1.4 GHz to 40
keV, and show examples of the variety of SEDs encountered. In the near-infrared
to optical (rest frame -- 4000\AA), the photometry is complete for
the whole sample and the mean SED is derived from detections only. Reddening
and host galaxy contamination could account for a large fraction of the
observed SED variety. The SEDs are all available on-line.Comment: 22 pages, 22 figures, ApJ accepted, scheduled to be published October
20th, 2012, v75
Detections of CO Molecular Gas in 24um-Bright ULIRGs at z~2 in the Spitzer First Look Survey
We present CO observations of 9 ULIRGs at z~2 with S(24\mu m)>1mJy,
previously confirmed with the mid-IR spectra in the Spitzer First Look Survey.
All targets are required to have accurate redshifts from Keck/GEMINI near-IR
spectra. Using the Plateau de Bure millimeter-wave Interferometer (PdBI) at
IRAM, we detect CO J(3-2) [7 objects] or J(2-1) [1 object] line emission from 8
sources with integrated intensities Ic ~(5-9)sigma. The CO detected sources
have a variety of mid-IR spectra, including strong PAH, deep silicate
absorption and power-law continuum, implying that these molecular gas rich
objects at z~2 could be either starbursts or dust obscured AGNs. The measured
line luminosity L'[CO] is (1.28-3.77)e+10[K km/s pc^2]. The averaged molecular
gas mass M(H2) is 1.7e+10Msun, assuming CO-to-H2 conversion factor of
0.8Msun/[K km/s pc^2]. Three sources (33%) -- MIPS506, MIPS16144 & MIPS8342 --
have double peak velocity profiles. The CO double peaks in MIPS506 and
MIPS16144 show spatial separations of 45kpc and 10.9kpc, allowing the estimates
of the dynamical masses of 3.2e+11*sin^(-2)(i)Msun and 5.4e+11*sin^{-2}(i)Msun
respectively. The implied gas fraction, M(gas)/M(dyn), is 3% and 4%, assuming
an average inclination angle. Finally, the analysis of the HST/NIC2 images,
mid-IR spectra and IR SED revealed that most of our sources are mergers,
containing dust obscured AGNs dominating the luminosities at (3-6)um. Together,
these results provide some evidence suggesting SMGs, bright 24um z~2 ULIRGs and
QSOs could represent three different stages of a single evolutionary sequence,
however, a complete physical model would require much more data, especially
high spatial resolution spectroscopy.Comment: 15 pages, 8 figures, accepted for publication in ApJ
Deep GALEX Imaging of the HST/COSMOS Field: A First Look at the Morphology of z~0.7 Star-forming Galaxies
We present a study of the morphological nature of redshift z~0.7 star-forming
galaxies using a combination of HST/ACS, GALEX and ground-based images of the
COSMOS field. Our sample consists of 8,146 galaxies, 5,777 of which are
detected in the GALEX near-ultraviolet band down to a limiting magnitude of
25.5 (AB). We make use of the UV to estimate star formation rates, correcting
for the effect of dust using the UV-slope, and compute, from the ACS F814W
images, the C,A,S,G,M20 morphological parameters for all objects in our sample.
We observe a morphological bimodality in the galaxy population and show that it
has a strong correspondence with the FUV - g color bimodality. We conclude that
UV-optical color predominantly evolves concurrently with morphology. We observe
many of the most star-forming galaxies to have morphologies approaching that of
early-type galaxies, and interpret this as evidence that strong starburst
events are linked to bulge growth and constitute a process through which
galaxies can be brought from the blue to the red sequence while simultaneously
modifying their morphology accordingly. We conclude that the red sequence has
continued growing at z~<0.7. We also observe z~0.7 galaxies to have physical
properties similar to that of local galaxies, except for higher star formation
rates. Whence we infer that the dimming of star-forming galaxies is responsible
for most of the evolution in the star formation rate density of the Universe
since that redshift, although our data are also consistent with a mild number
evolution. [abridged]Comment: 29 pages including 22 figures. Accepted for publication in ApJS
COSMOS Special Issue. A copy of the paper with high resolution figures is
available at http://www.astro.columbia.edu/~michel/galex_cosmos_paper.pd
Photometric Redshifts of Galaxies in COSMOS
We measure photometric redshifts and spectral types for galaxies in the
COSMOS survey. We use template fitting technique combined with luminosity
function priors and with the option to simultaneously estimate dust extinction
(i.e. E(B-V)) for each galaxy.Our estimated redshifts are accurate to i<25 and
z~1.2.
Using simulations with sampling and noise characteristics similar to those in
COSMOS, the accuracy and reliability is estimated for the photometric redshifts
as a function of the magnitude limits of the sample, S/N ratios and the number
of bands used. From the simulations we find that the ratio of derived 95%
confidence interval in the redshift probability distribution to the estimated
photometric redshift (D95) can be used to identify and exclude the catastrophic
failures in the photometric redshift estimates.
We compare the derived redshifts with high-reliability spectroscopic
redshifts for a sample of 868 normal galaxies with z < 1.2 from zCOSMOS.
Considering different scenarios, depending on using prior, no prior and/or
extinction, we compare the photometric and spectroscopic redshifts for this
sample. This corresponds to an rms scatter of 0.031, with a small number of
outliers (<2.5%). We also find good agreement (rms=0.10) between photometric
and spectroscopic redshifts for Type II AGNs.
We compare results from our photometric redshift procedure with three other
independent codes and find them in excellent agreement. We show preliminary
results, based on photometric redshifts for the entire COSMOS sample (to i < 25
mag.).Comment: 38 pages; 14 Figures; 7 Tables. Accepted for Publication in ApJS.
COSMOS Special Issu
HST Morphologies of z ~ 2 Dust-Obscured Galaxies II: Bump Sources
We present Hubble Space Telescope (HST) imaging of 22 ultra-luminous infrared
galaxies (ULIRGs) at z~2 with extremely red R-[24] colors (called dust-obscured
galaxies, or DOGs) which have a local maximum in their spectral energy
distribution (SED) at rest-frame 1.6um associated with stellar emission. These
sources, which we call "bump DOGs", have star-formation rates of 400-4000
Msun/yr and have redshifts derived from mid-IR spectra which show strong
polycyclic aromatic hydrocarbon emission --- a sign of vigorous on-going
star-formation. Using a uniform morphological analysis, we look for
quantifiable differences between bump DOGs, power-law DOGs (Spitzer-selected
ULIRGs with mid-IR SEDs dominated by a power-law and spectral features that are
more typical of obscured active galactic nuclei than starbursts),
sub-millimeter selected galaxies (SMGs), and other less-reddened ULIRGs from
the Spitzer extragalactic First Look Survey (XFLS). Bump DOGs are larger than
power-law DOGs (median Petrosian radius of 8.4 +/- 2.7 kpc vs. 5.5 +/- 2.3 kpc)
and exhibit more diffuse and irregular morphologies (median M_20 of -1.08 +/-
0.05 vs. -1.48 +/- 0.05). These trends are qualitatively consistent with
expectations from simulations of major mergers in which merging systems during
the peak star-formation rate period evolve from M_20 = -1.0 to M_20 = -1.7.
Less obscured ULIRGs (i.e., non-DOGs) tend to have more regular, centrally
peaked, single-object morphologies rather than diffuse and irregular
morphologies. This distinction in morphologies may imply that less obscured
ULIRGs sample the merger near the end of the peak star-formation rate period.
Alternatively, it may indicate that the intense star-formation in these
less-obscured ULIRGs is not the result of a recent major merger.Comment: Accepted to ApJ; 22 pages, 8 Figures, 7 Table
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The XMM-Newton Wide field survey in the COSMOS field: redshift evolution of AGN bias and subdominant role of mergers in triggering moderate luminosity AGN at redshift up to 2.2
We present a study of the redshift evolution of the projected correlation
function of 593 X-ray selected AGN with I_AB<23 and spectroscopic redshifts
z<4, extracted from the 0.5-2 keV X-ray mosaic of the 2.13 deg^2 XMM-COSMOS
survey. We introduce a method to estimate the average bias of the AGN sample
and the mass of AGN hosting halos, solving the sample variance using the halo
model and taking into account the growth of the structure over time. We find
evidence of a redshift evolution of the bias factor for the total population of
XMM-COSMOS AGN from b(z=0.92)=2.30 +/- 0.11 to b(z=1.94)=4.37 +/- 0.27 with an
average mass of the hosting DM halos logM [h^-1 M_sun] ~ 13.12 +/- 0.12 that
remains constant at all z < 2. Splitting our sample into broad optical lines
AGN (BL), AGN without broad optical lines (NL) and X-ray unobscured and
obscured AGN, we observe an increase of the bias with redshift in the range
z=0.7-2.25 and z=0.6-1.5 which corresponds to a constant halo mass logM [h^-1
M_sun] ~ 13.28 +/- 0.07 and logM [h^-1 M_sun] ~ 13.00 +/- 0.06 for BL /X-ray
unobscured AGN and NL/X-ray obscured AGN, respectively. The theoretical models
which assume a quasar phase triggered by major mergers can not reproduce the
high bias factors and DM halo masses found for X-ray selected BL AGN with L_BOL
~ 2e45 erg s^-1. Our work extends up to z ~ 2.2 the z <= 1 statement that, for
moderate luminosity X-ray selected BL AGN, the contribution from major mergers
is outnumbered by other processes, possibly secular such as tidal disruptions
or disk instabilities.Comment: 16 emulateapj pages, 18 figures and 3 tables. Accepted for the
publication in The Astrophysical Journa
The First Release COSMOS Optical and Near-IR Data and Catalog
We present imaging data and photometry for the COSMOS survey in 15
photometric bands between 0.3um and 2.4um. These include data taken on the
Subaru 8.3m telescope, the KPNO and CTIO 4m telescopes, and the CFHT 3.6m
telescope. Special techniques are used to ensure that the relative photometric
calibration is better than 1% across the field of view. The absolute
photometric accuracy from standard star measurements is found to be 6%. The
absolute calibration is corrected using galaxy spectra, providing colors
accurate to 2% or better. Stellar and galaxy colors and counts agree well with
the expected values. Finally, as the first step in the scientific analysis of
these data we construct panchromatic number counts which confirm that both the
geometry of the universe and the galaxy population are evolving.Comment: 19 pages, 13 figures, 14 tables, Accepted to ApJS for COSMOS speciall
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