274 research outputs found
A re-evaluation of the central velocity-dispersion profile in NGC 6388
Recently, two independent groups found very different results when measuring
the central velocity dispersion of the galactic globular cluster NGC 6388 with
different methods. While L\"utzgendorf et al. (2011) found a rising profile and
a high central velocity dispersion (23.3 km/s), measurements obtained by
Lanzoni et al. (2013) showed a value 40% lower. The value of the central
velocity dispersion has a serious impact on the mass and possible presence of
an intermediate-mass black hole at the center of NGC 6388. We use a photometric
catalog of NGC 6388 to create a simulated SINFONI and ARGUS dataset. The
construction of the IFU data cube is done with different observing conditions
reproducing the conditions reported for the original observations as closely as
possible. In addition, we produce an N-body realization of a 10^6 M_SUN stellar
cluster with the same photometric properties as NGC 6388 to account for
unresolved stars. We find that the individual radial velocities, i.e. the
measurements from the simulated SINFONI data, are systematically biased towards
lower velocity dispersions. The reason is that due to the wings in the point
spread function the velocities get biased towards the mean cluster velocity.
This study shows that even with AO supported observations, individual radial
velocities in crowded fields are likely to be biased. The ARGUS observations do
not show this kind of bias but were found to have larger uncertainties than
previously obtained. We find a bias towards higher velocity dispersions in the
ARGUS pointing when fixing the extreme velocities of the three brightest stars
but find those variations are within the determined uncertainties. We rerun
Jeans models and fit the kinematic profile with the new uncertainties. This
yields a BH mass of M_BH = (2.8 +- 0.4) x 10^4 M_SUN and M/L ratio M/L = (1.6
+- 0.1) M_SUN/L_SUN, consistent with our previous results.Comment: 8 pages, 8 figure, accepted for publication in A&
The dynamical state of the Globular Cluster M10 (NGC 6254)
Studying the radial variation of the stellar mass function in globular
clusters (GCs) has proved a valuable tool to explore the collisional dynamics
leading to mass segregation and core collapse. In order to study the radial
dependence of the luminosity and mass function of M 10, we used ACS/HST deep
high resolution archival images, reaching out to approximately the cluster's
half-mass radius (rhm), combined with deep WFPC2 images that extend our radial
coverage to more than 2 rhm. From our photometry, we derived a radial mass
segregation profile and a global mass function that we compared with those of
simulated clusters containing different energy sources (namely hard binaries
and/or an IMBH) able to halt core collapse and to quench mass segregation. A
set of direct N-body simulations of GCs, with and without an IMBH of mass 1% of
the total cluster mass, comprising different initial mass functions (IMFs) and
primordial binary fractions, was used to predict the observed mass segregation
profile and mass function. The mass segregation profile of M 10 is not
compatible with cluster models without either an IMBH or primordial binaries,
as a source of energy appears to be moderately quenching mass segregation in
the cluster. Unfortunately, the present observational uncertainty on the binary
fraction in M10 does not allow us to confirm the presence of an IMBH in the
cluster, since an IMBH, a dynamically non-negligible binary fraction (~ 5%), or
both can equally well explain the radial dependence of the cluster mass
function.Comment: 15 pages, 8 figures, accepted for publication on Ap
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&
Mass Segregation in NGC 2298: limits on the presence of an Intermediate Mass Black Hole
[abridged] Theoretical investigations have suggested the presence of
Intermediate Mass Black Holes (IMBHs, with masses in the 100-10000 Msun range)
in the cores of some Globular Clusters (GCs). In this paper we present the
first application of a new technique to determine the presence or absence of a
central IMBH in globular clusters that have reached energy equipartition via
two-body relaxation. The method is based on the measurement of the radial
profile for the average mass of stars in the system, using the fact that a
quenching of mass segregation is expected when an IMBH is present. Here we
measure the radial profile of mass segregation using main-sequence stars for
the globular cluster NGC 2298 from resolved source photometry based on HST-ACS
data. The observations are compared to expectations from direct N-body
simulations of the dynamics of star clusters with and without an IMBH. The mass
segregation profile for NGC 2298 is quantitatively matched to that inferred
from simulations without a central massive object over all the radial range
probed by the observations, that is from the center to about two half-mass
radii. Profiles from simulations containing an IMBH more massive than ~ 300-500
Msun (depending on the assumed total mass of NGC 2298) are instead inconsistent
with the data at about 3 sigma confidence, irrespective of the IMF and binary
fraction chosen for these runs. While providing a null result in the quest of
detecting a central black hole in globular clusters, the data-model comparison
carried out here demonstrates the feasibility of the method which can also be
applied to other globular clusters with resolved photometry in their cores.Comment: 21 pages, 3 figures, ApJ accepte
Intermediate-mass black holes in Globular Clusters
For a sample of nine Galactic globular clusters we measured the inner
kinematic profiles with integral-field spectroscopy that we combined with
existing outer kinematic measurements and HST luminosity profiles. With this
information we are able to detect the crucial rise in the velocity-dispersion
profile which indicates the presence of a central black hole. In addition,
N-body simulations compared to our data will give us a deeper insight in the
properties of clusters with black holes and stronger selection criteria for
further studies. For the first time, we obtain a homogeneous sample of globular
cluster integral- field spectroscopy which allows a direct comparison between
clusters with and without an intermediate-mass black hole.Comment: 4 pages, 2 figures. To appear in the conference proceedings "Reading
the book of globular clusters with the lens of stellar evolution", Mem. S. A.
It. Eds. P. Ventura, C. Charbonnel, M. Castellani and M. Di Criscienz
On the Fundamental Line of Galactic and Extragalactic Globular Clusters
In a previous paper we found that the Globular Clusters of our Galaxy lie
around a line in the log(Re), SBe, log(sigma) parameter space, with a moderate
degree of scatter and remarkable axi-symmetry. This implies the existence of a
purely photometric scaling law obtained by projecting such a line onto the
log(Re), SBe plane. Such photometric quantities are readily available for large
samples of clusters, as opposed to stellar velocity dispersion data. We study a
sample of 129 Galactic and extragalactic clusters on such photometric plane in
the V-band. We look for a linear relation between SBe and log(Re) and study how
the scatter around it is influenced by age and dynamical environment. We
interpret our results as a test on the evolutionary versus primordial origin of
the Fundamental Line. We perform a detailed analysis of surface brightness
profiles, which allows us to present a catalogue of structural properties,
without relying on a given dynamical model. We find a linear relation between
SBe and log(Re), in the form SBe = (5.25 +- 0.44) log(Re) + (15.58 +- 0.28),
where SBe is measured in mag/arcsec^2 and Re in parsec. Both young and old
clusters lie on the scaling law, with a scatter of approximately 1 mag in SBe.
However, young clusters display more scatter and a clear trend of such scatter
with age, which old clusters do not. Such trend becomes tighter if cluster age
is measured in units of the cluster half-light relaxation time. Two-body
relaxation therefore plays a major role, together with passive stellar
population evolution, in shaping the relation between SBe, log(Re), and cluster
age. We argue that the log(Re)-SBe relation and hence the Fundamental Line
scaling law is not primordially set at cluster formation, but rather is the
result of combined stellar evolution and collisional dynamical evolution.Comment: Accepted for publication on Astronomy and Astrophysics, official
acceptance date November 2, 200
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&
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&
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