182 research outputs found
Biases in the determination of dynamical parameters of star clusters: today and in the Gaia era
The structural and dynamical properties of star clusters are generally
derived by means of the comparison between steady-state analytic models and the
available observables. With the aim of studying the biases of this approach, we
fitted different analytic models to simulated observations obtained from a
suite of direct N-body simulations of star clusters in different stages of
their evolution and under different levels of tidal stress to derive mass, mass
function and degree of anisotropy. We find that masses can be
under/over-estimated up to 50% depending on the degree of relaxation reached by
the cluster, the available range of observed masses and distances of radial
velocity measures from the cluster center and the strength of the tidal field.
The mass function slope appears to be better constrainable and less sensitive
to model inadequacies unless strongly dynamically evolved clusters and a
non-optimal location of the measured luminosity function are considered. The
degree and the characteristics of the anisotropy developed in the N-body
simulations are not adequately reproduced by popular analytic models and can be
detected only if accurate proper motions are available. We show how to reduce
the uncertainties in the mass, mass-function and anisotropy estimation and
provide predictions for the improvements expected when Gaia proper motions will
be available in the near future.Comment: 14 pages, 8 figures, accepted for publication by MNRA
Modeling the dynamical evolution of the M87 globular cluster system
We study the dynamical evolution of the M87 globular cluster system (GCS)
with a number of numerical simulations. We explore a range of different initial
conditions for the GCS mass function (GCMF), for the GCS spatial distribution
and for the GCS velocity distribution. We confirm that an initial power-law
GCMF like that observed in young cluster systems can be readily transformed
through dynamical processes into a bell-shaped GCMF. However,only models with
initial velocity distributions characterized by a strong radial anisotropy
increasing with the galactocentric distance are able to reproduce the observed
constancy of the GCMF at all radii.We show that such strongly radial orbital
distributions are inconsistent with the observed kinematics of the M87 GCS. The
evolution of models with a bell-shaped GCMF with a turnover similar to that
currently observed in old GCS is also investigated. We show that models with
this initial GCMF can satisfy all the observational constraints currently
available on the GCS spatial distribution,the GCS velocity distribution and on
the GCMF properties.In particular these models successfully reproduce both the
lack of a radial gradient of the GCS mean mass recently found in an analysis of
HST images of M87 at multiple locations, and the observed kinematics of the M87
GCS.Our simulations also show that evolutionary processes significantly affect
the initial GCS properties by leading to the disruption of many clusters and
changing the masses of those which survive.The preferential disruption of inner
clusters flattens the initial GCS number density profile and it can explain the
rising specific frequency with radius; we show that the inner flattening
observed in the M87 GCS spatial distribution can be the result of the effects
of dynamical evolution on an initially steep density profile. (abridged)Comment: 15 pages,14 figures;accepted for publication in The Astrophysical
Journa
Mass modelling globular clusters in the Gaia era: a method comparison using mock data from an N-body simulation of M 4
As we enter a golden age for studies of internal kinematics and dynamics of Galactic globular clusters (GCs), it is timely to assess the performance of modelling techniques in recovering the mass, mass profile, and other dynamical properties of GCs. Here, we compare different mass-modelling techniques (distribution function (DF)-based models, Jeans models, and a grid of N-body models) by applying them to mock observations from a star-by-star N-body simulation of the GC M 4 by Heggie. The mocks mimic existing and anticipated data for GCs: surface brightness or number density profiles, local stellar mass functions, line-of-sight velocities, and Hubble Space Telescope-and Gaia-like proper motions. We discuss the successes and limitations of the methods. We find that multimass DF-based models, Jeans, and N-body models provide more accurate mass profiles compared to single-mass DF-based models. We highlight complications in fitting the kinematics in the outskirts due to energetically unbound stars associated with the cluster ('potential escapers', captured neither by truncated DF models nor by N-body models of clusters in isolation), which can be avoided with DF-based models including potential escapers, or with Jeans models. We discuss ways to account for mass segregation. For example, three-component DF-based models with freedom in their mass function are a simple alternative to avoid the biases of single-mass models (which systematically underestimate the total mass, half-mass radius, and central density), while more realistic multimass DF-based models with freedom in the remnant content represent a promising avenue to infer the total mass and the mass function of remnants
Star cluster dynamics
Dynamical evolution plays a key role in shaping the current properties of
star clusters and star cluster systems. A detailed understanding of the effects
of evolutionary processes is essential to be able to disentangle the properties
which result from dynamical evolution from those imprinted at the time of
cluster formation. In this review, we focus our attention on globular clusters
and review the main physical ingredients driving their early and long-term
evolution, describe the possible evolutionary routes and show how cluster
structure and stellar content are affected by dynamical evolution.Comment: 20 pages, 2 figures. To appear as invited review article in a special
issue of the Phil. Trans. Royal Soc. A: Ch. 7 "Star clusters as tracers of
galactic star-formation histories" (ed. R. de Grijs). Fully peer reviewed.
LaTeX, requires rspublic.cls style fil
Spatially Resolved Kinematics of an Ultra-Compact Dwarf Galaxy
We present the internal kinematics of UCD3, the brightest known ultra-compact
dwarf galaxy (UCD) in the Fornax cluster, making this the first UCD with
spatially resolved spectroscopy. Our study is based on seeing-limited
observations obtained with the ARGUS Integral Field Unit of the VLT/FLAMES
spectrograph under excellent seeing conditions (0.5 - 0.67 arcsec FWHM). The
velocity field of UCD3 shows the signature of weak rotation, comparable to that
found in massive globular clusters. Its velocity dispersion profile is fully
consistent with an isotropic velocity distribution and the assumption that mass
follows the light distribution obtained from Hubble Space Telescope imaging. In
particular, there is no evidence for the presence of an extended dark matter
halo contributing a significant (>~33 per cent within R < 200 pc) mass
fraction, nor for a central black hole more massive than ~5 per cent of the
UCD's mass. While this result does not exclude a galaxian origin for UCD3, we
conclude that its internal kinematics are fully consistent with it being a
massive star cluster.Comment: 5 pages, 3 figures; accepted for publication in MNRAS Letter
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
N-body simulations of gravitational dynamics
We describe the astrophysical and numerical basis of N-body simulations, both
of collisional stellar systems (dense star clusters and galactic centres) and
collisionless stellar dynamics (galaxies and large-scale structure). We explain
and discuss the state-of-the-art algorithms used for these quite different
regimes, attempt to give a fair critique, and point out possible directions of
future improvement and development. We briefly touch upon the history of N-body
simulations and their most important results.Comment: invited review (28 pages), to appear in European Physics Journal Plu
Dynamical Processes in Globular Clusters
Globular clusters are among the most congested stellar systems in the
Universe. Internal dynamical evolution drives them toward states of high
central density, while simultaneously concentrating the most massive stars and
binary systems in their cores. As a result, these clusters are expected to be
sites of frequent close encounters and physical collisions between stars and
binaries, making them efficient factories for the production of interesting and
observable astrophysical exotica. I describe some elements of the competition
among stellar dynamics, stellar evolution, and other processes that control
globular cluster dynamics, with particular emphasis on pathways that may lead
to the formation of blue stragglers.Comment: Chapter 10, in Ecology of Blue Straggler Stars, H.M.J. Boffin, G.
Carraro & G. Beccari (Eds), Astrophysics and Space Science Library, Springe
The Nature of UCDs: Internal Dynamics from an Expanded Sample and Homogeneous Database
We have obtained high-resolution spectra of 23 ultra-compact dwarf galaxies
(UCDs) in the Fornax cluster with -10.4>M_V>-13.5 mag (10^6<M/M_*<10^8), using
FLAMES/Giraffe at the VLT. This is the largest homogeneous data set of UCD
internal dynamics assembled to date. We derive dynamical M/L ratios for 15 UCDs
covered by HST imaging. In the M_V-sigma plane, UCDs with M_V<-12 mag are
consistent with the extrapolated Faber-Jackson relation for luminous
ellipticals, while fainter UCDs are closer to the extrapolated globular cluster
(GC) relation. At a given metallicity, Fornax UCDs have on average 30-40% lower
M/L ratios than Virgo UCDs, suggesting possible differences in age or dark
matter content between Fornax and Virgo UCDs. For our sample of Fornax UCDs we
find no significant correlation between M/L ratio and mass. We combine our data
with available M/L ratio measurements of compact stellar systems with
10^4<M/M_*<10^8, and normalise all M/L estimates to solar metallicity. We find
that UCDs (M > 2*10^6 M_*) have M/L ratios twice as large as GCs (M < 2*10^6
M_*). We show that stellar population models tend to under-predict dynamical
M/L ratios of UCDs and over-predict those of GCs. Considering the scaling
relations of stellar spheroids, UCDs align well along the 'Fundamental
Manifold', constituting the small-scale end of the galaxy sequence. The
alignment for UCDs is especially clear for r_e >~ 7 pc, which corresponds to
dynamical relaxation times that exceed a Hubble time. In contrast, GCs exhibit
a broader scatter and do not appear to align along the manifold. We argue that
UCDs are the smallest dynamically un-relaxed stellar systems, with M > 2*10^6
M_* and 7<r_e<100 pc. Future studies should aim at explaining the elevated M/L
ratios of UCDs and the environmental dependence of their properties.Comment: 17 pages, 14 figures, accepted for publication in A&A. V3 taking into
account proof corrections: Table 3 radial velocity entries corrected by
heliocentric correction, updated sigma entries in Table 5 for a few CenA
sources, updated references for G1 and omega Ce
Evidence for Environmentally Dependent Cluster Disruption in M83
Using multi-wavelength imaging from the Wide Field Camera 3 on the Hubble
Space Telescope we study the stellar cluster populations of two adjacent fields
in the nearby face-on spiral galaxy, M83. The observations cover the galactic
centre and reach out to ~6 kpc, thereby spanning a large range of environmental
conditions, ideal for testing empirical laws of cluster disruption. The
clusters are selected by visual inspection to be centrally concentrated,
symmetric, and resolved on the images. We find that a large fraction of objects
detected by automated algorithms (e.g. SExtractor or Daofind) are not clusters,
but rather are associations. These are likely to disperse into the field on
timescales of tens of Myr due to their lower stellar densities and not due to
gas expulsion (i.e. they were never gravitationally bound). We split the sample
into two discrete fields (inner and outer regions of the galaxy) and search for
evidence of environmentally dependent cluster disruption. Colour-colour
diagrams of the clusters, when compared to simple stellar population models,
already indicate that a much larger fraction of the clusters in the outer field
are older by tens of Myr than in the inner field. This impression is quantified
by estimating each cluster's properties (age, mass, and extinction) and
comparing the age/mass distributions between the two fields. Our results are
inconsistent with "universal" age and mass distributions of clusters, and
instead show that the ambient environment strongly affects the observed
populations.Comment: 6 pages, 3 figures, MNRAS in pres
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