1,113 research outputs found
Epidemiologic analysis of racial/ethnic disparities: A rejoinder to Frank and Exner
http://dx.doi.org/10.1016/j.socscimed.2007.11.04
Dynamical Masses of Young Star Clusters in NGC 4038/4039
In order to estimate the masses of the compact, young star clusters in the
merging galaxy pair, NGC 4038/4039 (``the Antennae''), we have obtained medium
and high resolution spectroscopy using ISAAC on VLT-UT1 and UVES on VLT-UT2 of
five such clusters. The velocity dispersions were estimated using the stellar
absorption features of CO at 2.29 microns and metal absorption lines at around
8500 \AA, including lines of the Calcium Triplet. The size scales and light
profiles were measured from HST images. From these data and assuming Virial
equilibrium, we estimated the masses of five clusters. The resulting masses
range from 6.5 x 10^5 to 4.7 x 10^6 M_sun. These masses are large, factor of a
few to more than 10 larger than the typical mass of a globular cluster in the
Milky Way. The mass-to-light ratios for these clusters in the V- and K-bands in
comparison with stellar synthesis models suggest that to first order the IMF
slopes are approximately consistent with Salpeter for a mass range of 0.1 to
100 M_sun. However, the clusters show a significant range of possible IMF
slopes or lower mass cut-offs and that these variations may correlate with the
interstellar environment of the cluster. Comparison with the results of
Fokker-Planck simulations of compact clusters with properties similar to the
clusters studied here, suggest that they are likely to be long-lived and may
lose a substantial fraction of their total mass. This mass loss would make the
star clusters obtain masses which are comparable to the typical mass of a
globular cluster.Comment: 16 pages, 12 figures, A&A accepte
Fokker-Planck Models for M15 without a Central Black Hole: The Role of the Mass Function
We have developed a set of dynamically evolving Fokker-Planck models for the
collapsed-core globular star cluster M15, which directly address the issue of
whether a central black hole is required to fit Hubble Space Telescope (HST)
observations of the stellar spatial distribution and kinematics. As in our
previous work reported by Dull et al., we find that a central black hole is not
needed. Using local mass-function data from HST studies, we have also inferred
the global initial stellar mass function. As a consequence of extreme mass
segregation, the local mass functions differs from the global mass function at
every location. In addition to reproducing the observed mass functions, the
models also provide good fits to the star-count and velocity-dispersion
profiles, and to the millisecond pulsar accelerations. We address concerns
about the large neutron star populations adopted in our previous Fokker-Planck
models for M15. We find that good model fits can be obtained with as few as
1600 neutron stars; this corresponds to a retention fraction of 5% of the
initial population for our best fit initial mass function. The models contain a
substantial population of massive white dwarfs, that range in mass up to 1.2
solar masses. The combined contribution by the massive white dwarfs and neutron
stars provides the gravitational potential needed to reproduce HST measurements
of the central velocity dispersion profile.Comment: 10 pages, 7 figure
The environment and redshift dependence of accretion onto dark matter halos and subhalos
A dark-matter-only Horizon Project simulation is used to investigate the
environment- and redshift- dependence of accretion onto both halos and
subhalos. These objects grow in the simulation via mergers and via accretion of
diffuse non-halo material, and we measure the combined signal from these two
modes of accretion. It is found that the halo accretion rate varies less
strongly with redshift than predicted by the Extended Press-Schechter (EPS)
formalism and is dominated by minor-merger and diffuse accretion events at z=0,
for all halos. These latter growth mechanisms may be able to drive the
radio-mode feedback hypothesised for recent galaxy-formation models, and have
both the correct accretion rate and form of cosmological evolution. The low
redshift subhalo accretors in the simulation form a mass-selected subsample
safely above the mass resolution limit that reside in the outer regions of
their host, with ~70% beyond their host's virial radius, where they are
probably not being significantly stripped of mass. These subhalos accrete, on
average, at higher rates than halos at low redshift and we argue that this is
due to their enhanced clustering at small scales. At cluster scales, the mass
accretion rate onto halos and subhalos at low redshift is found to be only
weakly dependent on environment and we confirm that at z~2 halos accrete
independently of their environment at all scales, as reported by other authors.
By comparing our results with an observational study of black hole growth, we
support previous suggestions that at z>1, dark matter halos and their
associated central black holes grew coevally, but show that by the present day,
dark matter halos could be accreting at fractional rates that are up to a
factor 3-4 higher than their associated black holes.Comment: 16 pages, 11 figures. Accepted for publication in MNRA
Decrease in intestinal endocrine cells in Balb/c mice with CT-26 carcinoma cells
The density of intestinal endocrine cells, in Balb/c mice with colon 26 (CT-26) carcinoma cells, were examined immunohistochemically at 28 days after implantation. After CT-26 cell administration there was a significant decrease in most of the intestinal endocrine cells (p < 0.01) compared with the control group. The significant quantitative changes in the intestinal endocrine cell density might contribute to the development of the gastrointestinal symptoms commonly encountered in cancer patients
Improved Mass and Radius Constraints for Quiescent Neutron Stars in Omega Cen and NGC 6397
We use Chandra and XMM observations of the globular clusters Cen and
NGC 6397 to measure the spectrum of their quiescent neutron stars (NSs), and
thus to constrain the allowed ranges of mass and radius for each. We also use
Hubble Space Telescope photometry of NGC 6397 to identify a potential optical
companion to the quiescent NS, and find evidence that the companion lacks
hydrogen. We carefully consider a number of systematic problems, and show that
the choices of atmospheric composition, interstellar medium abundances, and
cluster distances can have important effects on the inferred NS mass and
radius. We find that for typical NS masses, the radii of both NSs are
consistent with the 10-13 km range favored by recent nuclear physics
experiments. This removes the evidence suggested by Guillot and collaborators
for an unusually small NS radius, which relied upon the small inferred radius
of the NGC 6397 NS.Comment: Submitted to MNRAS. 17 page
Supervised Classification: Quite a Brief Overview
The original problem of supervised classification considers the task of
automatically assigning objects to their respective classes on the basis of
numerical measurements derived from these objects. Classifiers are the tools
that implement the actual functional mapping from these measurements---also
called features or inputs---to the so-called class label---or output. The
fields of pattern recognition and machine learning study ways of constructing
such classifiers. The main idea behind supervised methods is that of learning
from examples: given a number of example input-output relations, to what extent
can the general mapping be learned that takes any new and unseen feature vector
to its correct class? This chapter provides a basic introduction to the
underlying ideas of how to come to a supervised classification problem. In
addition, it provides an overview of some specific classification techniques,
delves into the issues of object representation and classifier evaluation, and
(very) briefly covers some variations on the basic supervised classification
task that may also be of interest to the practitioner
The Observed Growth of Massive Galaxy Clusters I: Statistical Methods and Cosmological Constraints
(Abridged) This is the first of a series of papers in which we derive
simultaneous constraints on cosmological parameters and X-ray scaling relations
using observations of the growth of massive, X-ray flux-selected galaxy
clusters. Our data set consists of 238 clusters drawn from the ROSAT All-Sky
Survey, and incorporates extensive follow-up observations using the Chandra
X-ray Observatory. Here we describe and implement a new statistical framework
required to self-consistently produce simultaneous constraints on cosmology and
scaling relations from such data, and present results on models of dark energy.
In spatially flat models with a constant dark energy equation of state, w, the
cluster data yield Omega_m=0.23 +- 0.04, sigma_8=0.82 +- 0.05, and w=-1.01 +-
0.20, marginalizing over conservative allowances for systematic uncertainties.
These constraints agree well and are competitive with independent data in the
form of cosmic microwave background (CMB) anisotropies, type Ia supernovae
(SNIa), cluster gas mass fractions (fgas), baryon acoustic oscillations (BAO),
galaxy redshift surveys, and cosmic shear. The combination of our data with
current CMB, SNIa, fgas, and BAO data yields Omega_m=0.27 +- 0.02, sigma_8=0.79
+- 0.03, and w=-0.96 +- 0.06 for flat, constant w models. For evolving w
models, marginalizing over transition redshifts in the range 0.05-1, we
constrain the equation of state at late and early times to be respectively
w_0=-0.88 +- 0.21 and w_et=-1.05 +0.20 -0.36. The combined data provide
constraints equivalent to a DETF FoM of 15.5. Our results highlight the power
of X-ray studies to constrain cosmology. However, the new statistical framework
we apply to this task is equally applicable to cluster studies at other
wavelengths.Comment: 16 pages, 7 figures. v4: final version (typographic corrections).
Results can be downloaded at
https://www.stanford.edu/group/xoc/papers/xlf2009.htm
The Observed Growth of Massive Galaxy Clusters III: Testing General Relativity on Cosmological Scales
This is the third of a series of papers in which we derive simultaneous
constraints on cosmological parameters and X-ray scaling relations using
observations of the growth of massive, X-ray flux-selected galaxy clusters. Our
data set consists of 238 clusters drawn from the ROSAT All-Sky Survey, and
incorporates extensive follow-up observations using the Chandra X-ray
Observatory. Here we present improved constraints on departures from General
Relativity (GR) on cosmological scales, using the growth index, gamma, to
parameterize the linear growth rate of cosmic structure. Using the method of
Mantz et al. (2009a), we simultaneously and self-consistently model the growth
of X-ray luminous clusters and their observable-mass scaling relations,
accounting for survey biases, parameter degeneracies and systematic
uncertainties. We combine the cluster growth data with gas mass fraction, SNIa,
BAO and CMB data. This combination leads to a tight correlation between gamma
and sigma_8. Consistency with GR requires gamma~0.55. Under the assumption of
self-similar evolution and constant scatter in the scaling relations, and for a
flat LCDM model, we measure gamma(sigma_8/0.8)^6.8=0.55+0.13-0.10, with
0.79<sigma_8<0.89. Relaxing the assumptions on the scaling relations by
introducing two additional parameters to model possible evolution in the
normalization and scatter of the luminosity-mass relation, we obtain consistent
constraints on gamma that are only ~20% weaker than those above. Allowing the
dark energy equation of state, w, to take any constant value, we simultaneously
constrain the growth and expansion histories, and find no evidence for
departures from either GR or LCDM. Our results represent the most robust
consistency test of GR on cosmological scales to date. (Abridged)Comment: Accepted for publication in MNRAS. 11 pages, 5 figures, 1 table. New
figure added: Fig. 4 shows the tight constraints on gamma from the cluster
growth data alone compared with those from the other data sets combined
- …