13,808 research outputs found
Mining Circumgalactic Baryons in the Low-Redshift Universe
(Abridged) This paper presents an absorption-line study of the multiphase
circumgalactic medium (CGM) based on observations of Lya, CII, CIV, SiII,
SiIII, and SiIV absorption transitions in the vicinities of 195 galaxies at
redshift z<0.176. The galaxy sample is established based on a cross-comparison
between public galaxy and QSO survey data and is characterized by a median
redshift of =0.041, a median projected distance of =362 kpc to the
sightline of the background QSO, and a median stellar mass of log(M_star/M_sun)
= 9.7 \pm 1.1. Comparing the absorber features identified in the QSO apectra
with known galaxy properties has led to strong constraints for the CGM
absorption properties at z<~0.176. First, abundant hydrogen gas is observed out
to d~500 kpc, well beyond the dark matter halo radius Rh of individual
galaxies, with a mean covering fraction of ~60%. In contrast, no heavy elements
are detected at d>~0.7 Rh from either low-mass dwarfs or high-mass galaxies.
The lack of detected heavy elements in low- and high-ionization states suggests
that either there exists a chemical enrichment edge at d~0.7 Rh or gaseous
clumps giving rise to the observed absorption lines cannot survive at these
large distances. Considering all galaxies at d>Rh leads to a strict upper limit
for the covering fraction of heavy elements of ~3% (at a 95% confidence level)
over d=(1-9) Rh. At d<Rh, differential covering fraction between low- and
high-ionization gas is observed, suggesting that the CGM becomes progressively
more ionized from d<0.3 Rh to larger distances. Comparing CGM absorption
observations at low and high redshifts shows that at a fixed-fraction of Rh the
CGM exhibits stronger mean absorption at z=2.2 than at z~0. We discuss possible
pseudo-evolution of the CGM as a result of misrepresentation of halo radius.Comment: 25 pages, 13 figures; accepted for publication in MNRA
A Spatial Structural Derivative Model for Ultraslow Diffusion
This study investigates the ultraslow diffusion by a spatial structural
derivative, in which the exponential function exp(x)is selected as the
structural function to construct the local structural derivative diffusion
equation model. The analytical solution of the diffusion equation is a form of
Biexponential distribution. Its corresponding mean squared displacement is
numerically calculated, and increases more slowly than the logarithmic function
of time. The local structural derivative diffusion equation with the structural
function exp(x)in space is an alternative physical and mathematical modeling
model to characterize a kind of ultraslow diffusion.Comment: 13 pages, 3 figure
Statistical Properties of Multiple Optical Emission Components in Gamma-Ray Bursts and Implications
Well-sampled optical lightcurves of 146 gamma-ray bursts (GRBs) are complied
from the literature. Multiple optical emission components are extracted with
power-law function fits to these lightcurves. We present a systematical
analysis for statistical properties and their relations to prompt gamma-ray
emission and X-ray afterglow for each component. We show that peak luminosity
in the prompt and late flares are correlated and the evolution of the peak
luminosity may signal the evolution of the accretion rate. No tight correlation
between the shallow decay phase/plateau and prompt gamma-ray emission is found.
Assuming that they are due to a long-lasting wind injected by a compact object,
we show that the injected behavior favors the scenarios of a long-lasting wind
after the main burst episode. The peak luminosity of the afterglow onset is
tightly correlated with Eiso, and it is dimmer as peaking later. Assuming that
the onset bump is due to the fireball deceleration by the external medium, we
examine the Gamma_0-Eiso relation and find that it is confirmed with the
current sample. Optical re-brightening is observed in 30 GRBs in our sample. It
shares the same relation between the width and the peak time as found in the
onset bump, but no clear correlation between the peak luminosity and Eiso as
observed in the onset bumps is found. Although its peak luminosity also decays
with time, the slope is much shallower than that of the onset peak. We get L
t^{-1}_{p}$, being consistent with off-axis observations to an expanding
external fireball in a wind-like circum medium. The late re-brightening may
signal another jet component. Mixing of different emission components may be
the reason for the observed chromatic breaks in different energy bands.Comment: 10 pages, 5 figures, to be published by IJMPD (Proceedings of "The
Third Galileo - Xu Guangqi meeting", Beijing, October 11-15, 2011
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