3,712 research outputs found
Evolution of the Dark Matter Distribution at the Galactic Center
Annihilation radiation from neutralino dark matter at the Galactic center
(GC) would be greatly enhanced if the dark matter were strongly clustered
around the supermassive black hole (SBH). The existence of a dark-matter
"spike" is made plausible by the observed, steeply-rising stellar density near
the GC SBH. Here the time-dependent equations describing gravitational
interaction of the dark matter particles with the stars are solved. Scattering
of dark matter particles by stars would substantially lower the dark matter
density near the GC SBH over 10^10 yr, due both to kinetic heating, and to
capture of dark matter particles by the SBH. This result suggests that
enhancements in the dark matter density around a SBH would be modest whether or
not the host galaxy had experienced the scouring effects of a binary SBH.Comment: 5 pages, 3 figures. Submitted to Physical Review Letter
RR Lyrae Variables in Two Fields in the Spheroid of M31
We present Hubble Space Telescope observations taken with the Advanced Camera for Surveys Wide Field Channel of two fields near M32âbetween 4 and 6 kpc from the center of M31. The data cover a time baseline sufficient for the identification and characterization of 681 RR Lyrae variables of which 555 are ab-type and 126 are c-type. The mean magnitude of these stars is = 25.29 ± 0.05, where the uncertainty combines both the random and systematic errors. The location of the stars in the Bailey diagram and the ratio of c-type RR Lyraes to all types are both closer to RR Lyraes in Oosterhoff type I globular clusters in the Milky Way as compared with Oosterhoff II clusters. The mean periods of the ab-type and c-type RR Lyraes are = 0.557 ± 0.003 and = 0.327 ± 0.003, respectively, where the uncertainties in each case represent the standard error of the mean. When the periods and amplitudes of the ab-type RR Lyraes in our sample are interpreted in terms of metallicity, we find the metallicity distribution function to be indistinguishable from a Gaussian with a peak at = â1.50 ± 0.02, where the quoted uncertainty is the standard error of the mean. Using a relation between RR Lyrae luminosity and metallicity along with a reddening of E(B â V) = 0.08 ± 0.03, we find a distance modulus of (m â M)_0 = 24.46 ± 0.11 for M31. We examine the radial metallicity gradient in the environs of M31 using published values for the bulge and halo of M31 as well as the abundances of its dwarf spheroidal companions and globular clusters. In this context, we conclude that the RR Lyraes in our two fields are more likely to be halo objects rather than associated with the bulge or disk of M31, in spite of the fact that they are located at 4-6 kpc in projected distance from the center
Hubble Space Telescope Planetary Camera Images of NGC 1316
We present HST Planetary Camera V and I~band images of the central region of
the peculiar giant elliptical galaxy NGC 1316. The inner profile is well fit by
a nonisothermal core model with a core radius of 0.41" +/- 0.02" (34 pc). At an
assumed distance of 16.9 Mpc, the deprojected luminosity density reaches \sim
2.0 \times 10^3 L_{\sun} pc.
Outside the inner two or three arcseconds, a constant mass-to-light ratio of
is found to fit the observed line width measurements. The
line width measurements of the center indicate the existence of either a
central dark object of mass 2 \times 10^9 M_{\sun}, an increase in the
stellar mass-to-light ratio by at least a factor of two for the inner few
arcseconds, or perhaps increasing radial orbit anisotropy towards the center.
The mass-to-light ratio run in the center of NGC 1316 resembles that of many
other giant ellipticals, some of which are known from other evidence to harbor
central massive dark objects (MDO's).
We also examine twenty globular clusters associated with NGC 1316 and report
their brightnesses, colors, and limits on tidal radii. The brightest cluster
has a luminosity of 9.9 \times 10^6 L_{\sun} (), and the
faintest detectable cluster has a luminosity of 2.4 \times 10^5 L_{\sun}
(). The globular clusters are just barely resolved, but their core
radii are too small to be measured. The tidal radii in this region appear to be
35 pc. Although this galaxy seems to have undergone a substantial merger
in the recent past, young globular clusters are not detected.Comment: 21 pages, latex, postscript figures available at
ftp://delphi.umd.edu/pub/outgoing/eshaya/fornax
The Luminosity Profiles of Brightest Cluster Galaxies
(Abridged) We have derived detailed R band luminosity profiles and structural
parameters for a total of 430 brightest cluster galaxies (BCGs), down to a
limiting surface brightness of 24.5 mag/arcsec^2. Light profiles were initially
fitted with a Sersic's R^(1/n) model, but we found that 205 (~48) BCGs require
a double component model to accurately match their light profiles. The best fit
for these 205 galaxies is an inner Sersic model, with indices n~1-7, plus an
outer exponential component.
Thus, we establish the existence of two categories of the BCGs luminosity
profiles: single and double component profiles. We found that double profile
BCGs are brighter ~0.2 mag than single profile BCG. In fact, the
Kolmogorov-Smirnov test applied to these subsamples indicates that they have
different total magnitude distributions, with mean values M_R=-23.8 +/- 0.6 mag
for single profile BCGs and M_R=-24.0 +/- 0.5 mag for double profile BCGs. We
find that partial luminosities for both subsamples are indistinguishable up to
r = 15 kpc, while for r > 20 kpc the luminosities we obtain are on average 0.2
mag brighter for double profile BCGs. This result indicates that extra-light
for double profile BCGs does not come from the inner region but from the outer
regions of these galaxies.
The best fit slope of the Kormendy relation for the whole sample is a = 3.13
+/- 0.04$. However, when fitted separately, single and double profile BCGs show
different slopes: a_(single) = 3.29 +/- 0.06 and a_(double)= 2.79 +/- 0.08.
On the other hand, we did not find differences between these two BCGs
categories when we compared global cluster properties such as the BCG-projected
position relative to the cluster X-ray center emission, X-ray luminosity, or
BCG orientation with respect to the cluster position angle.Comment: August 2011 issue of ApJS, volume 195, 15
http://iopscience.iop.org/0067-0049/195/2/1
A Search for the Most Massive Galaxies. III. Global and Central Structure
We used the Advanced Camera for Surveys on board the Hubble Space Telescope
to obtain high resolution i-band images of the centers of 23 single galaxies,
which were selected because they have SDSS velocity dispersions larger than 350
km/s. The surface brightness profiles of the most luminous of these objects
(M_i<-24) have well-resolved `cores' on scales of 150-1000 pc, and share
similar properties to BCGs. The total luminosity of the galaxy is a better
predictor of the core size than is the velocity dispersion. The correlations of
luminosity and velocity dispersion with core size agree with those seen in
previous studies of galaxy cores. Because of high velocity dispersions, our
sample of galaxies can be expected to harbor the most massive black holes, and
thus have large cores with large amounts of mass ejection. The mass-deficits
inferred from core-Sersic fits to the surface-brightness profiles are
approximately double the black-hole masses inferred from the M_bh-sigma
relation and the same as those inferred from the M_bh-L relation. The less
luminous galaxies (M_i>-23) tend to have steeper `power-law' inner profiles,
higher-ellipticity, diskier isophotes, and bulge-to-total ratios of order 0.5
-- all of which suggest that they are `fast-rotators' and rotational motions
could have contaminated the velocity dispersion estimate. There are obvious
dust features within about 300 pc of the center in about 35% of the sample,
predominantly in power-law rather than core galaxies.Comment: 27 Pages, 22 Figures, 2 Tables, Accepted for Publication in MNRA
Impaired Chronotropic Response to Exercise Stress Testing in Patients With Diabetes Predicts Future Cardiovascular Events
OBJECTIVESâ To assess the association between impaired chronotropic response (CR) and adverse events among patients with diabetes referred for exercise treadmill testing (ETT)
Supermassive Black Holes and the Evolution of Galaxies
Black holes, an extreme consequence of the mathematics of General Relativity,
have long been suspected of being the prime movers of quasars, which emit more
energy than any other objects in the Universe. Recent evidence indicates that
supermassive black holes, which are probably quasar remnants, reside at the
centers of most galaxies. As our knowledge of the demographics of these relics
of a violent earlier Universe improve, we see tantalizing clues that they
participated intimately in the formation of galaxies and have strongly
influenced their present-day structure.Comment: 20 pages, - This is a near-duplicate of the paper in Nature 395, A14,
1998 (Oct 1
- âŠ