281,364 research outputs found
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
Searching for Standard Clocks in the Primordial Universe
Classically oscillating massive fields can be used as "standard clocks" in
the primordial universe. They generate features in primordial density
perturbations that directly record the scale factor evolution a(t). Detecting
and measuring these "fingerprint" signals is challenging but would provide a
direct evidence for a specific primordial universe paradigm. In this paper,
such a search is performed for the power spectrum of the Cosmic Microwave
Background (CMB) anisotropies using the WMAP7 data. Although a good fit to the
data privileges a scale around k=0.01 Mpc^(-1), we do not find statistical
significance for, neither against, the presence of any feature. We then
forecast the expected constraints a Planck-like CMB experiment can impose on
the fingerprint parameters by using Markov-Chain-Monte-Carlo (MCMC) methods on
mock data. We exhibit a high sensitivity zone for wavenumbers ranging from 0.01
Mpc^(-1) to 0.1 Mpc^(-1) in which fingerprints show up first on the posterior
probability distribution of the wavenumber at which they occur, and then on the
modulation frequency. Within the sensitivity zone, we show that the
inflationary paradigm can be inferred from a single feature generating at least
a 20% modulation of the primordial power spectrum. This minimal value
sensitively depends on the modulation frequency.Comment: 22 pages, 13 figures, uses jcappub. References added, matches
published versio
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