2,492 research outputs found
Cosmic rays, lithium abundance and excess entropy in galaxy clusters
We consider the production of Li in spallation reactions by cosmic rays
in order to explain the observed abundance in halo metal-poor stars. We show
that heating of ambient gas by cosmic rays is an inevitable consequence of this
process, and estimate the energy input required to reproduce the observed
abundance of Li/H to be of order a few hundred eV per
particle. We draw attention to the possibility that this could explain the
excess entropy in gas in galaxy groups and clusters. The evolution of Li
and the accompanying heating of gas is calculated for structures collapsing at
the present epoch with injection of cosmic rays at high redshift. We determine
the energy required to explain the abundance of Li at
corresponding to the formation epoch of halo metal-poor stars, and also an
increased entropy level of keV cm necessary to explain X-ray
observations of clusters. The energy budget for this process is consistent with
the expected energy output of radio-loud AGNs, and the diffusion length scale
of cosmic-ray protons responsible for heating is comparable to the size of
regions with excess entropy. We also discuss the constraints imposed by the
extragalactic gamma-ray background.Comment: 5 pages, 1 Figure, Accepted for publication in MNRAS (Letters
A late-time transition in the equation of state versus Lambda-CDM
We study a model of the dark energy which exhibits a rapid change in its
equation of state w(z), such as occurs in vacuum metamorphosis. We compare the
model predictions with CMB, large scale structure and supernova data and show
that a late-time transition is marginally preferred over standard Lambda-CDM.Comment: 4 pages, 1 figure, to appear in the proceedings of XXXVIIth
Rencontres de Moriond, "The Cosmological Model", March 200
Probing the circumgalactic baryons through cross-correlations
We study the cross-correlation of distribution of galaxies, the
Sunyaev-Zel'dovich (SZ) and X-ray power spectra of galaxies from current and
upcoming surveys and show these to be excellent probes of the nature, i.e.
extent, evolution and energetics, of the circumgalactic medium (CGM). The
SZ-galaxy cross-power spectrum, especially at large multipoles, depends on the
steepness of the pressure profile of the CGM. This property of the SZ signal
can, thus, be used to constrain the pressure profile of the CGM. The X-ray
cross power spectrum also has a similar shape. However, it is much more
sensitive to the underlying density profile. We forecast the detectability of
the cross-correlated galaxy distribution, SZ and X-ray signals by combining
South Pole Telescope-Dark Energy Survey (SPT-DES) and eROSITA-DES/eROSITA-LSST
(extended ROentgen Survey with an Imaging Telescope Array-Large Synoptic Survey
Telescope) surveys, respectively. We find that, for the SPT-DES survey, the
signal-to-noise ratio (SNR) peaks at high mass and redshift with SNR
around and for flat
density and temperature profiles. The SNR peaks at for the
eROSITA-DES (eROSITA-LSST) surveys. We also perform a Fisher matrix analysis to
find the constraint on the gas fraction in the CGM in the presence or absence
of an unknown redshift evolution of the gas fraction. Finally, we demonstrate
that the cross-correlated SZ-galaxy and X-ray-galaxy power spectrum can be used
as powerful probes of the CGM energetics and potentially discriminate between
different feedback models recently proposed in the literature; for example, one
can distinguish a `no active galactic nuclei feedback' scenario from a CGM
energized by `fixed-velocity hot winds' at greater than .Comment: 14 pages, 10 figures, 4 tables, accepted for publication in MNRA
Heating of the intergalactic medium due to structure formation
We estimate the heating of the intergalactic medium due to shocks arising
from structure formation. Heating of the gas outside the collapsed regions,
with small overdensities () is considered here,
with the aid of Zel'dovich approximation. We estimate the equation of state of
this gas, relating the density with its temperature, and its evolution in time,
considering the shock heating due to one- density peaks as being the
most dominant. We also estimate the mass fraction of gas above a given
temperature as a function of redshift. We find that the baryon fraction above
K at is . We estimate the integrated Sunyaev-Zel'dovich
distortion from this gas at present epoch to be of order .Comment: 5 pages (3 figs), To appear in MNRAS (pink pages
Reionization Constraints on the Contribution of Primordial Compact Objects to Dark Matter
Many lines of evidence suggest that nonbaryonic dark matter constitutes
roughly 30% of the critical closure density, but the composition of this dark
matter is unknown. One class of candidates for the dark matter is compact
objects formed in the early universe, with typical masses M between 0.1 and 1
solar masses to correspond to the mass scale of objects found with microlensing
observing projects. Specific candidates of this type include black holes formed
at the epoch of the QCD phase transition, quark stars, and boson stars. Here we
show that accretion onto these objects produces substantial ionization in the
early universe, with an optical depth to Thomson scattering out to z=1100 of
approximately tau=2-4 [f_CO\epsilon_{-1}(M/Msun)]^{1/2} (H_0/65)^{-1}, where
\epsilon_{-1} is the accretion efficiency \epsilon\equiv L/{\dot M}c^2 divided
by 0.1 and f_CO is the fraction of matter in the compact objects. The current
upper limit to the scattering optical depth, based on the anisotropy of the
microwave background, is approximately 0.4. Therefore, if accretion onto these
objects is relatively efficient, they cannot be the main component of
nonbaryonic dark matter.Comment: 12 pages including one figure, uses aaspp4, submitted to Ap
Primordial black holes as generators of cosmic structures
Primordial black holes (PBHs) could provide the dark matter in various mass
windows below and those of might explain the
LIGO events. PBHs much larger than this might have important consequences even
if they provide only a small fraction of the dark matter. In particular, they
could generate cosmological structure either individually through through the
`seed' effect or collectively through the `Poisson' effect, thereby alleviating
some problems associated with the standard CDM scenario. If the PBHs all have a
similar mass and make a small contribution to the dark matter, then the seed
effect dominates on small scales, in which case PBHs could seed the
supermassive black holes in galactic nuclei or even galaxies themselves. If
they have a similar mass and provide the dark matter, the Poisson effect
dominates on all scales and the first bound clouds would form earlier than in
the usual scenario, with interesting observational consequences. If the PBHs
have an extended mass spectrum, which is more likely, they could fulfill all
three roles - providing the dark matter, binding the first bound clouds and
generating galaxies. In this case, the galactic mass function naturally has the
observed form, with the galaxy mass being simply related to the black hole
mass. The stochastic gravitational wave background from the PBHs in this
scenario would extend continuously from the LIGO frequency to the LISA
frequency, offering a potential goal for future surveys.Comment: 48 pages, 3 figures, accepted for publication in Monthly Notices of
Royal Astronomical Societ
A compressed cloud in the Vela supernova remnant
To elucidate the nature of the interstellar medium in the vicinity of the Vela supernova remnants (SNR) an extensive study with the International Ultraviolet Explorer of interstellar absorption lines toward 35 stars in the vicinity of the Vela SNR was undertaken. Observations of interstellar absorption, in particular of CI, towards one of these stars, HD 72350 (type B4 III), is of particular interest
Self-Regulated Growth of Supermassive Black Holes in Galaxies as the Origin of the Optical and X-ray Luminosity Functions of Quasars
We postulate that supermassive black-holes grow in the centers of galaxies
until they unbind the galactic gas that feeds them. We show that the
corresponding self-regulation condition yields a correlation between black-hole
mass (Mbh) and galaxy velocity dispersion (sigma) as inferred in the local
universe, and recovers the observed optical and X-ray luminosity functions of
quasars at redshifts up to z~6 based on the hierarchical evolution of galaxy
halos in a Lambda-CDM cosmology. With only one free parameter and a simple
algorithm, our model yields the observed evolution in the number density of
optically bright or X-ray faint quasars between 2<z<6 across 3 orders of
magnitude in bolometric luminosity and 3 orders of magnitude in comoving
density per logarithm of luminosity. The self-regulation condition identifies
the dynamical time of galactic disks during the epoch of peak quasar activity
(z~2.5) as the origin of the inferred characteristic quasar lifetime of ~10
million years. Since the lifetime becomes comparable to the Salpeter e-folding
time at this epoch, the model also implies that the Mbh-sigma relation is a
product of feedback regulated accretion during the peak of quasar activity. The
mass-density in black-holes accreted by that time is consistent with the local
black-hole mass density of ~(0.8-6.3) times 10^5 solar masses per cubic Mpc,
which we have computed by combining the Mbh-sigma relation with the measured
velocity dispersion function of SDSS galaxies (Sheth et al.~2003). Applying a
similar self-regulation principle to supernova-driven winds from starbursts, we
find that the ratio between the black hole mass and the stellar mass of
galactic spheroids increases with redshift as (1+z)^1.5 although the Mbh-sigma
relation is redshift-independent.Comment: 10 pages, 5 figures, submitted to Ap
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