278 research outputs found
Precision constraints on radiative neutrino decay with CMB spectral distortion
We investigate the radiative decay of the cosmic neutrino background, and its
impact on the spectrum of the cosmic microwave background (CMB) that is known
to be a nearly perfect black body. We derive exact formulae for the decay of a
heavier neutrino into a lighter neutrino and a photon, , and of absorption as its inverse, , by
accounting for the precise form of the neutrino momentum distribution. Our
calculations show that if the neutrinos are heavier than eV,
the exact formulae give results that differ by 50%, compared with
approximate ones where neutrinos are assumed to be at rest. We also find that
spectral distortion due to absorption is more important for heavy neutrino
masses (by a factor of 10 going from a neutrino mass of 0.01 eV to 0.1
eV). By analyzing the CMB spectral data measured with COBE-FIRAS, we obtain
lower limits on the neutrino lifetime of s
(95% C.L.) for the smaller mass splitting and s for the larger mass splitting. These represent up to one order of
magnitude improvement over previous CMB constraints. With future CMB
experiments such as PIXIE, these limits will improve by roughly 4 orders of
magnitude. This translates to a projected upper limit on the neutrino magnetic
moment (for certain neutrino masses and decay modes) of , where is the Bohr magneton. Such constraints would
make future precision CMB measurements competitive with lab-based constraints
on neutrino magnetic moments.Comment: 14 pages, 9 figures. v2: Added a number of references and
clarifications. Matches version published in PR
Impact of dark matter subhalos on extended HI disks of galaxies: Possible formation of HI fine structures and stars
Recent observations have discovered star formation activities in the extreme
outer regions of disk galaxies. However it remains unclear what physical
mechanisms are responsible for triggering star formation in such low-density
gaseous environments of galaxies. In order to understand the origin of these
outer star-forming regions, we numerically investigate how the impact of dark
matter subhalos orbiting a gas-rich disk galaxy embedded in a massive dark
matter halo influences the dynamical evolution of outer HI gas disk of the
galaxy. We find that if the masses of the subhalos () in a galaxy
with an extended HI gas disk are as large as , where
is the total mass of the galaxy's dark halo, local fine structures
can be formed in the extended HI disk. We also find that the gas densities of
some apparently filamentary structures can exceed a threshold gas density for
star formation and thus be likely to be converted into new stars in the outer
part of the HI disk in some models with larger . These results thus
imply that the impact of dark matter subhalos (``dark impact'') can be
important for better understanding the origin of recent star formation
discovered in the extreme outer regions of disk galaxies. We also suggest that
characteristic morphologies of local gaseous structures formed by the dark
impact can indirectly prove the existence of dark matter subhalos in galaxies.
We discuss the origin of giant HI holes observed in some gas-rich galaxies
(e.g., NGC 6822) in the context of the dark impact.Comment: 8 pages, 4 figures, accepted by ApJ
Identifying the Environment and Redshift of GRB Afterglows from the Time-Dependence of Their Absorption Spectra
The discovery of Gamma-Ray Burst (GRB) afterglows revealed a new class of
variable sources at optical and radio wavelengths. At present, the environment
and precise redshift of the detected afterglows are still unknown. We show that
if a GRB source resides in a compact (<100pc) gas-rich environment, the
afterglow spectrum will show time-dependent absorption features due to the
gradual ionization of the surrounding medium by the afterglow radiation.
Detection of this time-dependence can be used to constrain the size and density
of the surrounding gaseous system. For example, the MgII absorption line
detected in GRB970508 should have weakened considerably during the first month
if the absorption occurred in a gas cloud of size <100pc around the source. The
time-dependent HI or metal absorption features provide a precise determination
of the GRB redshift.Comment: 13 pages, 4 figures, submitted to ApJ
Modeling Non-Circular Motions in Disk Galaxies: Application to NGC 2976
We present a new procedure to fit non-axisymmetric flow patterns to 2-D
velocity maps of spiral galaxies. We concentrate on flows caused by bar-like or
oval distortions to the total potential that may arise either from a
non-axially symmetric halo or a bar in the luminous disk. We apply our method
to high-quality CO and Halpha data for the nearby, low-mass spiral NGC 2976
previously obtained by Simon et al., and find that a bar-like model fits the
data at least as well as their model with large radial flows. We find
supporting evidence for the existence of a bar in the baryonic disk. Our model
suggests that the azimuthally averaged central attraction in the inner part of
this galaxy is larger than estimated by these authors. It is likely that the
disk is also more massive, which will limit the increase to the allowed dark
halo density. Allowance for bar-like distortions in other galaxies may either
increase or decrease the estimated central attraction.Comment: 12 pages, 6 figures, accepted for publication in ApJ. v2: minor
changes to match proofs. For version with high-resolution figures, see
http://www.physics.rutgers.edu/~spekkens/papers/noncirc.pd
The Structure of Dark Matter Haloes in Dwarf Galaxies
Recent observations indicate that dark matter haloes have flat central
density profiles. Cosmological simulations with non-baryonic dark matter
predict however self similar haloes with central density cusps. This
contradiction has lead to the conclusion that dark matter must be baryonic.
Here it is shown that the dark matter haloes of dwarf spiral galaxies represent
a one parameter family with self similar density profiles. The observed global
halo parameters are coupled with each other through simple scaling relations
which can be explained by the standard cold dark matter model if one assumes
that all the haloes formed from density fluctuations with the same primordial
amplitude. We find that the finite central halo densities correlate with the
other global parameters. This result rules out scenarios where the flat halo
cores formed subsequently through violent dynamical processes in the baryonic
component. These cores instead provide important information on the origin and
nature of dark matter in dwarf galaxies.Comment: uuencoded Z-compressed postscript file, 10 pages, 3 figures included,
to appear in ApJ Letter
Origin of structural and kinematical properties of the Small Magellanic Cloud
We investigate structural, kinematical, and chemical properties of stars and
gas in the Small Magellanic Cloud (SMC) interacting with the Large Magellanic
Cloud (LMC) and the Galaxy based on a series of self-consistent chemodynamical
simulations. We adopt a new "dwarf spheroidal model" in which the SMC initially
has both old stars with a spherical spatial distribution and an extended HI gas
disk. We mainly investigate SMC's evolution for the last 3 Gyr within which the
Magellanic stream (MS) and the Magellanic bridge (MB) can be formed as a result
of the LMC-SMC-Galaxy interaction. Our principal results, which can be tested
against observations, are as follows. The final spatial distribution of the old
stars projected onto the sky is spherical even after the strong LMC-SMC-Galaxy
interaction, whereas that of the new ones is significantly flattened and
appears to form a bar structure. Old stars have the line-of-sight velocity
dispersion (sigma) of ~ 30 km/s and slow rotation with the maximum rotational
velocity (V) of less than slow rotation with the maximum rotational velocity
(V) of less than 20 km/s and show asymmetry in the radial profiles. New stars
have a smaller sigma than old ones and a significant amount of rotation
(V/sigma >1). HI gas shows velocity dispersions of sigma = 10-40 km/s a high
maximum rotational velocity (V ~ 50 km/s), and the spatial distribution similar
to that of new stars. The new stars with ages younger than 3 Gyr show a
negative metallicity gradient in the sense that more metal-rich stars are
located in the inner regions of the SMC.Comment: 21 pages, 21 figures (5 color), accepted by PAS
Rotational Widths for Use in the Tully-Fisher Relation. II. The Impact of Surface Brightness
Using a large sample of spiral galaxies for which 21 cm single-dish and/or
long-slit optical spectra are available, we make a detailed comparison between
various estimates of rotational widths. Different optical width estimators are
considered and their limitations discussed, with emphasis on biases associated
with rotation curve properties (shape and extent) and disk central surface
brightness. The best match with HI rotational velocities is obtained with
Polyex widths, which are measured at the optical radius (encompassing a fixed
fraction of the total light of the galaxy) from a model fit to the rotation
curve. In contrast with Polyex widths, optical rotational velocities measured
at 2.15 disk scale lengths r_d deviate from HI widths by an amount that
correlates with the central surface brightness of the disk. This bias occurs
because the rotation curves of galaxies are in general still rising at 2.15
r_d, and the fraction of total mass contained within this radius decreases with
increasing disk surface brightness. Statistical corrections, parameterized by
the radial extent of the observed rotation curve, are provided to reduce Polyex
and HI width measurements into a homogeneous system. This yields a single
robust estimate of rotational velocity to be used for applications of disk
scaling relations.Comment: 13 pages, 8 figures. To appear in the Astronomical Journal (August
2007
Scattering of low energy noble gas ions from a metal surface:a study of the neutralization and ionization phenomena for keV argon and neon particles scattered from a monocrystalline copper (100) surface
Het werk beschreven in dit proefschrift heeft betrekking op
de verstrooiing van lage energie ( 0.1 - 10 keV) edelgas ionen
door metaal oppervlakken. Deze techniek wordt gebruikt om de
samenstelling en de structuur van de allerbuitenste oppervlakte laag te bepalen Dit is van belang voor de bestudering van o.a. katalyse en corrosie. ...
Zie: Smanvatting
The Cloudy Universe
Modelling of Extreme Scattering Events suggests that the Galaxy's dark matter
is an undetected population of cold, AU-sized, planetary-mass gas clouds. None
of the direct observational constraints on this picture -- thermal/non-thermal
emission, extinction and lensing -- are problematic. The theoretical situation
is less comfortable, but still satisfactory. Galactic clouds can survive in
their current condition for billions of years, but we do not have a firm
description for either their origin or their evolution to the present epoch. We
hypothesise that the proto-clouds formed during the quark-hadron phase
transition, thereby introducing the inhomogeneity necessary for compatibility
with light element nucleosynthesis in a purely baryonic universe. We outline
the prospects for directly detecting the inferred cloud population. The most
promising signatures are cosmic-ray-induced H-alpha emission from clouds in the
solar neighbourhood, optical flashes arising from cloud-cloud collisions,
ultraviolet extinction, and three varieties of lensing phenomena.Comment: 16 pages, LaTeX, no figures, to appear in Pub. Ast. Soc. Au
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