1,185 research outputs found
The Distribution of Mass and Light in Cluster Infall Regions
The CAIRNS (Cluster And Infall Region Nearby Survey) project is a large
spectroscopic survey of the infall regions surrounding nine nearby rich
clusters of galaxies. I describe the survey and use the kinematics of galaxies
in the infall regions to estimate the cluster mass profiles. At small radii,
these mass profiles are consistent with independent mass estimates from X-ray
observations and Jeans analysis. I demonstrate the dependence of mass-to-light
ratios on environment by combining these mass profiles with Two-Micron All-Sky
Survey (2MASS) photometry. Near-infrared light is more extended than mass in
these clusters, suggesting that dense cluster cores are less efficient at
forming galaxies and/or more efficient at disrupting them. At large radii,
galaxy populations in cluster infall regions closely resemble those in the
field. The mass-to-light ratio at these radii should therefore be a good probe
of the global mass-to-light ratio. The mass-to-light ratio in the infall region
yields a surprisingly low estimate of .Comment: 7 pages, 4 figures, to appear in the Proceedings of IAU Colloquium
195: "Outskirts of Galaxy Clusters: Intense Life in the Suburbs", Torino,
Italy, March 2004, ed. A. Diaferi
Comparison of Hectospec Virial Masses with SZE Measurements
We present the first comparison of virial masses of galaxy clusters with
their Sunyaev-Zel'dovich Effect (SZE) signals. We study 15 clusters from the
Hectospec Cluster Survey (HeCS) with MMT/Hectospec spectroscopy and published
SZE signals. We measure virial masses of these clusters from an average of 90
member redshifts inside the radius . The virial masses of the clusters
are strongly correlated with their SZE signals (at the 99% confidence level
using a Spearman rank-sum test). This correlation suggests that can be
used as a measure of virial mass. Simulations predict a powerlaw scaling of
with 1.6. Observationally, we
find =1.110.16, significantly shallower (given the formal
uncertainty) than the theoretical prediction. However, the selection function
of our sample is unknown and a bias against less massive clusters cannot be
excluded (such a selection bias could artificially flatten the slope).
Moreover, our sample indicates that the relation between velocity dispersion
(or virial mass estimate) and SZE signal has significant intrinsic scatter,
comparable to the range of our current sample. More detailed studies of scaling
relations are therefore needed to derive a robust determination of the relation
between cluster mass and SZE.Comment: 6 pages, 2 figures, accepted to ApJ Letters, minor revisions,
shortened titl
Redshift-space limits of bound structures
An exponentially expanding Universe, possibly governed by a cosmological
constant, forces gravitationally bound structures to become more and more
isolated, eventually becoming causally disconnected from each other and forming
so-called "island universes". This new scenario reformulates the question about
which will be the largest structures that will remain gravitationally bound,
together with requiring a systematic tool that can be used to recognize the
limits and mass of these structures from observational data, namely redshift
surveys of galaxies. Here we present a method, based on the spherical collapse
model and N-body simulations, by which we can estimate the limits of bound
structures as observed in redshift space. The method is based on a theoretical
criterion presented in a previous paper that determines the mean density
contrast that a spherical shell must have in order to be marginally bound to
the massive structure within it. Understanding the kinematics of the system, we
translated the real-space limiting conditions of this "critical" shell to
redshift space, producing a projected velocity envelope that only depends on
the density profile of the structure. From it we created a redshift-space
version of the density contrast that we called "density estimator", which can
be calibrated from N-body simulations for a reasonable projected velocity
envelope template, and used to estimate the limits and mass of a structure only
from its redshift-space coordinates.Comment: Contains 12 pages, 12 figures and 8 table
Infall Regions and Scaling Relations of X-ray Selected Groups
We use the Fifth Data Release of the Sloan Digital Sky Survey to study
X-ray-selected galaxy groups and compare their properties to clusters. We
search for infall patterns around the groups and use these to measure group
mass profiles to large radii. In previous work, we analyzed infall patterns for
an X-ray-selected sample of 72 clusters from the ROSAT All-Sky Survey. Here, we
extend this approach to a sample of systems with smaller X-ray fluxes selected
from the 400 Square Degree serendipitous survey of clusters and groups in ROSAT
pointed observations. We identify 16 groups with SDSS DR5 spectroscopy, search
for infall patterns, and compute mass profiles out to 2-6 Mpc from the group
centers with the caustic technique. No other mass estimation methods are
currently available at such large radii for these low-mass groups, because the
virial estimate requires dynamical equilibrium and the gravitational lensing
signal is too weak. Despite the small masses of these groups, most display
recognizable infall patterns. We use caustic and virial mass estimates to
measure the scaling relations between different observables, extending these
relations to smaller fluxes and luminosities than many previous surveys. Close
inspection reveals that three of the groups are subclusters in the outskirts of
larger clusters. A fourth group is apparently undergoing a group-group merger.
These four merging groups represent the most extreme outliers in the scaling
relations. Excluding these groups, we find ,
consistent with previous determinations for both clusters and groups.
Understanding cluster and group scaling relations is crucial for measuring
cosmological parameters from clusters.Comment: published in AJ Feb 2010, significantly revised in response to
referee report, title edite
Measuring the escape velocity and mass profiles of galaxy clusters beyond their virial radius
The caustic technique uses galaxy redshifts alone to measure the escape
velocity and mass profiles of galaxy clusters to clustrocentric distances well
beyond the virial radius, where dynamical equilibrium does not necessarily
hold. We provide a detailed description of this technique and analyse its
possible systematic errors. We apply the caustic technique to clusters with
mass M_200>=10^{14}h^{-1} M_sun extracted from a cosmological hydrodynamic
simulation of a LambdaCDM universe. With a few tens of redshifts per squared
comoving megaparsec within the cluster, the caustic technique, on average,
recovers the profile of the escape velocity from the cluster with better than
10 percent accuracy up to r~4 r_200. The caustic technique also recovers the
mass profile with better than 10 percent accuracy in the range (0.6-4) r_200,
but it overestimates the mass up to 70 percent at smaller radii. This
overestimate is a consequence of neglecting the radial dependence of the
filling function F_beta(r). The 1-sigma uncertainty on individual escape
velocity profiles increases from ~20 to ~50 percent when the radius increases
from r~0.1 r_200 to ~4 r_200. Individual mass profiles have 1-sigma uncertainty
between 40 and 80 percent within the radial range (0.6-4) r_200. We show that
the amplitude of these uncertainties is completely due to the assumption of
spherical symmetry, which is difficult to drop. Alternatively, we can apply the
technique to synthetic clusters obtained by stacking individual clusters: in
this case, the 1-sigma uncertainty on the escape velocity profile is smaller
than 20 percent out to 4 r_200. The caustic technique thus provides reliable
average profiles which extend to regions difficult or impossible to probe with
other techniques.Comment: MNRAS accepted, 20 page
An Extremely Massive Dry Galaxy Merger in a Moderate Redshift Cluster
We have identified perhaps the largest major galaxy merger ever seen. While
analysing Spitzer IRAC images of CL0958+4702, an X-ray selected cluster at
z=0.39, we discovered an unusual plume of stars extending 110 kpc
outward from the bright central galaxy (BCG). Three galaxies 1-1.5 mag fainter
than the BCG lie within 17 kpc (projected) of the BCG and are probably
participating in the merger. The plume is detected in all four IRAC channels
and at optical wavelengths in images from the WIYN telescope; the surface
brightness is remarkably high (24.8 mag arcsec at 50 kpc).
The optical and infrared colors are consistent with those of other BCGs,
suggesting that the plume is composed of old stars and negligible recent star
formation (hence a "dry merger"). The luminosity in the plume is at least
equivalent to a 4L^* galaxy. A diffuse halo extending 110 kpc from the BCG in
one IRAC image suggests the total amount of diffuse light is L_r\sim
1.3x10^{11}h^{-2} L_sun. A Chandra observation shows an X-ray image and
spectrum typical of moderate-mass clusters. We use MMT/Hectospec to measure 905
redshifts in a 1 deg^2 region around the cluster. The velocities of two of the
BCG companions indicate a merger timescale for the companion galaxies of
110 Myr and 0.5-1 Gyr for the plume. We conclude that the BCG and
intracluster light of CL0958 is formed by major mergers at moderate redshifts.
After the major merger is complete, CL0958 will likely become a fossil cluster.Comment: 5 pages, 2 figures, to appear in ApJ
Mapping the Universe: The 2010 Russell Lecture
Redshift surveys are a powerful tool of modern cosmology. We discuss two
aspects of their power to map the distribution of mass and light in the
universe: (1) measuring the mass distribution extending into the infall regions
of rich clusters and (2) applying deep redshift surveys to the selection of
clusters of galaxies and to the identification of very large structures (Great
Walls). We preview the HectoMAP project, a redshift survey with median redshift
z = 0.34 covering 50 square degrees to r= 21. We emphasize the importance and
power of spectroscopy for exploring and understanding the nature and evolution
of structure in the universe.Comment: 19 pages, 5 figures (2 videos available in the on-line journal
article
MOSFETs with Variable Gate Oxide thickness by Selective Nitrogen Ion Implantation
The incorporation of nitrogen in silicon has been shown to retard the oxidation growth rate. The present study produced aluminum gate PMOSFETs with varied gate oxide thickness on the same chip through selective nitrogen ion implantation. The nitrogen implant dose of 4x1014 ions/cm2 at 35 keV prior to gate oxide growth reduced the oxidation rate between 10% and 60% at the oxidation schedules employed. This active area N-implant led to no degradation in electrical parameters such as gate delay, mobility, or subthreshold swing. MOSFETs with different gate oxide thicknesses allow for different threshold voltages on the same chip and increased nonminimum channel length MOSFET reliability
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