518 research outputs found
Dark Matter Subhalos in the Ursa Minor Dwarf Galaxy
Through numerical simulations, we study the dissolution timescale of the Ursa
Minor cold stellar clump, due to the combination of phase-mixing and
gravitational encounters with compact dark substructures in the halo of Ursa
Minor. We compare two scenarios; one where the dark halo is made up by a smooth
mass distribution of light particles and one where the halo contains 10% of its
mass in the form of substructures (subhalos). In a smooth halo, the stellar
clump survives for a Hubble time provided that the dark matter halo has a big
core. In contrast, when the point-mass dark substructures are added, the clump
survives barely for \sim 1.5 Gyr. These results suggest a strong test to the
\Lambda-cold dark matter scenario at dwarf galaxy scale.Comment: accepted for publication in Ap
Hot Disks And Delayed Bar Formation
We present observational evidence for the inhibition of bar formation in
dispersion-dominated (dynamically hot) galaxies by studying the relationship
between galactic structure and host galaxy kinematics in a sample of 257
galaxies between 0.1 z 0.84 from the All-Wavelength Extended Groth
Strip International Survey (AEGIS) and the Deep Extragalactic Evolutionary
Probe 2 (DEEP2) survey. We find that bars are preferentially found in galaxies
that are massive and dynamically cold (rotation-dominated) and on the stellar
Tully-Fisher relationship, as is the case for barred spirals in the local
Universe. The data provide at least one explanation for the steep (3)
decline in the overall bar fraction from z=0 to z=0.84 in L and brighter
disks seen in previous studies. The decline in the bar fraction at high
redshift is almost exclusively in the lower mass (10 log M(\Msun)
11), later-type and bluer galaxies. A proposed explanation for this
"downsizing" of the bar formation / stellar structure formation is that the
lower mass galaxies may not form bars because they could be dynamically hotter
than more massive systems from the increased turbulence of accreting gas,
elevated star formation, and/or increased interaction/merger rate at higher
redshifts. The evidence presented here provides observational support for this
hypothesis. However, the data also show that not every disk galaxy that is
massive and cold has a stellar bar, suggesting that mass and dynamic coldness
of a disk are necessary but not sufficient conditions for bar formation -- a
secondary process, perhaps the interaction history between the dark matter halo
and the baryonic matter, may play an important role in bar formation.Comment: In press, ApJ, 13 pages, 5 figures (3 color
Progress on stochastic background search codes for LIGO
One of the types of signals for which the LIGO interferometric gravitational
wave detectors will search is a stochastic background of gravitational
radiation. We review the technique of searching for a background using the
optimally-filtered cross-correlation statistic, and describe the state of plans
to perform such cross-correlations between the two LIGO interferometers as well
as between LIGO and other gravitational-wave detectors, in particular the
preparation of software to perform such data analysis.Comment: 7 pages, 1 encapsulated PostScript figure, uses IOP class files,
submitted to the proceedings of the 4th Amaldi meeting (which will be
published in Classical and Quantum Gravity
Methodology to improve mode identification and modal parameter extraction for rotor dynamic analysis
This work presents the development of a methodology that, through the use of the coordinate transformation method, identifies the ideal modal parameters that should be used during the modal balancing process as well as allows eliminating the computational modes generated during the rotor response diagram extraction process. There is currently a wide variety of methods for structures that allow extracting limited modal parameters, such as: previous knowledge of the number of modes, adjustment of computational or spurious modes, close mode identification problems, and others. However, localizing the phase angle in rotation systems in any angular position and through complex coupling of the vibration modes does not ensure that the methods developed for structures conserve the same performance during the adjustment process. As regards the line of investigation into modal balancing, a method is proposed that allows ensuring that the modes found are real modes of the system and that through direction tracking where a single vibration mode is excited, the optimum extraction position of the modal parameters used in the balancing process can be determined. The proposed methodology was developed using a linear model and was applied in a field turbogenerator to identify the vibration modes present in the response diagrams
Halo concentrations in the standard LCDM cosmology
We study the concentration of dark matter halos and its evolution in N-body
simulations of the standard LCDM cosmology. The results presented in this paper
are based on 4 large N-body simulations with about 10 billion particles each:
the Millennium-I and II, Bolshoi, and MultiDark simulations. The MultiDark (or
BigBolshoi) simulation is introduced in this paper. This suite of simulations
with high mass resolution over a large volume allows us to compute with
unprecedented accuracy the concentration over a large range of scales (about
six orders of magnitude in mass), which constitutes the state-of-the-art of our
current knowledge on this basic property of dark matter halos in the LCDM
cosmology. We find that there is consistency among the different simulation
data sets. We confirm a novel feature for halo concentrations at high
redshifts: a flattening and upturn with increasing mass. The concentration
c(M,z) as a function of mass and the redshift and for different cosmological
parameters shows a remarkably complex pattern. However, when expressed in terms
of the linear rms fluctuation of the density field sigma(M,z), the halo
concentration c(sigma) shows a nearly-universal simple U-shaped behaviour with
a minimum at a well defined scale at sigma=0.71. Yet, some small dependences
with redshift and cosmology still remain. At the high-mass end (sigma < 1) the
median halo kinematic profiles show large signatures of infall and highly
radial orbits. This c-sigma(M,z) relation can be accurately parametrized and
provides an analytical model for the dependence of concentration on halo mass.
When applied to galaxy clusters, our estimates of concentrations are
substantially larger -- by a factor up to 1.5 -- than previous results from
smaller simulations, and are in much better agreement with results of
observations. (abridged)Comment: Submitted to MNRA
Barred Galaxies in the Abell 901/2 Supercluster with STAGES
We present a study of bar and host disk evolution in a dense cluster
environment, based on a sample of ~800 bright (MV <= -18) galaxies in the Abell
901/2 supercluster at z~0.165. We use HST ACS F606W imaging from the STAGES
survey, and data from Spitzer, XMM-Newton, and COMBO-17. We identify and
characterize bars through ellipse-fitting, and other morphological features
through visual classification. (1) We explore three commonly used methods for
selecting disk galaxies. We find 625, 485, and 353 disk galaxies, respectively,
via visual classification, a single component S'ersic cut (n <= 2.5), and a
blue-cloud cut. In cluster environments, the latter two methods miss 31% and
51%, respectively, of visually-identified disks. (2) For moderately inclined
disks, the three methods of disk selection yield a similar global optical bar
fraction (f_bar-opt) of 34% +10%/-3%, 31% +10%/-3%, and 30% +10%/-3%,
respectively. (3) f_bar-opt rises in brighter galaxies and those which appear
to have no significant bulge component. Within a given absolute magnitude bin,
f_bar-opt is higher in visually-selected disk galaxies that have no bulge as
opposed to those with bulges. For a given morphological class, f_bar-opt rises
at higher luminosities. (4) For bright early-types, as well as faint late-type
systems with no evident bulge, the optical bar fraction in the Abell 901/2
clusters is comparable within a factor of 1.1 to 1.4 to that of field galaxies
at lower redshifts (5) Between the core and the virial radius of the cluster at
intermediate environmental densities, the optical bar fraction does not appear
to depend strongly on the local environment density and varies at most by a
factor of ~1.3. We discuss the implications of our results for the evolution of
bars and disks in dense environments.Comment: accepted for publication in ApJ, abstract abridged, for high
resolution figures see
http://www.as.utexas.edu/~marinova/STAGES/STAGES_bars.pd
The PN.S Elliptical Galaxy Survey: the dark matter in NGC 4494
We present new Planetary Nebula Spectrograph observations of the ordinary
elliptical galaxy NGC 4494, resulting in positions and velocities of 255 PNe
out to 7 effective radii (25 kpc). We also present new wide-field surface
photometry from MMT/Megacam, and long-slit stellar kinematics from VLT/FORS2.
The spatial and kinematical distributions of the PNe agree with the field stars
in the region of overlap. The mean rotation is relatively low, with a possible
kinematic axis twist outside 1 Re. The velocity dispersion profile declines
with radius, though not very steeply, down to ~70 km/s at the last data point.
We have constructed spherical dynamical models of the system, including Jeans
analyses with multi-component LCDM-motivated galaxies as well as logarithmic
potentials. These models include special attention to orbital anisotropy, which
we constrain using fourth-order velocity moments. Given several different sets
of modelling methods and assumptions, we find consistent results for the mass
profile within the radial range constrained by the data. Some dark matter (DM)
is required by the data; our best-fit solution has a radially anisotropic
stellar halo, a plausible stellar mass-to-light ratio, and a DM halo with an
unexpectedly low central density. We find that this result does not
substantially change with a flattened axisymmetric model.
Taken together with other results for galaxy halo masses, we find suggestions
for a puzzling pattern wherein most intermediate-luminosity galaxies have very
low concentration halos, while some high-mass ellipticals have very high
concentrations. We discuss some possible implications of these results for DM
and galaxy formation.Comment: 29 pages, 17 figures. MNRAS, accepte
Could the Fermi-LAT detect gamma-rays from dark matter annihilation in the dwarf galaxies of the Local Group?
The detection of gamma-rays from dark matter (DM) annihilation is among the
scientific goals of the Fermi Large Area Telescope (formerly known as GLAST)
and Cherenkov telescopes. In this paper we investigate the existence of
realistic chances of such a discovery selecting some nearby dwarf spheroidal
galaxies (dSph) as a target. We study the detectability with the Fermi-LAT of
the gamma-ray flux from DM annihilation in Draco, Ursa Minor, Carina, and
Sextans, for which the state-of-art DM density profiles were available. We
assume the DM is made of Weakly Interacting Massive Particles such as the
Lightest Supersymmetric Particle (LSP) and compute the expected gamma-ray flux
for optimistic choices of the unknown underlying particle physics parameters.
We then compute the boost factors due to the presence of DM clumps and of a
central supermassive black hole. Finally, we compare our predictions with the
Fermi-LAT sensitivity maps. We find that the dSph galaxies shine above the
Galactic smooth halo: e.g., the Galactic halo is brighter than the Draco dSph
only for angles smaller than 2.3 degrees above the Galactic Center. We also
find that the presence of a cusp or a constant density core in the DM mass
density profile does not produce any relevant effect in the gamma-ray flux due
to the fortunate combination of the geometrical acceptance of the Fermi-LAT
detector and the distance of the galaxies and that no significant enhancement
is given by the presence of a central black hole or a population of
sub-subhalos. We conclude that, even for the most optimistic scenario of
particle physics, the gamma-ray flux from DM annihilation in the dSph galaxies
of the LG would be too low to be detected with the Fermi-LAT.Comment: 11 pages, 11 figures, accepted for publication in A&
Search for Gravitational Waves Associated with 39 Gamma-Ray Bursts Using Data from the Second, Third, and Fourth LIGO Runs
We present the results of a search for short-duration gravitational-wave
bursts associated with 39 gamma-ray bursts (GRBs) detected by gamma-ray
satellite experiments during LIGO's S2, S3, and S4 science runs. The search
involves calculating the crosscorrelation between two interferometer data
streams surrounding the GRB trigger time. We search for associated
gravitational radiation from single GRBs, and also apply statistical tests to
search for a gravitational-wave signature associated with the whole sample. For
the sample examined, we find no evidence for the association of gravitational
radiation with GRBs, either on a single-GRB basis or on a statistical basis.
Simulating gravitational-wave bursts with sine-gaussian waveforms, we set upper
limits on the root-sum-square of the gravitational-wave strain amplitude of
such waveforms at the times of the GRB triggers. We also demonstrate how a
sample of several GRBs can be used collectively to set constraints on
population models. The small number of GRBs and the significant change in
sensitivity of the detectors over the three runs, however, limits the
usefulness of a population study for the S2, S3, and S4 runs. Finally, we
discuss prospects for the search sensitivity for the ongoing S5 run, and beyond
for the next generation of detectors.Comment: 24 pages, 10 figures, 14 tables; minor changes to text and Fig. 2;
accepted by Phys. Rev.
Search for gravitational waves from binary inspirals in S3 and S4 LIGO data
We report on a search for gravitational waves from the coalescence of compact
binaries during the third and fourth LIGO science runs. The search focused on
gravitational waves generated during the inspiral phase of the binary
evolution. In our analysis, we considered three categories of compact binary
systems, ordered by mass: (i) primordial black hole binaries with masses in the
range 0.35 M(sun) < m1, m2 < 1.0 M(sun), (ii) binary neutron stars with masses
in the range 1.0 M(sun) < m1, m2 < 3.0 M(sun), and (iii) binary black holes
with masses in the range 3.0 M(sun)< m1, m2 < m_(max) with the additional
constraint m1+ m2 < m_(max), where m_(max) was set to 40.0 M(sun) and 80.0
M(sun) in the third and fourth science runs, respectively. Although the
detectors could probe to distances as far as tens of Mpc, no gravitational-wave
signals were identified in the 1364 hours of data we analyzed. Assuming a
binary population with a Gaussian distribution around 0.75-0.75 M(sun), 1.4-1.4
M(sun), and 5.0-5.0 M(sun), we derived 90%-confidence upper limit rates of 4.9
yr^(-1) L10^(-1) for primordial black hole binaries, 1.2 yr^(-1) L10^(-1) for
binary neutron stars, and 0.5 yr^(-1) L10^(-1) for stellar mass binary black
holes, where L10 is 10^(10) times the blue light luminosity of the Sun.Comment: 12 pages, 11 figure
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