571 research outputs found
Unbiased Cluster Lens Reconstruction
Weak lensing observations measure the shear field and hence the gradient of
the dimensionless surface density . We present several new algorithms
to recover {}from shear estimates on a finite region and compare how
they perform with realistically noisy data. The reconstruction methods studied
here are divided into 2 classes: direct reconstruction and regularized
inversion. Direct reconstruction techniques express as a 2D integral
of the shear field. This yields an estimator for as a discrete sum
over background galaxy ellipticities which is straightforward to implement, and
allows a simple estimate of the noise. We study 3 types of direct
reconstruction methods: 1) -estimators that measure the surface density
at any given target point relative to the mean value in a reference region 2) a
method that explicitly attempts to minimize the rotational part of that is due to noise and 3) a novel, exact Fourier-space inverse
gradient operator. We also develop two `regularized maximum likelihood'
methods, one of which employs the conventional discrete Laplacian operator as a
regularizer and the other uses regularization of all components in Fourier
space. We compare the performance of all the estimators by means of simulations
and noise power analysis. A general feature of these unbiased methods is an
enhancement of the low frequency power which, for some of the methods, can be
quite severe. We find the best performance is provided by the maximum
likelihood method with Fourier space regularization although some of the other
methods perform almost as well.Comment: 37 pages, latex, figures include
The Origin of [O II] Emission in Recently Quenched Active Galaxy Nucleus Hosts
We have employed emission-line diagnostics derived from DEIMOS and NIRSPEC spectroscopy to determine the origin of the [O II] emission line observed in six active galactic nucleus (AGN) hosts at z ~ 0.9. These galaxies are a subsample of AGN hosts detected in the Cl1604 supercluster that exhibit strong Balmer absorption lines in their spectra and appear to be in a post-starburst or post-quenched phase, if not for their [O II] emission. Examining the flux ratio of the [N II] to Hα lines, we find that in five of the six hosts the dominant source of ionizing flux is AGN continuum emission. Furthermore, we find that four of the six galaxies have over twice the [O II] line luminosity that could be generated by star formation alone given their Hα line luminosities. This strongly suggests that AGN-excited narrow-line emission is contaminating the [O II] line flux. A comparison of star formation rates calculated from extinction-corrected [O II] and Hα line luminosities indicates that the former yields a five-fold overestimate of the current activity in these galaxies. Our findings reveal the [O II] line to be a poor indicator of star formation activity in a majority of these moderate-luminosity Seyferts. This result bolsters our previous findings that an increased fraction of AGN at high redshifts is hosted by galaxies in a post-starburst phase. The relatively high fraction of AGN hosts in the Cl1604 supercluster that show signs of recently truncated star formation activity may suggest that AGN feedback plays an increasingly important role in suppressing ongoing activity in large-scale structures at high redshift
The Violent Youth of Bright and Massive Cluster Galaxies and their Maturation over 7 Billion Years
In this study we investigate the formation and evolution mechanisms of the
brightest cluster galaxies (BCGs) over cosmic time. At high redshift
(), we selected BCGs and most massive cluster galaxies (MMCGs) from
the Cl1604 supercluster and compared them to low-redshift ()
counterparts drawn from the MCXC meta-catalog, supplemented by SDSS imaging and
spectroscopy. We observed striking differences in the morphological, color,
spectral, and stellar mass properties of the BCGs/MMCGs in the two samples.
High-redshift BCGs/MMCGs were, in many cases, star-forming, late-type galaxies,
with blue broadband colors, properties largely absent amongst the low-redshift
BCGs/MMCGs. The stellar mass of BCGs was found to increase by an average factor
of from to . Through this and other
comparisons we conclude that a combination of major merging (mainly wet or
mixed) and \emph{in situ} star formation are the main mechanisms which build
stellar mass in BCGs/MMCGs. The stellar mass growth of the BCGs/MMCGs also
appears to grow in lockstep with both the stellar baryonic and total mass of
the cluster. Additionally, BCGs/MMCGs were found to grow in size, on average, a
factor of , while their average S\'ersic index increased by 0.45
from to , also supporting a scenario involving major
merging, though some adiabatic expansion is required. These observational
results are compared to both models and simulations to further explore the
implications on processes which shape and evolve BCGs/MMCGs over the past
7 Gyr.Comment: Accepted for publication in MNRA
Mass along the Line of Sight to the Gravitational Lens B1608+656: Galaxy Groups and Implications for H_0
We report the discovery of four groups of galaxies along the line of sight to the B1608+656 gravitational lens system. One group is at the redshift of the primary lensing galaxy (z = 0.631) and appears to have a low mass, with eight spectroscopically confirmed members and an estimated velocity dispersion of 150 ± 60 km s^(-1). The three other groups are in the foreground of the lens. These groups contain ~10 confirmed members each and are located at redshifts of 0.265, 0.426, and 0.52. Two of the three additional groups are centered roughly on the lens system, while the third is centered ~1' south of the lens. We investigate the effect of each of the four groups on the gravitational lensing potential of the B1608+656 system, with a particular focus on the implications for the value of H_0 derived from this system. We find that each group provides an external convergence of ~0.005-0.060, depending on the assumptions made in the calculation. For the B1608+656 system, the stellar velocity dispersion of the lensing galaxy has been measured, thus breaking the mass sheet degeneracy due to the group that is physically associated with the lens. The effect of the other groups along the line of sight can be folded into the overall uncertainties due to large-scale structure (LSS) along the line of sight. Because B1608+656 appears to lie along an overdense line of sight, the LSS will cause the measurement of H_0 to be biased high for this system. This effect could be 5% or greater
Galaxy Groups Associated with Gravitational Lenses and H_0 from B1608+656
Compact groups of galaxies recently have been discovered in association with
several strong gravitational lens systems. These groups provide additional
convergence to the lensing potential and thus affect the value of H_0 derived
from the systems. Lens system time delays are now being measured with
uncertainties of only a few percent or better. Additionally, vast improvements
are being made in incorporating observational constraints such as Einstein ring
structures and stellar velocity dispersions into the lens models. These
advances are reducing the uncertainties on H_0 to levels at which the the
effects of associated galaxy groups may contribute significantly to the overall
error budget. We describe a dedicated multiwavelength program, using Keck, HST,
and Chandra, to find such groups and measure their properties. We present, as a
case study, results obtained from observations of the CLASS lens system
B1608+656 and discuss the implications for the value of H_0 derived from this
system.Comment: To appear in Proceedings of IAU Symposium 225: Impact of
Gravitational Lensing on Cosmology, 6 pages, 2 figure
An X-ray Selected Galaxy Cluster at z=1.11 in the Rosat Deep Cluster Survey
We report the discovery of an X-ray luminous galaxy cluster at z =1.11. RDCS
J0910+5422 was selected as an X-ray cluster candidate in the ROSAT Deep Cluster
Survey on the basis of its spatial extent in a Rosat PSPC image. Deep optical
and near-IR imaging reveal a red galaxy overdensity around the peak of the
X-ray emission, with a significant excess of objects with J-K and I-K colors
typical of elliptical galaxies at z ~ 1.0. Spectroscopic observations at the
Keck II telescope secured 9 galaxy redshifts in the range 1.095<z<1.120
yielding a mean cluster redshift of =1.106. Eight of these galaxies lie
within a 30 arcsec radius around the peak X-ray emission. A deep Chandra ACIS
exposure on this field shows extended X-ray morphology and allows the X-ray
spectrum of the intracluster medium to be measured. The cluster has a
bolometric luminosity L_x = 2.48^{+0.33}_{-0.26} x 10^44 ergs/s, a temperature
of kT = 7.2^{+2.2}_{-1.4} keV, and a mass within r = 1 Mpc of 7.0 x 10^14 M_sun
(H_0=65 km/s/Mpc, Omega_m = 0.3, and Lambda = 0.7). The spatial distribution of
the cluster members is elongated, which is not due to an observational
selection effect, and followed by the X-ray morphology. The X-ray surface
brightness profile and the spectrophotometric properties of the cluster members
suggest that this is an example of a massive cluster in an advanced stage of
formation with a hot ICM and an old galaxy population already in place at z >
1.Comment: 19 pages, 7 figures: Figures 1,4,6 included as separate jpg files.
Accepted for publication in The Astronomical Journa
The Properties of Radio Galaxies and the Effect of Environment in Large Scale Structures at
In this study we investigate 89 radio galaxies that are
spectroscopically-confirmed to be members of five large scale structures in the
redshift range of . Based on a two-stage classification
scheme, the radio galaxies are classified into three sub-classes: active
galactic nucleus (AGN), hybrid, and star-forming galaxy (SFG). We study the
properties of the three radio sub-classes and their global and local
environmental preferences. We find AGN hosts are the most massive population
and exhibit quiescence in their star-formation activity. The SFG population has
a comparable stellar mass to those hosting a radio AGN but are unequivocally
powered by star formation. Hybrids, though selected as an intermediate
population in our classification scheme, were found in almost all analyses to
be a unique type of radio galaxies rather than a mixture of AGN and SFGs. They
are dominated by a high-excitation radio galaxy (HERG) population. We discuss
environmental effects and scenarios for each sub-class. AGN tend to be
preferentially located in locally dense environments and in the cores of
clusters/groups, with these preferences persisting when comparing to galaxies
of similar colour and stellar mass, suggesting that their activity may be
ignited in the cluster/group virialized core regions. Conversely, SFGs exhibit
a strong preference for intermediate-density global environments, suggesting
that dusty starbursting activity in LSSs is largely driven by galaxy-galaxy
interactions and merging.Comment: 28 pages, 10 figures, accepted to MNRA
Pushing the Boundaries of the Cl 1604 Supercluster at z~0.9
The Cl 1604 supercluster at z~0.9 is known to contain at least four distinct
member clusters, separated in both projection and redshift. In this paper we
present deep, multicolor wide-field imaging of a region spanning ~45' on a
side, corresponding to 21 h^{-1}_{70} Mpc (physical) at the supercluster
redshift. We select galaxies whose colors correspond to those of
spectroscopically confirmed cluster members in the r' vs. (r'-i')
color-magnitude diagram. Using an adaptive kernel, we generate a map of the
projected red galaxy density and identify numerous new candidate clusters which
are likely supercluster members. Assuming that all of the density peaks are
associated with the supercluster, its transverse size is ~10 h^{-1}_{70} Mpc,
which is still significantly smaller than the nearly 93 h^{-1}_{70} Mpc depth
in redshift space.Comment: 5 pages, 5 figures. Accepted for publication in AJ. Figures and text
updated with correction
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