95 research outputs found
Relaxing and Virializing a Dark Matter Halo
Navarro, Frenk, and White have suggested that the density profiles of
simulated dark matter halos have a ``universal'' shape so that a given halo can
be characterized by a single free parameter which fixes its mass. In this
paper, we revisit the spherical infall model in the hope of recognizing in
detail the existence and origin of any such universality. A system of particles
is followed from linear perturbation, through first shell crossing, then
through an accretion or infall phase, and finally to virialization. During the
accretion phase, the system relaxes through a combination of phase mixing,
phase space instability, and moderate violent relation. It is driven quickly,
by the flow of mass through its surface, toward self-similar evolution. The
self-similar solution plays its usual role of intermediate attractor and can be
recognized from a virial-type theorem in scaled variables and from our
numerical simulations. The transition to final equilibrium state once infall
has ceased is relatively gentle, an observation which leads to an approximate
form for the distribution function of the final system. The infall phase fixes
the density profile in intermediate regions of the halo to be close to r^{-2}.
We make contact with the standard hierarchical clustering scenario and explain
how modifications of the self-similar infall model might lead to density
profiles in agreement with those found in numerical simulations.Comment: 26 pages, Latex, plus 11 figure
Black Holes and Galactic Density Cusps Spherically Symmetric Anisotropic Cusps
Aims: In this paper we study density cusps that may contain central black
holes. The actual co-eval self-similar growth would not distinguish between the
central object and the surroundings. Methods: To study the environment of a
growing black hole we seek descriptions of steady `cusps' that may contain a
black hole and that retain at least a memory of self-similarity. We refer to
the environment in brief as the `bulge' and on smaller scales, the `halo'.
Results: We find simple descriptions of the simulations of collisionless matter
by comparing predicted densities, velocity dispersions and distribution
functions with the simulations. In some cases central point masses may be
included by iteration. We emphasize that the co-eval self-similar growth allows
an explanation of the black hole bulge mass correlation between approximately
similar collisionless systems. Conclusions: We have derived our results from
first principles assuming adiabatic self-similarity and either self-similar
virialisation or normal steady virialisation. We conclude that distribution
functions that retain a memory of self-similar evolution provide an
understanding of collisionless systems. The implied energy relaxation of the
collisionless matter is due to the time dependence. Phase mixing relaxation may
be enhanced by clump-clump interactions.Comment: 9 pp, 3 figs, accepted by A\&
Hydrogen Clouds and the MACHO/EROS Events
We propose that the recently reported MACHO/EROS events correspond to
gravitational amplification by dark clouds rather than compact objects. These
clouds must be very dense with and R\la 10^{14}~{\rm
cm}. In all likelihood, the clouds will be members of a family of objects with
different sizes and masses. We therefore expect events of longer duration than
the ones reported by the MACHO and EROS groups but with light curves very
different from the ones derived assuming point mass lenses. We suggest that one
such event has already been observed in radio measurements of the quasar
1502+106. The abundances of free electrons, metals, complex molecules, and dust
grains are constrained to be very small suggesting that the clouds are formed
from a primordial mixture of hydrogen and helium. Cosmic rays and background UV
radiation ionize a halo around the cloud. Radio waves from distant sources will
be scattered by the electrons in this halo, an effect which may have already
been observed in quasars such as 1502+106. We argue that dark clouds are a
viable alternative to compact objects for baryonic dark matter in the halo.Comment: 11 pages, LaTe
Protostellar Evolution during Time Dependent, Anisotropic Collapse
The formation and collapse of a protostar involves the simultaneous infall
and outflow of material in the presence of magnetic fields, self-gravity, and
rotation. We use self-similar techniques to self-consistently model the
anisotropic collapse and outflow by a set of angle-separated self-similar
equations. The outflow is quite strong in our model, with the velocity
increasing in proportion to radius, and material formally escaping to infinity
in the finite time required for the central singularity to develop.
Analytically tractable collapse models have been limited mainly to
spherically symmetric collapse, with neither magnetic field nor rotation. Other
analyses usually employ extensive numerical simulations, or either perturbative
or quasistatic techniques. Our model is unique as an exact solution to the
non-stationary equations of self-gravitating MHD, which features co-existing
regions of infall and outflow.
The velocity and magnetic topology of our model is quadrupolar, although
dipolar solutions may also exist. We provide a qualitative model for the origin
and subsequent evolution of such a state. However, a central singularity forms
at late times, and we expect the late time behaviour to be dominated by the
singularity rather than to depend on the details of its initial state. Our
solution may, therefore, have the character of an attractor among a much more
general class of self-similarity.Comment: 11 pages, 5 figures, To appear in MNRAS, Memorial paper for M.
Aburiha
CHANG-ES V: Nuclear Radio Outflow in a Virgo Cluster Spiral after a Tidal Disruption Event
We have observed the Virgo Cluster spiral galaxy, NGC~4845, at 1.6 and 6 GHz
using the Karl G. Jansky Very Large Array, as part of the `Continuum Halos in
Nearby Galaxies -- an EVLA Survey' (CHANG-ES). The source consists of a bright
unresolved core with a surrounding weak central disk (1.8 kpc diameter). The
core is variable over the 6 month time scale of the CHANG-ES data and has
increased by a factor of 6 since 1995. The wide bandwidths of
CHANG-ES have allowed us to determine the spectral evolution of this core which
peaks {\it between} 1.6 and 6 GHz (it is a GigaHertz-peaked spectrum source).We
show that the spectral turnover is dominated by synchrotron self-absorption and
that the spectral evolution can be explained by adiabatic expansion (outflow),
likely in the form of a jet or cone. The CHANG-ES observations serendipitously
overlap in time with the hard X-ray light curve obtained by Nikolajuk \& Walter
(2013) which they interpret as due to a tidal disruption event (TDE) of a
super-Jupiter mass object around a black hole. We outline a
standard jet model, provide an explanation for the observed circular
polarization, and quantitatively suggest a link between the peak radio and peak
X-ray emission via inverse Compton upscattering of the photons emitted by the
relativistic electrons. We predict that it should be possible to resolve a
young radio jet via VLBI as a result of this nearby TDE.Comment: 45 pages, 10 figures, accepted July 2, 2015 to the Astrophysical
Journa
On Universal Halos and the Radial Orbit Instability
The radial orbit instability drives dark matter halos toward a universal
structure. This conclusion, first noted by Huss, Jain, and Steinmetz, is
explored in detail through a series of numerical experiments involving the
collapse of an isolated halo into the non-linear regime. The role played by the
radial orbit instability in generating the density profile, shape, and orbit
structure is carefully analyzed and, in all cases, the instability leads to
universality independent of initial conditions. New insights into the
underlying physics of the radial orbit instability are presented.Comment: 31 pages, 11 figures, submitted to the Astrophysical Journa
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