583 research outputs found
Dark Matter from Early Decays
Two leading dark matter candidates from supersymmetry and other theories of
physics beyond the standard model are WIMPs and weak scale gravitinos. If the
lightest stable particle is a gravitino, then a WIMP will decay into it with a
natural lifetime of order a month ~ M_{pl}^2/M_{weak}^3. We show that if the
bulk of dark matter today came from decays of neutral particles with lifetimes
of order a year or smaller, then it could lead to a reduction in the amount of
small scale substructure, less concentrated halos and constant density cores in
the smallest mass halos. Such beneficial effects may therefore be realized
naturally, as discussed by Cembranos, Feng, Rajaraman, and Takayama, in the
case of supersymmetry.Comment: Matches version accepted for publication in PRD. Added a paragraph to
Sec V. 9 pages, 3 figure
The Compression of Dark Matter Halos by Baryonic Infall
The initial radial density profiles of dark matter halos are laid down by
gravitational collapse in hierarchical structure formation scenarios and are
subject to further compression as baryons cool and settle to the halo centers.
We here describe an explicit implementation of the algorithm, originally
developed by Young, to calculate changes to the density profile as the result
of adiabatic infall in a spherical halo model. Halos with random motion are
more resistant to compression than are those in which random motions are
neglected, which is a key weakness of the simple method widely employed.
Young's algorithm results in density profiles in excellent agreement with those
from N-body simulations. We show how the algorithm may be applied to determine
the original uncompressed halos of real galaxies, a step which must be computed
with care in order to enable a confrontation with theoretical predictions from
theories such as LCDM.Comment: Revised version for ApJ. 8 pages, 8 figures, latex uses emulateap
Unstable Disk Galaxies. I. Modal Properties
I utilize the Petrov-Galerkin formulation and develop a new method for
solving the unsteady collisionless Boltzmann equation in both the linear and
nonlinear regimes. In the first order approximation, the method reduces to a
linear eigenvalue problem which is solved using standard numerical methods. I
apply the method to the dynamics of a model stellar disk which is embedded in
the field of a soft-centered logarithmic potential. The outcome is the full
spectrum of eigenfrequencies and their conjugate normal modes for prescribed
azimuthal wavenumbers. The results show that the fundamental bar mode is
isolated in the frequency space while spiral modes belong to discrete families
that bifurcate from the continuous family of van Kampen modes. The population
of spiral modes in the bifurcating family increases by cooling the disk and
declines by increasing the fraction of dark to luminous matter. It is shown
that the variety of unstable modes is controlled by the shape of the dark
matter density profile.Comment: Accepted for publication in The Astrophysical Journa
The Dark Matter at the End of the Galaxy
Dark matter density profiles based upon Lambda-CDM cosmology motivate an
ansatz velocity distribution function with fewer high velocity particles than
the Maxwell-Boltzmann distribution or proposed variants. The high velocity tail
of the distribution is determined by the outer slope of the dark matter halo,
the large radius behavior of the Galactic dark matter density. N-body
simulations of Galactic halos reproduce the high velocity behavior of this
ansatz. Predictions for direct detection rates are dramatically affected for
models where the threshold scattering velocity is within 30% of the escape
velocity.Comment: 10 pages, 5 figure
Variation of Galactic Bar Length with Amplitude and Density as Evidence for Bar Growth over a Hubble Time
K_s-band images of 20 barred galaxies show an increase in the peak amplitude
of the normalized m=2 Fourier component with the R_25-normalized radius at this
peak. This implies that longer bars have higher amplitudes. The long bars
also correlate with an increased density in the central parts of the disks, as
measured by the luminosity inside 0.25R_25 divided by the cube of this radius
in kpc. Because denser galaxies evolve faster, these correlations suggest that
bars grow in length and amplitude over a Hubble time with the fastest evolution
occurring in the densest galaxies. All but three of the sample have early-type
flat bars; there is no clear correlation between the correlated quantities and
the Hubble type.Comment: ApJ Letters, 670, L97, preprint is 7 pages, 4 figure
Warm dark matter at small scales: peculiar velocities and phase space density
We study the scale and redshift dependence of the power spectra for density
perturbations and peculiar velocities, and the evolution of a coarse grained
phase space density for (WDM) particles that decoupled during the radiation
dominated stage. The (WDM) corrections are obtained in a perturbative expansion
valid in the range of redshifts at which N-body simulations set up initial
conditions, and for a wide range of scales. The redshift dependence is
determined by the kurtosis of the distribution function at
decoupling. At large redshift there is an enhancement of peculiar velocities
for that contributes to free streaming and leads to further
suppression of the matter power spectrum and an enhancement of the peculiar
velocity autocorrelation function at scales smaller than the free streaming
scale. Statistical fluctuations of peculiar velocities are also suppressed on
these scales by the same effect. In the linearized approximation, the coarse
grained phase space density features redshift dependent (WDM) corrections from
gravitational perturbations determined by the power spectrum of density
perturbations and . For it \emph{grows
logarithmically} with the scale factor as a consequence of the suppression of
statistical fluctuations. Two specific models for WDM are studied in detail.
The (WDM) corrections relax the bounds on the mass.Comment: 22 pages, 9 figs, more explanations. Published versio
Constraints on dark matter particles from theory, galaxy observations and N-body simulations
Mass bounds on dark matter (DM) candidates are obtained for particles
decoupling in or out of equilibrium with {\bf arbitrary} isotropic and
homogeneous distribution functions. A coarse grained Liouville invariant
primordial phase space density is introduced. Combining its
value with recent photometric and kinematic data on dwarf spheroidal satellite
galaxies in the Milky Way (dShps), the DM density today and -body
simulations, yields upper and lower bounds on the mass, primordial phase space
densities and velocity dispersion of the DM candidates. The mass of the DM
particles is bound in the few keV range. If chemical freeze out occurs before
thermal decoupling, light bosonic particles can Bose-condense. Such
Bose-Einstein {\it condensate} is studied as a dark matter candidate. Depending
on the relation between the critical()and decoupling()temperatures, a
BEC light relic could act as CDM but the decoupling scale must be {\it higher}
than the electroweak scale. The condensate tightens the upper bound on the
particle's mass. Non-equilibrium scenarios that describe particle production
and partial thermalization, sterile neutrinos produced out of equilibrium and
other DM models are analyzed in detail obtaining bounds on their mass,
primordial phase space density and velocity dispersion. Light thermal relics
with and sterile neutrinos lead to a
primordial phase space density compatible with {\bf cored} dShps and disfavor
cusped satellites. Light Bose condensed DM candidates yield phase space
densities consistent with {\bf cores} and if also with cusps.
Phase space density bounds from N-body simulations suggest a potential tension
for WIMPS with .Comment: 27 pages 8 figures. Version to appear in Phys. Rev.
Hybrid nature of 0846+51W1: a BL Lac object with a narrow line Seyfert 1 nucleus
We have found a NLS1 nucleus in the extensively studied eruptive BL Lac,
0846+51W1, out of a large sample of NLS1 compiled from the spectroscopic
dataset of SDSS DR1. Its optical spectrum can be well decomposed into three
components, a power law component from the relativistic jet, a stellar
component from the host galaxy, and a component from a typical NLS1 nucleus.
The emission line properties of 0846+51W1, FWHM(Hbeta) ~ 1710 km s^-1 and
[OIII]5007/Hbeta ~ 0.32 when it was in faint state, fulfil the conventional
definition of NLS1. Strong FeII emission is detected in the SDSS spectrum,
which is also typical of NLS1s. We try to estimate its central black hole mass
using various techniques and find that 0846+51W1 is very likely emitting at a
few times 10% L_Edd. We speculate that Seyfert-like nuclei, including NLS1s,
might be concealed in a significant fraction of BL Lacs but have not been
sufficiently explored due to the fact that, by definition, the optical-UV
continuum of such kind of objects are often overwhelmed by the synchrotron
emission.Comment: ChJAA accepte
Elliptical Galaxies with Emission Lines from the Sloan Digital Sky Survey
We present the results of 11 elliptical galaxies with strong nebular emission
lines during our study of star formation history along the Hubble sequence.
After removing the dilution from the underlying old stellar populations by use
of stellar population synthesis model, we derive the accurate fluxes of all
emission lines for these objects, which are later classified with emission line
ratios into one Seyfert 2, six LINERs and four HII galaxies. We also identify
one HII galaxy (A1216+04) as a hitherto unknown Wolf-Rayet galaxy from the
presence of the Wolf-Rayet broad bump at 4650 \AA. We propose that the
star-forming activities in elliptical galaxies are triggered by either
galaxy-galaxy interaction or the merging of a small satellite/a massive star
cluster, as already suggested by recent numerical simulations
Triaxial orbit based galaxy models with an application to the (apparent) decoupled core galaxy NGC 4365
We present a flexible and efficient method to construct triaxial dynamical
models of galaxies with a central black hole, using Schwarzschild's orbital
superposition approach. Our method is general and can deal with realistic
luminosity distributions, which project to surface brightness distributions
that may show position angle twists and ellipticity variations. The models are
fit to measurements of the full line-of-sight velocity distribution (wherever
available). We verify that our method is able to reproduce theoretical
predictions of a three-integral triaxial Abel model. In a companion paper (van
de Ven, de Zeeuw & van den Bosch), we demonstrate that the method recovers the
phase-space distribution function. We apply our method to two-dimensional
observations of the E3 galaxy NGC 4365, obtained with the integral-field
spectrograph SAURON, and study its internal structure, showing that the
observed kinematically decoupled core is not physically distinct from the main
body and the inner region is close to oblate axisymmetric.Comment: 21 Pages, 14 (Colour) Figures, Companion paper is arXiv:0712.0309
Accepted to MNRAS. Full resolution version at
http://www.strw.leidenuniv.nl/~bosch/papers/RvdBosch_triaxmethod.pd
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