2,030 research outputs found
Angular Sizes of Faint Field Disk Galaxies: Intrinsic Luminosity Evolution
In order to explain the small scale-lengths detected in the recent deep field
observations performed from large ground-based telescopes and from the Hubble
Space Telescope, we investigate the predictions at high redshifts for disk
galaxies that formed by infall. Changes with redshift in the observed
properties of field galaxies are directly related to the evolution of the disks
and of the stellar populations. We see that changes in the rest frame
luminosity of a galaxy induce smaller values of half-light radii than are
predicted assuming no evolution. Comparisons are presented with two observed
samples from Mutz et al. (1994) and Smail et al. (1995).Comment: plain tex file + 3 postscript figures. To be published in ApJ
The Influences of Outflow on the Dynamics of Inflow
Both numerical simulations and observations indicate that in an
advection-dominated accretion flow most of the accretion material supplied at
the outer boundary will not reach the inner boundary. Rather, they are lost via
outflow. Previously, the influence of outflow on the dynamics of inflow is
taken into account only by adopting a radius-dependent mass accretion rate
with . In this paper, based on a 1.5
dimensional description to the accretion flow, we investigate this problem in
more detail by considering the interchange of mass, radial and azimuthal
momentum, and the energy between the outflow and inflow. The physical
quantities of the outflow is parameterized based on our current understandings
to the properties of outflow mainly from numerical simulations of accretion
flows. Our results indicate that under reasonable assumptions to the properties
of outflow, the main influence of outflow has been properly included by
adopting .Comment: 16 pages, 5 figures. accepted for publication in Ap
Secondary CMB anisotropies in a universe reionized in patches
In a universe reionized in patches, the Doppler effect from Thomson
scattering off free electrons generates secondary cosmic microwave background
(CMB) anisotropies. For a simple model with small patches and late
reionization, we analytically calculate the anisotropy power spectrum. Patchy
reionization can, in principle, be the main source of anisotropies on arcminute
scales. On larger angular scales, its contribution to the CMB power spectrum is
a small fraction of the primary signal and is only barely detectable in the
power spectrum with even an ideal, i.e. cosmic variance limited, experiment and
an extreme model of reionization. Consequently patchy reionization is unlikely
to affect cosmological parameter estimation from the acoustic peaks in the CMB.
Its detection on small angles would help determine the ionization history of
the universe, in particular the typical size of the ionized region and the
duration of the reionization process.Comment: 7 pages, 2 figures, submitted to Ap
Can we avoid dark energy?
The idea that we live near the centre of a large, nonlinear void has
attracted attention recently as an alternative to dark energy or modified
gravity. We show that an appropriate void profile can fit both the latest
cosmic microwave background and supernova data. However, this requires either a
fine-tuned primordial spectrum or a Hubble rate so low as to rule these models
out. We also show that measurements of the radial baryon acoustic scale can
provide very strong constraints. Our results present a serious challenge to
void models of acceleration.Comment: 5 pages, 4 figures; minor changes; version published in Phys. Rev.
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Large Scale Structure Forecast Constraints on Particle Production During Inflation
Bursts of particle production during inflation provide a well-motivated
mechanism for creating bump like features in the primordial power spectrum.
Current data constrains these features to be less than about 5% the size of the
featureless primordial power spectrum at wavenumbers of about 0.1 h Mpc^{-1}.
We forecast that the Planck cosmic microwave background experiment will be able
to strengthen this constraint to the 0.5% level. We also predict that adding
data from a square kilometer array (SKA) galaxy redshift survey would improve
the constraint to about the 0.1% level. For features at larger wave-numbers,
Planck will be limited by Silk damping and foregrounds. While, SKA will be
limited by non-linear effects. We forecast for a Cosmic Inflation Probe (CIP)
galaxy redshift survey, similar constraints can be achieved up to about a
wavenumber of 1 h Mpc^{-1}.Comment: 10 pages. Matches PRD accepted versio
Maximum black-hole spin from quasi-circular binary mergers
Black holes of mass M must have a spin angular momentum S below the Kerr
limit chi = S/M^2 < 1, but whether astrophysical black holes can attain this
limiting spin depends on their accretion history. Gas accretion from a thin
disk limits the black-hole spin to chi_gas < 0.9980 +- 0.0002, as
electromagnetic radiation from this disk with retrograde angular momentum is
preferentially absorbed by the black hole. Extrapolation of
numerical-relativity simulations of equal-mass binary black-hole mergers to
maximum initial spins suggests these mergers yield a maximum spin chi_eq <
0.95. Here we show that for smaller mass ratios q = m/M << 1, the superradiant
extraction of angular momentum from the larger black hole imposes a fundamental
limit chi_lim < 0.9979 +- 0.0001 on the final black-hole spin even in the
test-particle limit q -> 0 of binary black-hole mergers. The nearly equal
values of chi_gas and chi_lim imply that measurement of supermassive black-hole
spins cannot distinguish a black hole built by gas accretion from one assembled
by the gravitational inspiral of a disk of compact stellar remnants. We also
show how superradiant scattering alters the mass and spin predicted by models
derived from extrapolating test-particle mergers to finite mass ratios.Comment: final version accepted in PRD, new Fig.4 and discussio
Star Captures by Quasar Accretion Disks: A Possible Explanation of the M-sigma Relation
A new theory of quasars is presented in which the matter of thin accretion
disks around black holes is supplied by stars that plunge through the disk.
Stars in the central part of the host galaxy are randomly perturbed to highly
radial orbits, and as they repeatedly cross the disk they lose orbital energy
by drag, eventually merging into the disk. Requiring the rate of stellar mass
capture to equal the mass accretion rate into the black hole, a relation
between the black hole mass and the stellar velocity dispersion is predicted of
the form M_{BH} \propto sigma_*^{30/7}. The normalization depends on various
uncertain parameters such as the disk viscosity, but is consistent with
observation for reasonable assumptions. We show that a seed central black hole
in a newly formed stellar system can grow at the Eddington rate up to this
predicted mass via stellar captures by the accretion disk. Once this mass is
reached, star captures are insufficient to maintain an Eddington accretion
rate, and the quasar may naturally turn off as the accretion switches to a
low-efficiency advection mode. The model provides a mechanism to deliver mass
to the accretion disk at small radius, probably solving the problem of
gravitational instability to star formation in the disk at large radius. We
note that the matter from stars that is incorporated to the disk has an average
specific angular momentum that is very small or opposite to that of the disk,
and discuss how a rotating disk may be maintained as it captures this matter if
a small fraction of the accreted mass comes from stellar winds that form a disk
extending to larger radius. We propose several observational tests and
consequences of this theory.Comment: submitted to Ap
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