1,766 research outputs found
Can the WIMP annihilation boost factor be boosted by the Sommerfeld enhancement?
We demonstrate that the Sommerfeld correction to CDM annihilations can be
appreciable if even a small component of the dark matter is extremely cold.
Subhalo substructure provides such a possibility given that the smallest clumps
are relatively cold and contain even colder substructure due to incomplete
phase space mixing. Leptonic channels can be enhanced for plausible models and
the solar neighbourhood boost required to account for PAMELA/ATIC data is
plausibly obtained, especially in the case of a few TeV mass neutralino for
which the Sommerfeld-corrected boost is found to be Saturation
of the Sommerfeld effect is shown to occur below thereby
constraining the range of contributing substructures to be above We find that the associated diffuse gamma ray signal from
annihilations would exceed EGRET constraints unless the channels annihilating
to heavy quarks or to gauge bosons are suppressed. The lepton channel gamma
rays are potentially detectable by the FERMI satellite, not from the inner
galaxy where substructures are tidally disrupted, but rather as a
quasi-isotropic background from the outer halo, unless the outer substructures
are much less concentrated than the inner substructures and/or the CDM density
profile out to the virial radius steepens significantly.Comment: 8 pages, 5 figures. References added. Replaced to match published
versio
The Morphology-Density-Relation: Impact on the Satellite Fraction
In the past years several authors studied the abundance of satellites around
galaxies in order to better estimate the halo masses of host galaxies. To
investigate this connection, we analyze galaxies with
from the hydrodynamical cosmological
simulation Magneticum. We find that the satellite fraction of centrals is
independent of their morphology. With the exception of very massive galaxies at
low redshift, our results do not support the assumption that the dark matter
(DM) haloes of spheroidal galaxies are significantly more massive than those of
disc galaxies at fixed . We show that the
density-morphology-relation starts to build up at and is independent
of the star-formation properties of central galaxies. We conclude that
environmental quenching is more important for satellites than for centrals. Our
simulations indicate that conformity is already in place at , where
formation redshift and current star-formation rate (SFR) of central and
satellite galaxies correlate. Centrals with low SFRs have formed earlier (at
fixed ) while centrals with high SFR formed later, with
typical formation redshifts well in agreement with observations. However, we
confirm the recent observations that the apparent number of satellites of
spheroidal galaxies is significantly larger than for disc galaxies. This
difference completely originates from the inclusion of companion galaxies, i.e.
galaxies that do not sit in the potential minimum of a DM halo. Thus, due to
the density-morphological-relation the number of satellites is not a good
tracer for the halo mass, unless samples are restricted to the central galaxies
of DM haloes.Comment: 17 pages, submitted to MNRAS, www.magneticum.or
Indirect Signals from Dark Matter in Split Supersymmetry
We study the possibilities for the indirect detection of dark matter in Split
Supersymmetry from gamma-rays, positrons, and antiprotons. The most promising
signal is the gamma-ray line, which may be observable at the next generation of
detectors. For certain halo profiles and a high mass neutralino, the line can
even be visible in current experiments. The continuous gamma-ray signal may be
observable, if there is a central spike in the galactic halo density. The
signals are found to be similar to those in MSSM models. These indirect signals
complement other experiments, being most easily observable for regions of
parameter space, such as heavy wino and higgsino dominated neutralinos, which
are least accessible for direct detection and accelerator searches.Comment: 10 pages, 5 figures; experimental sensitivities added to figure 2,
revised version to appear in Phys. Rev.
Photometric Variability Among the Brightest Asymptotic Giant Branch Stars Near the Center of M32
Deep K' images with 0.1 arcsec angular resolution, obtained with ALTAIR+NIRI
on Gemini North, are used to investigate photometric variablity among the
brightest asymptotic giant branch (AGB) stars in the central regions of M32.
Based on a comparison with brightnesses obtained from the K-band data discussed
by Davidge et al. (2000, ApJ, 545, L89), it is concluded that (1) at least 60%
of bright AGB stars near the center of M32 are photometrically variable, and
(2) the amplitudes of the light variations are similar to those of long period
variables in the Galactic bulge. We do not find evidence for a population of
large amplitude variables, like those detected by IRAS in the Galactic bulge.
The technique discussed here may prove useful for conducting an initial
reconnaisance of photometric variability among AGB stars in spheroids in the
Virgo cluster and beyond, where the required long exposure times may restrict
observations to only a few epochs.Comment: 8 pages of text, 3 postscript figures. ApJ (letters) in pres
Triggered Star Formation in the Environment of Young Massive Stars
Recent observations with the Spitzer Space Telescope show clear evidence that
star formation takes place in the surrounding of young massive O-type stars,
which are shaping their environment due to their powerful radiation and stellar
winds. In this work we investigate the effect of ionising radiation of massive
stars on the ambient interstellar medium (ISM): In particular we want to
examine whether the UV-radiation of O-type stars can lead to the observed
pillar-like structures and can trigger star formation. We developed a new
implementation, based on a parallel Smooth Particle Hydrodynamics code (called
IVINE), that allows an efficient treatment of the effect of ionising radiation
from massive stars on their turbulent gaseous environment. Here we present
first results at very high resolution. We show that ionising radiation can
trigger the collapse of an otherwise stable molecular cloud. The arising
structures resemble observed structures (e.g. the pillars of creation in the
Eagle Nebula (M16) or the Horsehead Nebula B33). Including the effect of
gravitation we find small regions that can be identified as formation places of
individual stars. We conclude that ionising radiation from massive stars alone
can trigger substantial star formation in molecular clouds.Comment: 4 pages, 2 figures. To appear in: "Triggered Star Formation in a
Turbulent ISM", IAU Symposium 237, Prague, Czech Republic, August 2006; eds.
B.G.Elmegreen & J. Palou
Cloud Dispersal in Turbulent Flows
Cold clouds embedded in warm media are very common objects in astrophysics.
Their disruption timescale depends strongly on the dynamical configuration. We
discuss the evolution of an initially homogeneous cold cloud embedded in warm
turbulent gas. Within a couple of dynamical timescales, the filling factor of
the cold gas within the original cloud radius drops below 50%. Turbulent
diffusivities estimated from the time evolution of radial filling factor
profiles are not constant with time. Cold and warm gas are bodily transported
by turbulence and mixed. This is only mildly indicated by column density maps.
The radiation field within the cloud, however, increases by several orders of
magnitudes due to the mixing, with possible consequences for cloud chemistry
and evolution within a few dynamical timescales.Comment: 11 pages, 12 figures, accepted by MNRA
A light-front description of electromagnetic form factors for hadrons
A review of the hadron electromagnetic form factors obtained in a light-front
constituent quark model, based on the eigenfunctions of a mass operator, is
presented. The relevance of different components in the q-q interaction for the
description of hadron experimental form factors is analysed.Comment: 6 pages, Latex, 3 Postscript figures included. Proceedings of
"Nucleon 99", Frascati, June 1999. To appear in Nucl. Phys.
Magnetized Non-linear Thin Shell Instability: Numerical Studies in 2D
We revisit the analysis of the Non-linear Thin Shell Instability (NTSI)
numerically, including magnetic fields. The magnetic tension force is expected
to work against the main driver of the NTSI -- namely transverse momentum
transport. However, depending on the field strength and orientation, the
instability may grow. For fields aligned with the inflow, we find that the NTSI
is suppressed only when the Alfv\'en speed surpasses the (supersonic)
velocities generated along the collision interface. Even for fields
perpendicular to the inflow, which are the most effective at preventing the
NTSI from developing, internal structures form within the expanding slab
interface, probably leading to fragmentation in the presence of self-gravity or
thermal instabilities. High Reynolds numbers result in local turbulence within
the perturbed slab, which in turn triggers reconnection and dissipation of the
excess magnetic flux. We find that when the magnetic field is initially aligned
with the flow, there exists a (weak) correlation between field strength and gas
density. However, for transverse fields, this correlation essentially vanishes.
In light of these results, our general conclusion is that instabilities are
unlikely to be erased unless the magnetic energy in clouds is much larger than
the turbulent energy. Finally, while our study is motivated by the scenario of
molecular cloud formation in colliding flows, our results span a larger range
of applicability, from supernovae shells to colliding stellar winds.Comment: 12 pages, 17 figures, some of them at low resolution. Submitted to
ApJ, comments welcom
The Validity of the Adiabatic Contraction Approximation for Dark Matter Halos
We use high resolution numerical simulations to investigate the adiabatic
contraction of dark matter halos with a Hernquist density profile. We test the
response of the halos to the growth of additional axisymmetric disk potentials
with various central concentrations and the spherically symmetric potential of
a softened point mass. Adding the potentials on timescales that are long
compared to the dynamical time scale of the halo, the contracted halos have
density profiles that are in excellent agreement with analytical predictions
based on the conservation of the adiabatic invariant . This is
surprising as this quantity is strictly conserved only for particles on
circular orbits and in spherically symmetric potentials. If the same potentials
are added on timescales that are short compared to the dynamical timescale, the
result depends strongly on the adopted potential. The adiabatic approximation
still works for disk potentials. It does, however, fail for the central
potential.Comment: 7 pages, 3 figures, 1 table. Added reference. Accepted for
publication in ApJ
Properties of Early-Type, Dry Galaxy Mergers and the Origin of Massive Elliptical Galaxies
The luminosity dependence of kinematical and isophotal properties of
elliptical galaxies is investigated using numerical simulations of galaxy
merging, combined with semi-analytical models of hierarchical structure
formation. Mergers of spiral galaxies as the only formation mechanism for
elliptical galaxies can neither reproduce the kinematical and photometric
properties of very massive elliptical galaxies nor the change from rotationally
flattened disky to anisotropic boxy systems with increasing luminosity. We
present numerical simulations showing that binary mergers of early-type
galaxies open an additional channel for the formation of anisotropic, slowly
rotating and boxy ellipticals. Including this channel in a semi-analytical
model we can successfully reproduce the observed trend that more luminous giant
ellipticals are more boxy and less flattened by rotation. This trend can be
strengthened by suppressing residual gas infall and star formation for galaxies
with stellar bulge masses . Hence we
propose that mergers of early-type galaxies play an important role for the
assembly of massive elliptical galaxies.Comment: accepted for publication by ApJ
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