838 research outputs found
A measurement of large-scale peculiar velocities of clusters of galaxies: results and cosmological implications
Peculiar velocities of clusters of galaxies can be measured by studying the
fluctuations in the cosmic microwave background (CMB) generated by the
scattering of the microwave photons by the hot X-ray emitting gas inside
clusters. While for individual clusters such measurements result in large
errors, a large statistical sample of clusters allows one to study cumulative
quantities dominated by the overall bulk flow of the sample with the
statistical errors integrating down. We present results from such a measurement
using the largest all-sky X-ray cluster catalog combined to date and the 3-year
WMAP CMB data. We find a strong and coherent bulk flow on scales out to at
least > 300 h^{-1} Mpc, the limit of our catalog. This flow is difficult to
explain by gravitational evolution within the framework of the concordance LCDM
model and may be indicative of the tilt exerted across the entire current
horizon by far-away pre-inflationary inhomogeneities.Comment: Ap.J. (Letters), in press. 20 Oct issue (Vol. 686
Luminosity evolution in gamma-ray bursts
Wee stimatetheluminosit y evolution and formation ratefor over a 900 BATSE GRBs by using redshift and luminosity data calculated by Band, Norris, and Bonnell (ApJ, 613 (2004) 484B) via thelag-luminosit y correlation. By applying maximum likelihood techniques, we are able to infer the true distribution of the parent GRB population’s luminosity function and density distributions in a way that accounts for detector selection effects. We find that after accounting for data truncation, there still exists a significant correlation between the average luminosity
and redshift, indicating that distant GRBs are on average more luminous than nearby counterparts. This is consistent with previous studies showing strong source evolution and also recent observations of under luminous nearby GRBs. We find no evidence for beaming angle evolution in the current sample of GRBs with known redshift, suggesting that this increase in luminosity cannot be due to an evolution of the collimation of gamma-ray emission. The resulting luminosity function is well fit with a singlep ower law of index L
�−1.5, which is intermediate between the values predicted by the k = 2 power law and Gaussian structured jet models. We also find that the GRB comoving rate density rises steeply with a broad peak between 1 3. This rate density qualitatively
matches the current estimates of the star formation rate at high redshifts, favoring a short lived massive star progenitor model, or a binary model with a short delay
between the formation of the compact object and the eventual merger
Do We Expect Most AGN to Live in Disks?
Recent observations have indicated that a large fraction of the low to
intermediate luminosity AGN population lives in disk-dominated hosts, while the
more luminous quasars live in bulge-dominated hosts, in conflict with some
previous model predictions. We therefore build and compare a semi-empirical
model for AGN fueling which accounts for both merger and non-merger
'triggering.' In particular, we show that the 'stochastic accretion' model - in
which fueling in disk galaxies is essentially a random process arising whenever
dense gas clouds reach the nucleus - provides a good match to the present
observations at low/intermediate luminosities. However it falls short of the
high-luminosity population. We combine this with models for major
merger-induced AGN fueling, which lead to rarer but more luminous events, and
predict the resulting abundance of disk-dominated and bulge-dominated AGN host
galaxies as a function of luminosity and redshift. We compile and compare
observational constraints from z~0-2. The models and observations generically
show a transition from disk to bulge dominance in hosts near the Seyfert-quasar
transition, at all redshifts. 'Stochastic' fueling dominates AGN by number
(dominant at low luminosity), and dominates BH growth below the knee in the
present-day BH mass function (<10^7 M_sun). However it accounts for just ~10%
of BH mass growth at masses >10^8 M_sun. In total, fueling in disky hosts
accounts for ~30% of the total AGN luminosity density/BH mass density. The
combined model also accurately predicts the AGN luminosity function and
clustering/bias as a function of luminosity and redshift; however, we argue
that these are not sensitive probes of BH fueling mechanisms.Comment: 13 pages, 5 figures, PDF updated to match published versio
A measurement of large-scale peculiar velocities of clusters of galaxies: technical details
This paper presents detailed analysis of large-scale peculiar motions derived
from a sample of ~ 700 X-ray clusters and cosmic microwave background (CMB)
data obtained with WMAP. We use the kinematic Sunyaev-Zeldovich (KSZ) effect
combining it into a cumulative statistic which preserves the bulk motion
component with the noise integrated down. Such statistic is the dipole of CMB
temperature fluctuations evaluated over the pixels of the cluster catalog
(Kashlinsky & Atrio-Barandela 2000). To remove the cosmological CMB
fluctuations the maps are Wiener-filtered in each of the eight WMAP channels
(Q, V, W) which have negligible foreground component. Our findings are as
follows: The thermal SZ (TSZ) component of the clusters is described well by
the Navarro-Frenk-White profile expected if the hot gas traces the dark matter
in the cluster potential wells. Such gas has X-ray temperature decreasing
rapidly towards the cluster outskirts, which we demonstrate results in the
decrease of the TSZ component as the aperture is increased to encompass the
cluster outskirts. We then detect a statistically significant dipole in the CMB
pixels at cluster positions. Arising exclusively at the cluster pixels this
dipole cannot originate from the foreground or instrument noise emissions and
must be produced by the CMB photons which interacted with the hot intracluster
gas via the SZ effect. The dipole remains as the monopole component, due to the
TSZ effect, vanishes within the small statistical noise out to the maximal
aperture where we still detect the TSZ component. We demonstrate with
simulations that the mask and cross-talk effects are small for our catalog and
contribute negligibly to the measurements. The measured dipole thus arises from
the KSZ effect produced by the coherent large scale bulk flow motion.Comment: Minor changes to match the published version - Ap.J., 1 Feb 2009
issu
A Correlation Between the Intrinsic Brightness and Average Decay Rate of Gamma-ray Burst X-ray Afterglow Light Curves
We present a correlation between the average temporal decay
({\alpha}X,avg,>200s) and early-time luminosity (LX,200s) of X-ray afterglows
of gamma-ray bursts as observed by Swift-XRT. Both quantities are measured
relative to a rest frame time of 200 s after the {\gamma}-ray trigger. The
luminosity average decay correlation does not depend on specific temporal
behavior and contains one scale independent quantity minimizing the role of
selection effects. This is a complementary correlation to that discovered by
Oates et al. (2012) in the optical light curves observed by Swift-UVOT. The
correlation indicates that on average, more luminous X-ray afterglows decay
faster than less luminous ones, indicating some relative mechanism for energy
dissipation. The X-ray and optical correlations are entirely consistent once
corrections are applied and contamination is removed. We explore the possible
biases introduced by different light curve morphologies and observational
selection effects, and how either geometrical effects or intrinsic properties
of the central engine and jet could explain the observed correlation.Comment: Accepted for Publication in ApJ; 16 pages, 15 figures, 2 table
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