420 research outputs found
Star Formation in a Cosmological Simulation of Reionization
We study the luminosity functions of high-redshift galaxies in detailed
hydrodynamic simulations of cosmic reionization, which are designed to
reproduce the evolution of the Lyman-alpha forest between z=5 and z=6. We find
that the luminosity functions and total stellar mass densities are in agreement
with observations when plausible assumptions about reddenning at z=6 are made.
Our simulations support the conclusion that stars alone reionized the universe.Comment: Accepted for publication in Ap
Towards a complete accounting of energy and momentum from stellar feedback in galaxy formation simulations
Stellar feedback plays a key role in galaxy formation by regulating star
formation, driving interstellar turbulence and generating galactic scale
outflows. Although modern simulations of galaxy formation can resolve scales of
10-100 pc, star formation and feedback operate on smaller, "subgrid" scales.
Great care should therefore be taken in order to properly account for the
effect of feedback on global galaxy evolution. We investigate the momentum and
energy budget of feedback during different stages of stellar evolution, and
study its impact on the interstellar medium using simulations of local star
forming regions and galactic disks at the resolution affordable in modern
cosmological zoom-in simulations. In particular, we present a novel subgrid
model for the momentum injection due to radiation pressure and stellar winds
from massive stars during early, pre-supernova evolutionary stages of young
star clusters. Early injection of momentum acts to clear out dense gas in star
forming regions, hence limiting star formation. The reduced gas density
mitigates radiative losses of thermal feedback energy from subsequent supernova
explosions, leading to an increased overall efficiency of stellar feedback. The
detailed impact of stellar feedback depends sensitively on the implementation
and choice of parameters. Somewhat encouragingly, we find that implementations
in which feedback is efficient lead to approximate self-regulation of global
star formation efficiency. We compare simulation results using our feedback
implementation to other phenomenological feedback methods, where thermal
feedback energy is allowed to dissipate over time scales longer than the formal
gas cooling time. We find that simulations with maximal momentum injection
suppress star formation to a similar degree as is found in simulations adopting
adiabatic thermal feedback.Comment: ApJ submitted. For a high-resolution version of the paper, see
http://kicp.uchicago.edu/~agertz
Gamma-Ray Burst Phenomenon as Collapse of QED Magnetized Vacuum Bubble: Analogy with Sonoluminescence
We consider the phenomenon of a gamma-ray burst as a nonlinear collapse of a
magnetic cavity surrounding a neutron star with very strong magnetic field B =
10^15 - 10^16 G due to the process of the bubble shape instability in a
resonant MHD field of the accreting plasma. The QED effect of vacuum
polarizability by the strong magnetic field is taken into account. We develop
an analogy with the phenomenon of sonoluminescence (SL) when the gas bubble is
located in the surrounding liquid with a driven sound intensity. We show that
this analogy between GRB and SL phenomena really exists.Comment: 14 pages, submitted to Natur
Dynamics of ``Small Galaxies'' in the Hubble Deep Field
We have previously found in the Hubble Deep Field a significant angular
correlation of faint, high color-redshift objects on scales below one
arcsecond, or several kiloparsecs in metric size. We examine the correlation
and nearest neighbor statistics to conclude that 38% of these objects in the
HDF have a companion within one arcsecond, three times the number expected in a
random distribution with the same number of objects. We examine three dynamical
scenarios for these object multiplets: 1) the objects are star-forming regions
within normal galaxies, whose disks have been relatively dimmed by K-correction
and surface brightness dimming; 2) they are fragments merging into large
galaxies; 3) they are satellites accreting onto normal L_* galaxies. We find
that hypothesis 1 is most tenable. First, large galaxies in the process of a
merger formation would have accumulated too much mass in their centers (5e12
M_sun inside 2 kpc) to correspond to present day objects. Second, accretion by
dynamical friction occurs with a predictable density vs. radius slope, not seen
among the faint HDF objects. Since the dynamical friction time is roughly (1
Gyr), a steady-state should have been reached by redshift z < 5. Star-forming
regions within galaxies clearly present no dynamical problems. Since large
spirals would still appear as such in the HDF, we favor a scenario in which the
faint compact sources in the HDF are giant starforming regions within small
normal galaxies, such as Magellanic irregulars. Finally we checked that
reduction in mass-to-light from induced star-formation cannot alone explain the
luminosity overdensity.Comment: AASTeX 4.0 (preprint), 4 PostScript figure
Polarization of AGN in UV Spectral Range
We present the review of some new problems in cosmology and physics of stars
in connection with future launching of WSO. We discuss three problems. UV
observations of distant z > 6 quasars allow to obtain information on the soft <
1 KeV X-ray radiation of the accretion disk around a supermassive black hole
because of its cosmological redshift. Really the region of X-ray radiation is
insufficiently investigated because of high galactic absorption. In a result
one will get important information on the reionization zone of the Universe.
Astronomers from ESO revealed the effect of alignment of electric vectors of
polarized QSOs. One of the probable mechanism of such alignment is the
conversion of QSO radiation into low mass pseudoscalar particles (axions) in
the extragalactic magnetic field. These boson like particles have been
predicted by new SUSY particle physics theory. Since the probability of such
conversion is increasing namely in UV spectral range one can expect the strong
correlation between UV spectral energy distribution of QSO radiation and
polarimetric data in the optical range. In the stellar physics one of the
interesting problems is the origin of the X-ray sources with super Eddington
luminosities. The results of UV observations of these X-ray sources will allow
to find the origin of these sources as accreting intermediate mass black holes.Comment: 6 pages, 3 figure
Morphology of the Secondary CMB Anisotropies: the Key to "Smoldering" Reionization
We show how the morphological analysis of the maps of the secondary CMB
anisotropies can detect an extended period of ``smoldering'' reionization,
during which the universe remains partially ionized. Neither radio observations
of the redshifted 21cm line nor IR observations of the redshifted Lyman-alpha
forest will be able to detect such a period. The most sensitive to this kind of
non-gaussianity parameters are the number of regions in the excursion set, the
perimeter of the excursion set, and the genus of the largest (by area) region.
For example, if the universe reionized fully at z=6, but maintained about 1/3
mean ionized fraction since z=20, then a 2 arcmin map with 500x500 pixel
resolution and a signal-to-noise ratio S/N=1/2 allows to detect the
non-gaussianity due to reionization with better than 99% confidence level.Comment: submitted to MNRA
Dependence of the Inner DM Profile on the Halo Mass
I compare the density profile of dark matter (DM) halos in cold dark matter
(CDM) N-body simulations with 1 Mpc, 32 Mpc, 256 Mpc and 1024 Mpc box sizes. In
dimensionless units the simulations differ only for the initial power spectrum
of density perturbations. I compare the profiles when the most massive halos
are composed of about 10^5 DM particles. The DM density profiles of the halos
in the 1 Mpc box show systematically shallower cores with respect to the
corresponding halos in the 32 Mpc simulation that have masses, M_{dm}, typical
of the Milky Way and are fitted by a NFW profile. The DM density profiles of
the halos in the 256 Mpc box are consistent with having steeper cores than the
corresponding halos in the 32 Mpc simulation, but higher mass resolution
simulations are needed to strengthen this result. Combined, these results
indicate that the density profile of DM halos is not universal, presenting
shallower cores in dwarf galaxies and steeper cores in clusters. Physically the
result sustains the hypothesis that the mass function of the accreting
satellites determines the inner slope of the DM profile. In comoving
coordinates, r, the profile \rho_{dm} \propto 1/(X^\alpha(1+X)^{3-\alpha}),
with X=c_\Delta r/r_\Delta, r_\Delta is the virial radius and \alpha
=\alpha(M_{dm}), provides a good fit to all the DM halos from dwarf galaxies to
clusters at any redshift with the same concentration parameter c_\Delta ~ 7.
The slope, \gamma, of the outer parts of the halo appears to depend on the
acceleration of the universe: when the scale parameter is a=(1+z)^{-1} < 1, the
slope is \gamma ~ 3 as in the NFW profile, but \gamma ~ 4 at a > 1 when
\Omega_\Lambda ~ 1 and the universe is inflating.[abridged]Comment: Accepted for publication in MNRAS. 13 pages, including 11 figures and
2 tables. The revised version has an additional discussion section and work
on the velocity dispersion anisotrop
Mutual Constraints Between Reionization Models and Parameter Extraction From Cosmic Microwave Background Data
Spectroscopic studies of high-redshift objects and increasingly precise data
on the cosmic microwave background (CMB) are beginning to independently place
strong complementary bounds on the epoch of hydrogen reionization. Parameter
estimation from current CMB data continues, however, to be subject to several
degeneracies. Here, we focus on those degeneracies in CMB parameter forecasts
related to the optical depth to reionization. We extend earlier work on the
mutual constraints that such analyses of CMB data and a reionization model may
place on each other to a more general parameter set, and to the case of data
anticipated from the satellite. We focus in particular on a semi-analytic
model of reionization by the first stars, although the methods here are easily
extended to other reionization scenarios. A reionization model can provide
useful complementary information for cosmological parameter (CP) extraction
from the CMB, particularly for the degeneracies between the optical depth and
either of the amplitude and index of the primordial scalar power spectrum,
which are still present in the most recent data. Alternatively, by using a
reionization model, known limits on astrophysical quantities can reduce the
forecasted errors on CPs. Forthcoming CMB data also have the potential to
constrain the sites of early star formation, as well as the fraction of baryons
that participate in it. Finally, we examine the implications of an independent,
e.g., spectroscopic, determination of the epoch of reionization for the
determination of CPs from the CMB. This has the potential to significantly
strengthen limits from the CMB on parameters such as the index of the power
spectrum, while having the considerable advantage of being free of the choice
of the reionization model (abridged).Comment: Accepted on 15 Feb. 2002 for publication in ApJ, 22 pages with 4
figures; paper's text expanded significantly, new Discussion section, results
(table and figures) unchanged from version
Atmospheres and Spectra of Strongly Magnetized Neutron Stars -- III. Partially Ionized Hydrogen Models
We construct partially ionized hydrogen atmosphere models for magnetized
neutron stars in radiative equilibrium with surface fields B=10^12-5 \times
10^14 G and effective temperatures T_eff \sim a few \times 10^5-10^6 K. These
models are based on the latest equation of state and opacity results for
magnetized, partially ionized hydrogen plasmas that take into account various
magnetic and dense medium effects. The atmospheres directly determine the
characteristics of thermal emission from isolated neutron stars. For the models
with B=10^12-10^13 G, the spectral features due to neutral atoms lie at extreme
UV and very soft X-ray energy bands and therefore are difficult to observe.
However, the continuum flux is also different from the fully ionized case,
especially at lower energies. For the superstrong field models (B\ga 10^14 G),
we show that the vacuum polarization effect not only suppresses the proton
cyclotron line as shown previously, but also suppresses spectral features due
to bound species; therefore spectral lines or features in thermal radiation are
more difficult to observe when the neutron star magnetic field is \ga 10^14 G.Comment: 12 pages, 10 figures; ApJ, accepted (v599: Dec 20, 2003
Superfluid turbulence and pulsar glitch statistics
Experimental evidence is reviewed for the existence of superfluid turbulence
in a differentially rotating, spherical shell at high Reynolds numbers
(\Rey\gsim 10^3), such as the outer core of a neutron star. It is shown that
torque variability increases with \Rey, suggesting that glitch activity in
radio pulsars may be a function of \Rey as well. The \Rey distribution of
the 67 glitching radio pulsars with characteristic ages {\rm
yr} is constructed from radio timing data and cooling curves and compared with
the \Rey distribution of all 348 known pulsars with {\rm
yr}. The two distributions are different, with a Kolmogorov-Smirnov probability
. The conclusion holds for (modified) Urca and
nonstandard cooling, and for Newtonian and superfluid viscosities
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