839 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
A VLT/FORS2 Multi-Slit Search for Lyman-alpha Emitting Galaxies at z~6.5
We present results from a deep spectroscopic search in the 9150A atmospheric
window for z~6.5 Lyman-alpha emitting galaxies using the VLT/FORS2. Our
multi-slit+narrow-band filter survey covers a total spatial area of 17.6 sq.
arcmin in four different fields and reaches fluxes down to 5x10^(-18)
erg/s/cm^2 (7 sigma detection). Our detection limit is significantly fainter
than narrow-band searches at this redshift and fainter also than the unlensed
brightness of Hu et al.'s HCM6A at z=6.56, and thus provides better overlap
with surveys at much lower redshifts. Eighty secure emission line galaxies are
detected. However, based on their clear continuum emission shortward of the
line or the presence of multiple lines, none of these can be Ly-alpha emission
at z~6.5. Our null result of finding no z~6.5 Ly-alpha emitters suggests that
the number density of Ly-alpha emitters with L>2x10^(42) erg/s declines by ~2
between z~3 and z~6.5.Comment: accepted by ApJ Letters (originally submitted June 11, 2004
Clustering of Dust-Obscured Galaxies at z ~ 2
We present the angular autocorrelation function of 2603 dust-obscured
galaxies (DOGs) in the Bootes field of the NOAO Deep Wide-Field Survey. DOGs
are red, obscured galaxies, defined as having R-[24] \ge 14 (F_24/F_R \ga
1000). Spectroscopy indicates that they are located at 1.5 \la z \la 2.5. We
find strong clustering, with r_0 = 7.40^{+1.27}_{-0.84} Mpc/h for the full F_24
> 0.3 mJy sample. The clustering and space density of the DOGs are consistent
with those of submillimeter galaxies, suggestive of a connection between these
populations. We find evidence for luminosity-dependent clustering, with the
correlation length increasing to r_0 = 12.97^{+4.26}_{-2.64} Mpc/h for brighter
(F_24 > 0.6 mJy) DOGs. Bright DOGs also reside in richer environments than
fainter ones, suggesting these subsamples may not be drawn from the same parent
population. The clustering amplitudes imply average halo masses of log M =
12.2^{+0.3}_{-0.2} Msun for the full DOG sample, rising to log M =
13.0^{+0.4}_{-0.3} Msun for brighter DOGs. In a biased structure formation
scenario, the full DOG sample will, on average, evolve into ~ 3 L* present-day
galaxies, whereas the most luminous DOGs may evolve into brightest cluster
galaxies.Comment: ApJL in press; 4 pages, 3 figures, 1 tabl
First Weak-lensing Results from "See Change": Quantifying Dark Matter in the Two Z>1.5 High-redshift Galaxy Clusters SPT-CL J2040-4451 and IDCS J1426+3508
We present a weak-lensing study of SPT-CLJ2040-4451 and IDCSJ1426+3508 at
z=1.48 and 1.75, respectively. The two clusters were observed in our "See
Change" program, a HST survey of 12 massive high-redshift clusters aimed at
high-z supernova measurements and weak-lensing estimation of accurate cluster
masses. We detect weak but significant galaxy shape distortions using IR images
from the WFC3, which has not yet been used for weak-lensing studies. Both
clusters appear to possess relaxed morphology in projected mass distribution,
and their mass centroids agree nicely with those defined by both the galaxy
luminosity and X-ray emission. Using an NFW profile, for which we assume that
the mass is tightly correlated with the concentration parameter, we determine
the masses of SPT-CL J2040-4451 and IDCS J1426+3508 to be
M_{200}=8.6_{-1.4}^{+1.7}x10^14 M_sun and 2.2_{-0.7}^{+1.1}x10^14 M_sun,
respectively. The weak-lensing mass of SPT-CLJ2040-4451 shows that the cluster
is clearly a rare object. Adopting the central value, the expected abundance of
such a massive cluster at z>1.48 is only ~0.07 in the parent 2500 sq. deg.
survey. However, it is yet premature to claim that the presence of this cluster
creates a serious tension with the current LCDM paradigm unless that tension
will remain in future studies after marginalizing over many sources of
uncertainties such as the accuracy of the mass function and the
mass-concentration relation at the high mass end. The mass of IDCSJ1426+3508 is
in excellent agreement with our previous ACS-based weak-lensing result while
the much higher source density from our WFC3 imaging data makes the current
statistical uncertainty ~40% smaller.Comment: Accepted to Ap
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
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 most massive objects in the Universe
We calculate the most massive object in the Universe, finding it to be a
cluster of galaxies with total mass M_200=3.8e15 Msun at z=0.22, with the 1
sigma marginalized regions being 3.3e15 Msun<M<4.4e15 Msun and 0.12<z<0.36. We
restrict ourselves to self-gravitating bound objects, and base our results on
halo mass functions derived from N-body simulations. Since we consider the very
highest mass objects, the number of candidates is expected to be small, and
therefore each candidate can be extensively observed and characterized. If
objects are found with excessively large masses, or insufficient objects are
found near the maximum expected mass, this would be a strong indication of the
failure of LambdaCDM. The expected range of the highest masses is very
sensitive to redshift, providing an additional evolutionary probe of LambdaCDM.
We find that the three most massive clusters in the recent SPT 178 deg^2
catalog match predictions, while XMMU J2235.3--2557 is roughly 3 sigma
inconsistent with LambdaCDM. We discuss Abell 2163 and Abell 370 as candidates
for the most massive cluster in the Universe, although uncertainties in their
masses preclude definitive comparisons with theory. Our findings motivate
further observations of the highest mass end of the mass function. Future
surveys will explore larger volumes, and the most massive object in the
Universe may be identified within the next decade. The mass distribution of the
largest objects in the Universe is a potentially powerful test of LambdaCDM,
probing non-Gaussianity and the behavior of gravity on large scales.Comment: 4 pages, 2 figures; Abell 2163 and Abell 370 included as candidates
for the most massive cluster in the Univers
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