188 research outputs found
Central kinematics of the globular cluster NGC 2808: Upper limit on the mass of an intermediate-mass black hole
Globular clusters are an excellent laboratory for stellar population and
dynamical research. Recent studies have shown that these stellar systems are
not as simple as previously assumed. With multiple stellar populations as well
as outer rotation and mass segregation they turn out to exhibit high
complexity. This includes intermediate-mass black holes which are proposed to
sit at the centers of some massive globular clusters. Today's high angular
resolution ground based spectrographs allow velocity-dispersion measurements at
a spatial resolution comparable to the radius of influence for plausible IMBH
masses, and to detect changes in the inner velocity-dispersion profile.
Together with high quality photometric data from HST, it is possible to
constrain black-hole masses by their kinematic signatures. We determine the
central velocity-dispersion profile of the globular cluster NGC 2808 using
VLT/FLAMES spectroscopy. In combination with HST/ACS data our goal is to probe
whether this massive cluster hosts an intermediate-mass black hole at its
center and constrain the cluster mass to light ratio as well as its total mass.
We derive a velocity-dispersion profile from integral field spectroscopy in the
center and Fabry Perot data for larger radii. High resolution HST data are used
to obtain the surface brightness profile. Together, these data sets are
compared to dynamical models with varying parameters such as mass to light
ratio profiles and black-hole masses. Using analytical Jeans models in
combination with variable M/L profiles from N-body simulations we find that the
best fit model is a no black hole solution. After applying various Monte Carlo
simulations to estimate the uncertainties, we derive an upper limit of the back
hole mass of M_BH < 1 x 10^4 M_SUN (with 95 % confidence limits) and a global
mass-to-light ratio of M/L_V = (2.1 +- 0.2) M_SUN/L_SUN.Comment: 12 pages, 9 figures, 2 tables, accepted for publication in A&
Central rotations of Milky Way Globular Clusters
Most Milky Way globular clusters (GCs) exhibit measurable flattening, even if
on a very low level. Both cluster rotation and tidal fields are thought to
cause this flattening. Nevertheless, rotation has only been confirmed in a
handful of GCs, based mostly on individual radial velocities at large radii. We
are conducting a survey of the central kinematics of Galactic GCs using the new
Integral Field Unit instrument VIRUS-W. We detect rotation in all 11 GCs that
we have observed so far, rendering it likely that a large majority of the Milky
Way GCs rotate. We use published catalogs of the ACS survey of GCs to derive
central ellipticities and position angles. We show that in all cases where the
central ellipticity permits an accurate measurement of the position angle,
those angles are in excellent agreement with the kinematic position angles that
we derive from the VIRUS-W velocity fields. We find an unexpected tight
correlation between central rotation and outer ellipticity, indicating that
rotation drives flattening for the objects in our sample. We also find a tight
correlation between central rotation and published values for the central
velocity dispersion, most likely due to rotation impacting the old dispersion
measurements.Comment: 6 pages, 3 figures; accepted for publication in ApJ Letter
A Dynamical N-body Model for the Central Region of Centauri
Supermassive black holes (SMBHs) are fundamental keys to understand the
formation and evolution of their host galaxies. However, the formation and
growth of SMBHs are not yet well understood. One of the proposed formation
scenarios is the growth of SMBHs from seed intermediate-mass black holes
(IMBHs, 10^2 to 10^5 M_{\odot}) formed in star clusters. In this context, and
also with respect to the low mass end of the M-sigma relation for galaxies,
globular clusters are in a mass range that make them ideal systems to look for
IMBHs. Among Galactic star clusters, the massive cluster Centauri is a
special target due to its central high velocity dispersion and also its
multiple stellar populations. We study the central structure and dynamics of
the star cluster Centauri to examine whether an IMBH is necessary to
explain the observed velocity dispersion and surface brightness profiles. We
perform direct N-body simulations to follow the dynamical evolution of
Centauri. The simulations are compared to the most recent data-sets in order to
explain the present-day conditions of the cluster and to constrain the initial
conditions leading to the observed profiles. We find that starting from
isotropic spherical multi-mass King models and within our canonical
assumptions, a model with a central IMBH mass of 2% of the cluster stellar
mass, i.e. a 5x10^4 M_{\odot} IMBH, provides a satisfactory fit to both the
observed shallow cusp in surface brightness and the continuous rise towards the
center of the radial velocity dispersion profile. In our isotropic spherical
models, the predicted proper motion dispersion for the best-fit model is the
same as the radial velocity dispersion one. (abridged)Comment: 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
Rescaled mean spherical approximation for colloidal mixtures
In this work, the rescaled mean spherical approximation (RMSA) for colloidal mixtures interacting via a DLVO-type potential is developed, and its application to suspensions of highly charged macroions is illustrated. For this purpose we introduce a simple scheme to solve the mean spherical approximation (MSA) for Yukawa mixtures with factorized coupling parameters. This scheme consists of the mapping of the Yukawa system onto a corresponding primitive model system. Such a correspondence is used as a device for the calculation of the static structure functions of the original Yukawa mixture. Within this scheme, a straightforward implementation of the rescaling procedure is performed, which allows for the calculation of partial structure factors in strongly interacting mixtures. The rescaling procedure we use is an extension of that introduced by Hansen and Hayter for monodisperse suspensions. The structure factors obtained with the rescaled mean spherical approximation compare well with computer simulation results. The advantages and limitations of the RMSA are also discussed in some detail.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/28352/1/0000113.pd
General Non-equilibrium Theory of Colloid Dynamics
A non-equilibrium extension of Onsager's canonical theory of thermal
fluctuations is employed to derive a self-consistent theory for the description
of the statistical properties of the instantaneous local concentration profile
n(r,t) of a colloidal liquid in terms of the coupled time evolution equations
of its mean value n(r,t) and of the covariance {\sigma}(r,r';t) \equiv
of its fluctuations {\delta}n(r, t) = n(r, t) -
n(r, t). These two coarse-grained equations involve a local mobility function
b(r, t) which, in its turn, is written in terms of the memory function of the
two-time correlation function C(r, r' ; t, t') \equiv <{\delta}n(r,
t){\delta}n(r',t')>. For given effective interactions between colloidal
particles and applied external fields, the resulting self-consistent theory is
aimed at describing the evolution of a strongly correlated colloidal liquid
from an initial state with arbitrary mean and covariance n^0(r) and
{\sigma}^0(r,r') towards its equilibrium state characterized by the equilibrium
local concentration profile n^(eq)(r) and equilibrium covariance
{\sigma}^(eq)(r,r').
This theory also provides a general theoretical framework to describe
irreversible processes associated with dynamic arrest transitions, such as
aging, and the effects of spatial heterogeneities
Kinematic signature of an intermediate-mass black hole in the globular cluster NGC 6388
Intermediate-mass black holes (IMBHs) are of interest in a wide range of
astrophysical fields. In particular, the possibility of finding them at the
centers of globular clusters has recently drawn attention. IMBHs became
detectable since the quality of observational data sets, particularly those
obtained with HST and with high resolution ground based spectrographs, advanced
to the point where it is possible to measure velocity dispersions at a spatial
resolution comparable to the size of the gravitational sphere of influence for
plausible IMBH masses. We present results from ground based VLT/FLAMES
spectroscopy in combination with HST data for the globular cluster NGC 6388.
The aim of this work is to probe whether this massive cluster hosts an
intermediate-mass black hole at its center and to compare the results with the
expected value predicted by the scaling relation. The
spectroscopic data, containing integral field unit measurements, provide
kinematic signatures in the center of the cluster while the photometric data
give information of the stellar density. Together, these data sets are compared
to dynamical models and present evidence of an additional compact dark mass at
the center: a black hole. Using analytical Jeans models in combination with
various Monte Carlo simulations to estimate the errors, we derive (with 68%
confidence limits) a best fit black-hole mass of and a global mass-to-light ratio of $M/L_V = (1.6 \pm 0.3) \
M_{\odot}/L_{\odot}$.Comment: 12 pages, 12 figures, Accepted for publication in A&
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