202 research outputs found
High-velocity stars in the cores of globular clusters: The illustrative case of NGC 2808
We report the detection of five high-velocity stars in the core of the
globular cluster NGC 2808. The stars lie on the the red giant branch and show
total velocities between 40 and 45 km/s. For a core velocity dispersion sigma_c
= 13.4 km/s, this corresponds to up to 3.4 sigma_c. These velocities are close
to the estimated escape velocity (~ 50 km/s) and suggest an ejection from the
core. Two of these stars have been confirmed in our recent integral field
spectroscopy data and we will discuss them in more detail here. These two red
giants are located at a projected distance of ~ 0.3 pc from the center.
According to their positions on the color magnitude diagram, both stars are
cluster members. We investigate several possible origins for the high
velocities of the stars and conceivable ejection mechanisms. Since the
velocities are close to the escape velocity, it is not obvious whether the
stars are bound or unbound to the cluster. We therefore consider both cases in
our analysis. We perform numerical simulations of three-body dynamical
encounters between binaries and single stars and compare the resulting velocity
distributions of escapers with the velocities of our stars. We compare the
predictions for a single dynamical encounter with a compact object with those
of a sequence of two-body encounters due to relaxation. If the stars are
unbound, the encounter must have taken place recently, when the stars were
already in the giant phase. After including binary fractions and black-hole
retention fractions, projection effects, and detection probabilities from
Monte-Carlo simulations, we estimate the expected numbers of detections for all
the different scenarios. Based on these numbers, we conclude that the most
likely scenario is that the stars are bound and were accelerated by a single
encounter between a binary of main-sequence stars and a ~ 10 M_sun black hole.Comment: 13 pages, 12 figures, Accepted for publication in A&
Density Fluctuations in an Electrolyte from Generalized Debye-Hueckel Theory
Near-critical thermodynamics in the hard-sphere (1,1) electrolyte is well
described, at a classical level, by Debye-Hueckel (DH) theory with (+,-) ion
pairing and dipolar-pair-ionic-fluid coupling. But DH-based theories do not
address density fluctuations. Here density correlations are obtained by
functional differentiation of DH theory generalized to {\it non}-uniform
densities of various species. The correlation length diverges universally
at low density as (correcting GMSA theory). When
one has as
where the amplitudes compare informatively with experimental data.Comment: 5 pages, REVTeX, 1 ps figure included with epsf. Minor changes,
references added. Accepted for publication in Phys. Rev. Let
The Candidate Intermediate-Mass Black Hole in the Globular Cluster M54
Ibata et al. reported evidence for density and kinematic cusps in the
Galactic globular cluster M54, possibly due to the presence of a 9400
solar-mass black hole. Radiative signatures of accretion onto M54's candidate
intermediate-mass black hole (IMBH) could bolster the case for its existence.
Analysis of new Chandra and recent Hubble Space Telescope astrometry rules out
the X-ray counterpart to the candidate IMBH suggested by Ibata et al. If an
IMBH exists in M54, then it has an Eddington ratio of L(0.3-8 keV) / L(Edd) <
1.4 x 10^(-10), more similar to that of the candidate IMBH in M15 than that in
G1. From new imaging with the NRAO Very Large Array, the luminosity of the
candidate IMBH is L(8.5 GHz) < 3.6 x 10^29 ergs/s (3 sigma). Two background
active galaxies discovered toward M54 could serve as probes of its intracluster
medium.Comment: 4 pages; 2 figures; emulateapj.cls; to appear in A
Extreme mass ratio inspiral rates: dependence on the massive black hole mass
We study the rate at which stars spiral into a massive black hole (MBH) due
to the emission of gravitational waves (GWs), as a function of the mass M of
the MBH. In the context of our model, it is shown analytically that the rate
approximately depends on the MBH mass as M^{-1/4}. Numerical simulations
confirm this result, and show that for all MBH masses, the event rate is
highest for stellar black holes, followed by white dwarfs, and lowest for
neutron stars. The Laser Interferometer Space Antenna (LISA) is expected to see
hundreds of these extreme mass ratio inspirals per year. Since the event rate
derived here formally diverges as M->0, the model presented here cannot hold
for MBHs of masses that are too low, and we discuss what the limitations of the
model are.Comment: Accepted to CQG, special LISA issu
Internal properties of ultracompact dwarf galaxies in the Virgo cluster
We present new imaging and spectroscopic observations of six ultracompact dwarf (UCD) galaxies in the Virgo Cluster, along with reanalyzed data for five Fornax Cluster UCDs. These are the most luminous UCDs: -14 mag < M-V < -12 mag. Our Hubble Space Telescope imaging shows that most of the UCDs have shallow or steep cusps in their cores; only one UCD has a flat "King'' core. None of the UCDs show tidal cutoffs down to our limiting surface brightness. Spectroscopic analysis shows that Virgo UCDs are old ( older than 8 Gyr) and have metallicities in the range from [Z/H] = -1.35 to +0.35 dex. Five Virgo UCDs have supersolar [alpha/Fe] abundance ratios, and one Virgo UCD has a solar abundance ratio. The supersolar [alpha/Fe] abundances are typical of old stellar populations found in globular clusters and elliptical galaxies. We find that Virgo UCDs have structural and dynamical properties similar to Fornax UCDs. The Virgo and Fornax UCDs all have masses approximate to(2-9) x 10(7) M-circle dot and mass-to-light ratios approximate to(3-5) M-circle dot/L-circle dot,(V). The dynamical mass-to-light ratios for Virgo UCDs are consistent with simple stellar population model predictions: the Virgo UCDs do not require dark matter to explain their mass-to-light ratios. We conclude that the internal properties of Virgo UCDs are consistent with them being the high-mass/high-luminosity extreme of known globular cluster populations. We refrain from any firm conclusions on Fornax UCD origins until accurate age, metallicity, and alpha-abundance estimates are obtained for them. Some of our results, notably the fundamental plane projections, are consistent with the formation of UCDs by the simple removal of the halo from the nuclei of nucleated dwarf galaxies. However, the ages, metallicities, and abundances for Virgo UCDs are not consistent with this simple stripping model. It might be consistent with more sophisticated models of the stripping process that include the effects of gas removal on the chemical evolution of the nuclei
Gravitational Dynamics of Large Stellar Systems
Internal dynamical evolution can drive stellar systems into states of high
central density. For many star clusters and galactic nuclei, the time scale on
which this occurs is significantly less than the age of the universe. As a
result, such systems are expected to be sites of frequent interactions among
stars, binary systems, and stellar remnants, making them efficient factories
for the production of compact binaries, intermediate-mass black holes, and
other interesting and eminently observable astrophysical exotica. We describe
some elements of the competition among stellar dynamics, stellar evolution, and
other mechanisms to control the dynamics of stellar systems, and discuss
briefly the techniques by which these systems are modeled and studied.
Particular emphasis is placed on pathways leading to massive black holes in
present-day globular clusters and other potentially detectable sources of
gravitational radiation.Comment: 21 pages, invited talk presented at the 18th International Conference
on General Relativity and Gravitation (GRG18), Sydney, July 2007. To appear
in Classical and Quantum Gravit
Mechanism for the Suppression of Intermediate-Mass Black Holes
A model for the formation of supermassive primordial black holes in galactic
nuclei with the simultaneous suppression of the formation of intermediate-mass
black holes is presented. A bimodal mass function for black holes formed
through phase transitions in a model with a "Mexican hat" potential has been
found. The classical motion of the phase of a complex scalar field during
inflation has been taken into account. Possible observational manifestations of
primordial black holes in galaxies and constraints on their number are
discussed.Comment: 12 pages, 2 figure
Influence of hydrodynamics on many-particle diffusion in 2D colloidal suspensions
We study many-particle diffusion in 2D colloidal suspensions with full
hydrodynamic interactions through a novel mesoscopic simulation technique. We
focus on the behaviour of the effective scaled tracer and collective diffusion
coefficients and , where is the
single-particle diffusion coefficient, as a function of the density of the
colloids . At low Schmidt numbers , we find that
hydrodynamics has essentially no effect on the behaviour of . At
larger , is enhanced at all densities, although the
differences compared to the case without hydrodynamics are minor. The
collective diffusion coefficient, on the other hand, is much more strongly
coupled to hydrodynamical conservation laws and is distinctly different from
the purely dissipative case
Prospects for a Dark Matter annihilation signal towards the Sagittarius dwarf galaxy with ground based Cherenkov telescopes
Dwarf galaxies are widely believed to be among the best targets for indirect
dark matter searches using high-energy gamma rays; and indeed gamma-ray
emission from these objects has long been a subject of detailed study for
ground-based atmospheric Cherenkov telescopes. Here, we update current
exclusion limits obtained on the closest dwarf, the Sagittarius dwarf galaxy,
in light of recent realistic dark matter halo models. The constraints on the
velocity-weighted annihilation cross section of the dark matter particle are of
a few 10 cms in the TeV energy range for a 50 h exposure.
The limits are extrapolated to the sensitivities of future Cherenkov Telescope
Arrays. For 200 h of observation time, the sensitivity at 95% C.L. reaches
10 cms. Possible astrophysical backgrounds from gamma-ray
sources dissembled in Sagittarius dwarf are studied. It is shown that with
long-enough observation times, gamma-ray background from millisecond pulsars in
a globular cluster contained within Sagittarius dwarf may limit the sensitivity
to dark matter annihilations.Comment: 12 pages, 5 figures, 2 tables, accepted for publication in Ap
Compact stellar systems in the Fornax cluster: a UV perspective
In recent years, increasing evidence for chemical complexity and multiple
stellar populations in massive globular clusters (GCs) has emerged, including
extreme horizontal branches (EHBs) and UV excess. Our goal is to improve our
understanding of UV excess in the regime of both massive GCs and ultra-compact
dwarf galaxies (UCDs). To this end, we use deep archival GALEX data of the
central Fornax cluster to measure NUV and FUV magnitudes of UCDs and massive
GCs. We obtain NUV photometry for a sample of 35 compact objects with
-13.5<M_V<-10 mag. Of those, 21 objects also have FUV photometry. Roughly half
of the sources fall into the UCD luminosity regime (M_V <=-11 mag). We find
that seven out of 17 massive Fornax GCs exhibit a NUV excess with respect to
expectations from stellar population models, even for models with enhanced
Helium abundance. This suggests that not only He-enrichment has contributed to
forming the EHB population of these GCs. The GCs extend to stronger UV excess
than GCs in M31 and massive GCs in M87, at the 97% confidence level. Most of
the UCDs with FUV photometry also show evidence for UV excess, but their UV
colours can be matched by isochrones with enhanced Helium abundances and old
ages 12-14 Gyrs. We find that Fornax compact objects with X-ray emission
detected from Chandra images are almost disjunct in colour from compact objects
with GALEX UV detection, with only one X-ray source among the 35 compact
objects. However, since this source is one of the three most UV bright GCs, we
cannot exclude that the physical processes causing X-ray emission also
contribute to some of the observed UV excess.Comment: Research Note, 7 pages, 3 figures, accepted for publication in A&
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