3,405 research outputs found
Axisymmetric electrovacuum spacetimes with a translational Killing vector at null infinity
By using the Bondi-Sachs-van der Burg formalism we analyze the asymptotic
properties at null infinity of axisymmetric electrovacuum spacetimes with a
translational Killing vector and, in general, an infinite ``cosmic string''
(represented by a conical singularity) along the axis. Such spacetimes admit
only a local null infinity. There is a non-vanishing news function due to the
existence of the string even though there is no radiation.
We prove that if null infinity has a smooth compact cross section and the
spacetime is not flat in a neighbourhood of null infinity, then the
translational Killing vector must be timelike and the spacetime is stationary.
The other case in which an additional symmetry of axisymmetric spacetimes
admits compact cross sections of null infinity is the boost symmetry, which
leads to radiative spacetimes representing ``uniformly accelerated objects''.
These cases were analyzed in detail in our previous works. If the translational
Killing vector is spacelike or null, corresponding to cylindrical or plane
waves, some complete generators of null infinity are ``singular'' but null
infinity itself can be smooth apart from these generators.
As two explicit examples of local null infinity, Schwarzschild spacetime with
a string and a class of cylindrical waves with a string are discussed in detail
in the Appendix.Comment: 15 pages, RevTeX, submitted to Class. Quantum Gra
The Evolution of the M-sigma Relation
(Abridged) We examine the evolution of the black hole mass - stellar velocity
dispersion (M-sigma) relation over cosmic time using simulations of galaxy
mergers that include feedback from supermassive black hole growth. We consider
mergers of galaxies varying the properties of the progenitors to match those
expected at redshifts z=0-6. We find that the slope of the resulting M-sigma
relation is the same at all redshifts considered. For the same feedback
efficiency that reproduces the observed amplitude of the M-sigma relation at
z=0, there is a weak redshift-dependence to the normalization that results from
an increasing velocity dispersion for a given galactic stellar mass. We develop
a formalism to connect redshift evolution in the M-sigma relation to the
scatter in the local relation at z=0. We show that the scatter in the local
relation places severe constraints on the redshift evolution of both the
normalization and slope of the M-sigma relation. Furthermore, we demonstrate
that cosmic downsizing introduces a black hole mass-dependent dispersion in the
M-sigma relation and that the skewness of the distribution about the locally
observed M-sigma relation is sensitive to redshift evolution in the
normalization and slope. In principle, these various diagnostics provide a
method for differentiating between theories for producing the M-sigma relation.
In agreement with existing constraints, our simulations imply that hierarchical
structure formation should produce the relation with small intrinsic scatter.Comment: 12 pages, 6 figures, version accepted by Ap
Direct cosmological simulations of the growth of black holes and galaxies
We investigate the coupled formation and evolution of galaxies and their
embedded supermassive black holes using state-of-the-art hydrodynamic
simulations of cosmological structure formation. For the first time, we
self-consistently follow the dark matter dynamics, radiative gas cooling, star
formation, as well as black hole growth and associated feedback processes,
starting directly from initial conditions appropriate for the LambdaCDM
cosmology. Our modeling of the black hole physics is based on an approach we
have developed in simulations of isolated galaxy mergers. Here we examine: (i)
the predicted global history of black hole mass assembly (ii) the evolution of
the local black hole-host mass correlations and (iii) the conditions that allow
rapid growth of the first quasars, and the properties of their hosts and
descendants today. We find a total black hole mass density in good agreement
with observational estimates. The black hole accretion rate density peaks at
lower redshift and evolves more strongly at high redshift than the star
formation rate density, but the ratio of black hole to stellar mass densities
shows only a moderate evolution at low redshifts. We find strong correlations
between black hole masses and properties of the stellar systems, agreeing well
with the measured local M_BH-sigma and M_BH -M_* relationships, but also
suggesting (dependent on the mass range) a weak evolution with redshift in the
normalization and the slope. Our simulations also produce massive black holes
at high redshift, due to extended periods of exponential growth in regions that
collapse early and exhibit strong gas inflows. These first supermassive BH
systems however are not necessarily the most massive ones today, since they are
often overtaken in growth by quasars that form later. (abridged)Comment: 22 pages, 17 figures, submitted to Ap
The Extraordinary Abundances of QSO Broad Absorption Line Regions: A Matter of Novae?
The broad absorption lines (BALs) of QSOs indicate abundances of heavy
elements, relative to hydrogen, that are 1 to 2 orders of magnitude higher than
the solar values. In at least one QSO, an especially large enhancement of
phosphorus is observed. These abundances resemble those in Galactic novae, and
this suggests that novae may produce the BAL gas. The needed rate of nova
outbursts may come from single white dwarfs that accrete gas as they pass
through a supermassive accretion disk around a central black hole.Comment: 9 pages including 1 Postscript figure. Uses aaspp4.sty and
flushrt.sty. Uuencoded, gzipped tarfile. To appear in Astrophys. J.
(Letters), 1996 April 1
Breakdown of the linear approximation in the perturbative analysis of heat conduction in relativistic systems
We analyze the effects of thermal conduction in a relativistic fluid just
after its departure from spherical symmetry, on a time scale of the order of
relaxation time. Using first order perturbation theory, it is shown that, as in
spherical systems, at a critical point the effective inertial mass density of a
fluid element vanishes and becomes negative beyond that point. The impact of
this effect on the reliability of causality conditions is discussed.Comment: 11 pages (Latex2.09) To appear in Physics Letters
Laudatores Temporis Acti, or Why Cosmology is Alive and Well - A Reply to Disney
A recent criticism of cosmological methodology and achievements by Disney
(2000) is assessed. Some historical and epistemological fallacies in the said
article have been highlighted. It is shown that---both empirically and
epistemologically---modern cosmology lies on sounder foundations than it is
portrayed. A brief historical account demonstrates that this form of
unsatisfaction with cosmology has had a long tradition, and rather meagre
results in the course of the XX century.Comment: 11 pages, no figures; a criticism of astro-ph/0009020; Gen. Rel.
Grav., accepted for publicatio
The Evolution in the Faint-End Slope of the Quasar Luminosity Function
(Abridged) Based on numerical simulations of galaxy mergers that incorporate
black hole (BH) growth, we predict the faint end slope of the quasar luminosity
function (QLF) and its evolution with redshift. Our simulations have yielded a
new model for quasar lifetimes where the lifetime depends on both the
instantaneous and peak quasar luminosities. This motivates a new interpretation
of the QLF in which the bright end consists of quasars radiating at nearly
their peak luminosities, but the faint end is mostly made up of quasars in less
luminous phases of evolution. The faint-end QLF slope is then determined by the
faint-end slope of the quasar lifetime for quasars with peak luminosities near
the observed break. We determine this slope from the quasar lifetime as a
function of peak luminosity, based on a large set of simulations spanning a
wide variety of host galaxy, merger, BH, and ISM gas properties. Brighter peak
luminosity (higher BH mass) systems undergo more violent evolution, and expel
and heat gas more rapidly in the final stages of quasar evolution, resulting in
a flatter faint-end slope (as these objects fall below the observed break in
the QLF more rapidly). Therefore, as the QLF break luminosity moves to higher
luminosities with increasing redshift, implying a larger typical quasar peak
luminosity, the faint-end QLF slope flattens. From the quasar lifetime as a
function of peak luminosity and this interpretation of the QLF, we predict the
faint-end QLF slope and its evolution with redshift in good agreement with
observations. Although BHs grow anti-hierarchically (with lower-mass BHs formed
primarily at lower redshifts), the observed change in slope and differential or
luminosity dependent density evolution in the QLF is completely determined by
the luminosity-dependent quasar lifetime and physics of quasar feedback.Comment: 13 pages, 4 figures, submitted to ApJ (Replacement with minor
revisions and changed sign convention
CMB anisotropies seen by an off-center observer in a spherically symmetric inhomogeneous universe
The current authors have previously shown that inhomogeneous, but spherically
symmetric universe models containing only matter can yield a very good fit to
the SNIa data and the position of the first CMB peak. In this work we examine
how far away from the center of inhomogeneity the observer can be located in
these models and still fit the data well. Furthermore, we investigate whether
such an off-center location can explain the observed alignment of the lowest
multipoles of the CMB map. We find that the observer has to be located within a
radius of 15 Mpc from the center for the induced dipole to be less than that
observed by the COBE satellite. But for such small displacements from the
center, the induced quadru- and octopoles turn out to be insufficiently large
to explain the alignment.Comment: 8 pages (REVTeX4), 7 figures; v2: minor changes, matches published
versio
Accretion of low angular momentum material onto black holes: 2D magnetohydrodynamical case
We report on the second phase of our study of slightly rotating accretion
flows onto black holes. We consider magnetohydrodynamical (MHD) accretion flows
with a spherically symmetric density distribution at the outer boundary, but
with spherical symmetry broken by the introduction of a small,
latitude-dependent angular momentum and a weak radial magnetic field. We study
accretion flows by means of numerical 2D, axisymmetric, MHD simulations with
and without resistive heating. Our main result is that the properties of the
accretion flow depend mostly on an equatorial accretion torus which is made of
the material that has too much angular momentum to be accreted directly. The
torus accretes, however, because of the transport of angular momentum due to
the magnetorotational instability (MRI). Initially, accretion is dominated by
the polar funnel, as in the hydrodynamic inviscid case, where material has zero
or very low angular momentum. At the later phase of the evolution, the torus
thickens towards the poles and develops a corona or an outflow or both.
Consequently, the mass accretion through the funnel is stopped. The accretion
of rotating gas through the torus is significantly reduced compared to the
accretion of non-rotating gas (i.e., the Bondi rate). It is also much smaller
than the accretion rate in the inviscid, weakly rotating case.Our results do
not change if we switch on or off resistive heating. Overall our simulations
are very similar to those presented by Stone, Pringle, Hawley and Balbus
despite different initial and outer boundary conditions. Thus, we confirm that
MRI is very robust and controls the nature of radiatively inefficient accretion
flows.Comment: submitted in Ap
Special Theory of Relativity through the Doppler Effect
We present the special theory of relativity taking the Doppler effect as the
starting point, and derive several of its main effects, such as time dilation,
length contraction, addition of velocities, and the mass-energy relation, and
assuming energy and momentum conservation, we discuss how to introduce the
4-momentum in a natural way. We also use the Doppler effect to explain the
"twin paradox", and its version on a cylinder. As a by-product we discuss
Bell's spaceship paradox, and the Lorentz transformation for arbitrary
velocities in one dimension.Comment: 20 pages, 1 figur
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