241 research outputs found
Constraining the IMF using TeV gamma ray absorption
Gamma rays of ~TeV energies from distant sources suffer attenuation due to
pair production off of ~1 micron EBL photons. We may exploit this process in
order to indirectly measure the EBL and constrain models of galaxy formation.
Here, using semi-analytic models of galaxy formation, we examine how gamma ray
absorption may be used as an indirect probe of the stellar initial mass
function (IMF), although there is a degeneracy with dust modeling. We point out
that with the new generation of gamma ray telescopes including STACEE, MAGIC,
HESS, VERITAS, and Milagro, we should soon possess a wealth of new data and a
new method for probing the nature of the IMF.Comment: contribution to "TeV Astrophysics of Extragalactic Sources" VERITAS
workshop, editors M. Catanese, J. Quinn, T. Weekes; 3 pages 1 figur
Resolving the Outer Disks and Halos of Nearby Galaxies
In a hierarchical merging scenario, the outer parts of a galaxy are a fossil
record of the galaxy's early history. Observations of the outer disks and halos
of galaxies thus provide a tool to study individual galaxy histories and test
formation theories. Locally, an impressive effort has been made to understand
the halo of the Milky Way, Andromeda, and M33. However, due to the stochastic
nature of halo formation, a better understanding of this process requires a
large sample of galaxies with known halo properties. The GHOSTS project (PI: R.
de Jong) aims to characterize the halos and outer portions of 14 nearby (D=4-14
Mpc) spiral galaxies using the Hubble Space Telescope. Detection of individual
stars in the outer parts of these galaxies enables us to study both the
morphological properties of the galaxies, and determine the stars' metallicity
and age.Comment: Contributed talk; to appear in the proceedings of "Galaxies in the
Local Volume" Sydney 8-13 July 200
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The use of fractally-designed waveforms in electroforming
Pulsed electrodeposition offers the potential for superior control of deposit properties because of the additional control variables available. However, optimization of pulsed deposition processes is a challenge because of the complexity. E.g., the tendency of electroforms to acquire irregularities such as dendritic growths or other morphological instabilities, creates the need for methods to control these undesirable phenomena. One such method is periodic reverse pulses. Optimization of periodic reverse processes is not simple and can lead to local solutions that do not optimize all properties simultaneously. One method for global optimization that might, for example, control surface irregularities on several size scales, uses a periodic reverse design based on fractal time series. This incorporates deplating pulses of several lengths within one self- similar waveform. The properties of fractals permit control of highly complex designs with a small number of input variables. The creation of such waveforms, their properties, and their use in a lead- plating process are described. Speculation on the potential for further application of this method is offered. 26 figs, 11 refs
Constraining scalar fields with stellar kinematics and collisional dark matter
The existence and detection of scalar fields could provide solutions to
long-standing puzzles about the nature of dark matter, the dark compact objects
at the centre of most galaxies, and other phenomena. Yet, self-interacting
scalar fields are very poorly constrained by astronomical observations, leading
to great uncertainties in estimates of the mass and the
self-interacting coupling constant of these fields. To counter this,
we have systematically employed available astronomical observations to develop
new constraints, considerably restricting this parameter space. In particular,
by exploiting precise observations of stellar dynamics at the centre of our
Galaxy and assuming that these dynamics can be explained by a single boson
star, we determine an upper limit for the boson star compactness and impose
significant limits on the values of the properties of possible scalar fields.
Requiring the scalar field particle to follow a collisional dark matter model
further narrows these constraints. Most importantly, we find that if a scalar
dark matter particle does exist, then it cannot account for both the
dark-matter halos and the existence of dark compact objects in galactic nucleiComment: 23 pages, 8 figures; accepted for publication by JCAP after minor
change
Evolution and instabilities of disks harboring super massive black holes
The bar formation is still an open problem in modern astrophysics. In this
paper we present numerical simulation performed with the aim of analyzing the
growth of the bar instability inside stellar-gaseous disks, where the star
formation is triggered, and a central black hole is present. The aim of this
paper is to point out the impact of such a central massive black hole on the
growth of the bar. We use N-body-SPH simulations of the same isolated
disk-to-halo mass systems harboring black holes with different initial masses
and different energy feedback on the surrounding gas. We compare the results of
these simulations with the one of the same disk without black hole in its
center. We make the same comparison (disk with and without black hole) for a
stellar disk in a fully cosmological scenario. A stellar bar, lasting 10 Gyrs,
is present in all our simulations. The central black hole mass has in general a
mild effect on the ellipticity of the bar but it is never able to destroy it.
The black holes grow in different way according their initial mass and their
feedback efficiency, the final values of the velocity dispersions and of the
black hole masses are near to the phenomenological constraints.Comment: 10 pages, 8 figures, accepted for pubblication in "Astrophysics and
Space Science
Primordial black hole production due to preheating
During the preheating process at the end of inflation the amplification of
field fluctuations can lead to the amplification of curvature perturbations. If
the curvature perturbations on small scales are sufficiently large, primordial
black holes (PBHs) will be overproduced. In this paper we study PBH production
in the two-field preheating model with quadratic inflaton potential. We show
that for many values of the inflaton mass m, and coupling g, small scale
perturbations will be amplified sufficiently, before backreaction can shut off
preheating, so that PBHs will be overproduced during the subsequent radiation
dominated era.Comment: 5 pages, 3 eps figures. Minor changes to match version to appear in
PRD as a rapid communicatio
Substructures in lens galaxies: PG1115+080 and B1555+375, two fold configurations
We study the anomalous flux ratio which is observed in some four-image lens
systems, where the source lies close to a fold caustic. In this case two of the
images are close to the critical curve and their flux ratio should be equal to
unity, instead in several cases the observed value differs significantly. The
most plausible solution is to invoke the presence of substructures, as for
instance predicted by the Cold Dark Matter scenario, located near the two
images. In particular, we analyze the two fold lens systems PG1115+080 and
B1555+375, for which there are not yet satisfactory models which explain the
observed anomalous flux ratios. We add to a smooth lens model, which reproduces
well the positions of the images but not the anomalous fluxes, one or two
substructures described as singular isothermal spheres. For PG1115+080 we
consider a smooth model with the influence of the group of galaxies described
by a SIS and a substructure with mass as well as a
smooth model with an external shear and one substructure with mass . For B1555+375 either a strong external shear or two substructures
with mass reproduce the data quite well.Comment: 26 pages, updated bibliography, Accepted for publication in
Astrophysics & Space Scienc
CMB-Cluster Lensing
Clusters of galaxies are powerful cosmological probes, particularly if their
masses can be determined. One possibility for mass determination is to study
the cosmic microwave background (CMB) on small angular scales and observe
deviations from a pure gradient due to lensing of massive clusters. I show
that, neglecting contamination, this technique has the power to determine
cluster masses very accurately, in agreement with estimates by Seljak and
Zaldarriaga (1999). However, the intrinsic small scale structure of the CMB
significantly degrades this power. The resulting mass constraints are useless
unless one imposes a prior on the concentration parameter c. With even a modest
prior on c, an ambitious CMB experiment (0.5' resolution and 1 microK per
pixel) could determine masses of high redshift (z>0.5) clusters with ~ 30%
accuracy.Comment: 17 pages, 10 figure
Effect of halo modelling on WIMP exclusion limits
WIMP direct detection experiments are just reaching the sensitivity required
to detect galactic dark matter in the form of neutralinos. Data from these
experiments are usually analysed under the simplifying assumption that the
Milky Way halo is an isothermal sphere with maxwellian velocity distribution.
Observations and numerical simulations indicate that galaxy halos are in fact
triaxial and anisotropic. Furthermore, in the cold dark matter paradigm
galactic halos form via the merger of smaller subhalos, and at least some
residual substructure survives. We examine the effect of halo modelling on WIMP
exclusion limits, taking into account the detector response. Triaxial and
anisotropic halo models, with parameters motivated by observations and
numerical simulations, lead to significant changes which are different for
different experiments, while if the local WIMP distribution is dominated by
small scale clumps then the exclusion limits are changed dramatically.Comment: 9 pages, 9 figures, version to appear in Phys. Rev. D, minor change
Acceptability and Feasibility of Community-Based, Lay Navigator-Facilitated At-Home Self-Collection for Human Papillomavirus Testing in Underscreened Women
Objective: Women without regular health care providers or a medical home routinely fail to complete recommended cervical cancer screening. At-home self-collection of samples to test for high-risk strains of human papillomavirus (hrHPV) can improve screening rates. This study documents acceptability and feasibility of community lay navigator (LN)-facilitated at-home self-collection for underscreened women in Appalachian Virginia. Materials and Methods: This study used mixed methods in three phases. Phase I involved focus groups of LNs to ensure cultural acceptability of self-collection, and to enhance recruitment of medically underserved women. An environmental scan of community resources and climate was created in Phase II. During Phase III, underscreened women in Appalachian Virginia (the far southwest corner of Virginia) were recruited to complete hrHPV testing using LN-provided self-collection kits. Results: LN-facilitated at-home self-collection for HPV testing was deemed culturally acceptable and feasible to participants in this community-based pilot study. Self-kit training included 64 LNs, of which 35 engaged in the study and were provided 77 kits and instructions. A total of 59 self-kits were returned, of which 42 were correctly completed with valid HPV results, yielding a 16.6% hrHPV rate. Conclusions: Over a quarter of the women LNs recruited had no medical home, indicating this delivery model may have potential to reach women at increased risk of being underscreened for cervical cancer. Research is needed to identify optimal approaches to increase LN participation in outreach self-collection interventions
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