513 research outputs found
Obscuration by Gas and Dust in Luminous Quasars
We explore the connection between absorption by neutral gas and extinction by
dust in mid-infrared (IR) selected luminous quasars. We use a sample of 33
quasars at redshifts 0.7 < z < 3 in the 9 deg^2 Bo\"otes multiwavelength survey
field that are selected using Spitzer Space Telescope Infrared Array Camera
colors and are well-detected as luminous X-ray sources (with >150 counts) in
Chandra observations. We divide the quasars into dust-obscured and unobscured
samples based on their optical to mid-IR color, and measure the neutral
hydrogen column density N_H through fitting of the X-ray spectra. We find that
all subsets of quasars have consistent power law photon indices equal to 1.9
that are uncorrelated with N_H. We classify the quasars as gas-absorbed or
gas-unabsorbed if N_H > 10^22 cm^-2 or N_H < 10^22 cm^-2, respectively. Of 24
dust-unobscured quasars in the sample, only one shows clear evidence for
significant intrinsic N_H, while 22 have column densities consistent with N_H <
10^22 cm^-2. In contrast, of the nine dust-obscured quasars, six show evidence
for intrinsic gas absorption, and three are consistent with N_H < 10^22 cm^-2.
We conclude that dust extinction in IR-selected quasars is strongly correlated
with significant gas absorption as determined through X-ray spectral fitting.
These results suggest that obscuring gas and dust in quasars are generally
co-spatial, and confirm the reliability of simple mid-IR and optical
photometric techniques for separating quasars based on obscuration.Comment: 5 pages, 3 figure
The Subillimeter Properties of Extremely Red Objects in the CUDSS Fields
We discuss the submillimeter properties of Extremely Red Objects (EROs) in
the two Canada-UK Deep Submillimeter Survey (CUDSS) Fields. We measure the mean
submillimeter flux of the ERO population (to K < 20.7) and find 0.4 +/- 0.07
mJy for EROs selected by (I-K) > 4.0 and 0.56 +/- 0.09 mJy for EROs selected by
(R-K) > 5.3 but, these measurements are dominated by discrete, bright
submillimeter sources. We estimate that EROs produce 7-11% of the far-infrared
background at 850um. This is substantially less than a previous measurement by
Wehner, Barger & Kneib (2002) and we discuss possible reasons for this
discrepancy. We show that ERO counterparts to bright submillimeter sources lie
within the starburst region of the near-infrared color-color plot of Pozzetti &
Mannucci (2000). Finally, we claim that pairs or small groups of EROs with
separations of < 10 arcseconds often mark regions of strong submillimeter flux.Comment: 9 pages, 8 figures, accepted for publication in Ap
IDCS J1426.5+3508: The Most Massive Galaxy Cluster at
We present a deep (100 ks) Chandra observation of IDCS J1426.5+3508, a
spectroscopically confirmed, infrared-selected galaxy cluster at .
This cluster is the most massive galaxy cluster currently known at ,
based on existing Sunyaev-Zel'dovich (SZ) and gravitational lensing detections.
We confirm this high mass via a variety of X-ray scaling relations, including
-M, -M, -M and -M, finding a tight distribution of masses
from these different methods, spanning M = 2.3-3.3
M, with the low-scatter -based mass M. IDCS J1426.5+3508 is currently the
only cluster at for which X-ray, SZ and gravitational lensing mass
estimates exist, and these are in remarkably good agreement. We find a
relatively tight distribution of the gas-to-total mass ratio, employing total
masses from all of the aforementioned indicators, with values ranging from
= 0.087-0.12. We do not detect metals in the intracluster medium
(ICM) of this system, placing a 2 upper limit of . This upper limit on the metallicity suggests that this system may
still be in the process of enriching its ICM. The cluster has a dense,
low-entropy core, offset by 30 kpc from the X-ray centroid, which makes
it one of the few "cool core" clusters discovered at , and the first
known cool core cluster at . The offset of this core from the
large-scale centroid suggests that this cluster has had a relatively recent
(500 Myr) merger/interaction with another massive system.Comment: Minor changes to match accepted version, results unchanged; ApJ in
pres
The Spitzer South Pole Telescope Deep Field Survey: Linking galaxies and halos at z=1.5
We present an analysis of the clustering of high-redshift galaxies in the
recently completed 94 deg Spitzer-SPT Deep Field survey. Applying flux and
color cuts to the mid-infrared photometry efficiently selects galaxies at
in the stellar mass range , making this
sample the largest used so far to study such a distant population. We measure
the angular correlation function in different flux-limited samples at scales
(corresponding to physical distances Mpc) and
thereby map the one- and two-halo contributions to the clustering. We fit halo
occupation distributions and determine how the central galaxy's stellar mass
and satellite occupation depend on the halo mass. We measure a prominent peak
in the stellar-to-halo mass ratio at a halo mass of , 4.5 times higher than the value. This supports
the idea of an evolving mass threshold above which star formation is quenched.
We estimate the large-scale bias in the range and the satellite
fraction to be , showing a clear evolution compared to
. We also find that, above a given stellar mass limit, the fraction of
galaxies that are in similar mass pairs is higher at than at . In
addition, we measure that this fraction mildly increases with the stellar mass
limit at , which is the opposite of the behavior seen at low-redshift.Comment: 32 pages, 22 figures. Published in MNRA
IDCS J1433.2+3306: An IR-Selected Galaxy Cluster at z = 1.89
We report the discovery of an IR-selected galaxy cluster in the IRAC Distant
Cluster Survey (IDCS). New data from the Hubble Space Telescope
spectroscopically confirm IDCS J1433.2+3306 at z = 1.89 with robust
spectroscopic redshifts for seven members, two of which are based on the 4000
Angstrom break. Detected emission lines such as [OII] and Hbeta indicate star
formation rates of >20 solar masses per year for three galaxies within a 500
kpc projected radius of the cluster center. The cluster exhibits a red sequence
with a scatter and color indicative of a formation redshift z > 3.5. The
stellar age of the early-type galaxy population is approximately consistent
with those of clusters at lower redshift (1 < z < 1.5) suggesting that clusters
at these redshifts are experiencing ongoing or increasing star formation.Comment: Accepted in Ap
Near-Infrared Spectroscopy of 0.4<z<1.0 CFRS Galaxies: Oxygen Abundances, SFRs and Dust
Using new J-band VLT-ISAAC and Keck-NIRSPEC spectroscopy, we have measured
Halpha and [NII] line fluxes for 0.47<z<0.92 CFRS galaxies which have [OII],
Hbeta and [OIII]a line fluxes available from optical spectroscopy, to
investigate how the properties of the star forming gas in galaxies evolve with
redshift. We derive the extinction and oxygen abundances for the sample using a
method based on a set of ionisation parameter and oxygen abundance diagnostics,
simultaneously fitting the [OII], Hbeta,[OIII], Halpha, and [NII] line fluxes.
The individual reddening measurements allow us to accurately correct the
Halpha-based star formation rate (SFR) estimates for extinction. Our most
salient conclusions are: a) in all 30 CFRS galaxies the source of gas
ionisation is not due to AGN activity; b) we find a range of 0<AV<3, suggesting
that it is important to determine the extinction for every single galaxy in
order to reliably measure SFRs and oxygen abundances in high redshift galaxies;
c) high values of [NII]/Halpha >0.1 for most (but not all) of the CFRS galaxies
indicate that they lie on the high-metallicity branch of the R23 calibration;
d) about one third of the 0.47<z<0.92 CFRS galaxies in our sample have lower
metallicities than local galaxies with similar luminosities and star formation
rates; e) comparison with a chemical evolution model indicates that these low
metallicity galaxies are unlikely to be the progenitors of metal-poor dwarf
galaxies at z~0.Comment: Accepted for publication in the Astrophysical Journa
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