35 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&
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
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&
The central Blue Straggler population in four outer-halo globular clusters
Using HST/WFPC2 data, we have performed a comparative study of the Blue
Straggler Star (BSS) populations in the central regions of the globular
clusters AM 1, Eridanus, Palomar 3, and Palomar 4. Located at distances RGC >
50 kpc from the Galactic Centre, these are (together with Palomar 14 and NGC
2419) the most distant clusters in the Halo. We determine their
colour-magnitude diagrams and centres of gravity. The four clusters turn out to
have similar ages (10.5-11 Gyr), significantly smaller than those of the
inner-Halo globulars, and similar metallicities. By exploiting wide field
ground based data, we build the most extended radial density profiles from
resolved star counts ever published for these systems. These are well
reproduced by isotropic King models of relatively low concentration. BSSs
appear to be significantly more centrally segregated than red giants in all
globular clusters, in agreement with the estimated core and half-mass
relaxation times which are smaller than the cluster ages. Assuming that this is
a signature of mass segregation, we conclude that AM 1 and Eridanus are
slightly dynamically more evolved than Pal 3 and Pal 4.Comment: Accepted for publication in ApJ, references update
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A Deep View into the Nucleus of the Sagittarius Dwarf Spheroidal Galaxy with MUSE. III. Discrete Multicomponent Population-dynamical Models Based on the Jeans Equations
We present comprehensive multicomponent dynamical models of M54 (NGC 6715), the nuclear star cluster of the Sagittarius (Sgr) dwarf galaxy, which is undergoing a tidal disruption in the Milky Way halo. Previous papers in this series used a large MUSE mosaic data set to identify multiple stellar populations in the system and study their kinematic differences. Here, we use Jeans-based dynamical models that fit the population properties (mean age and metallicity), spatial distributions, and kinematics simultaneously. They provide a solid physical explanation for our previous findings. Population-dynamical models deliver a comprehensive view of the whole system, and allow us to disentangle the different stellar populations. We explore their dynamical interplay and confirm our previous findings about the build-up of Sgr’s nuclear cluster via contributions from globular cluster stars, Sgr inner field stars, and in situ star formation. We explore various parameterizations of the gravitational potential and show the importance of a radially varying mass-to-light ratio for the proper treatment of the mass profile. We find a total dynamical mass within M54's tidal radius (∼75 pc) of 1.60 ± 0.07 × 106 M ⊙ in excellent agreement with N-body simulations. Metal-poor globular cluster stars contribute about 65% of the total mass or 1.04 ± 0.05 × 106 M ⊙. Metal-rich stars can be further divided into young and intermediate-age populations, which contribute 0.32 ± 0.02 × 106 M ⊙ (20%) and 0.24 ± 0.02 × 106 M ⊙ (15%), respectively. Our population-dynamical models successfully distinguish the different stellar populations in Sgr’s nucleus because of their different spatial distributions, ages, metallicities, and kinematic features
Gas inflows in the polar ring of NGC 4111 : the birth of an AGN
We have used Hubble Space Telescope (HST) images, SAURON Integral Field Spectroscopy (IFS), and adaptative optics assisted Gemini NIFS near-infrared K-band IFS to map the stellar and gas distribution, excitation and kinematics of the inner few kpc of the nearby edge-on S0 galaxy NGC 4111. The HST images map its ≈450 pc diameter dusty polar ring, with an estimated gas mass ≥107 M . The NIFS data cube maps the inner 110 pc radius at ≈7 pc spatial resolution, revealing a ≈220 pc diameter polar ring in hot (2267 ± 166 K) molecular H2 1–0 S(1) gas embedded in the polar ring. The stellar velocity field shows disc-dominated kinematics along the galaxy plane both in the SAURON large scale and in the NIFS nuclear-scale data. The large-scale [O III] λ5007 Å velocity field shows a superposition of two disc kinematics: one similar to that of the stars and another along the polar ring, showing non-circular motions that seem to connect with the velocity field of the nuclear H2 ring, whose kinematics indicate accelerated inflow to the nucleus. The estimated mass inflow rate is enough not only to feed an active galactic nucleus (AGN) but also to trigger circumnuclear star formation in the near future. We propose a scenario in which gas from the polar ring, which probably originated from the capture of a dwarf galaxy, is moving inwards and triggering an AGN, as supported by the local X-ray emission, which seems to be the source of the H2 1–0 S(1) excitation. The fact that we see neither near-UV nor Br γ emission suggests that the nascent AGN is still deeply buried under the optically thick dust of the polar ring
Gas inflows in the polar ring of NGC 4111: the birth of an AGN
We have used Hubble Space Telescope (HST) images, SAURON Integral Field
Spectroscopy (IFS) and adaptative optics assisted Gemini NIFS near-infrared
K-band IFS to map the stellar and gas distribution, excitation and kinematics
of the inner few kpc of the nearby edge-on S0 galaxy NGC 4111. The HST images
map its 450 pc diameter dusty polar ring, with an estimated gas mass
M. The NIFS datacube maps the inner 110 pc radius at
7 pc spatial resolution revealing a 220 pc diameter polar
ring in hot ( K) molecular H 1-0 S(1) gas embedded in the polar
ring. The stellar velocity field shows disk-dominated kinematics along the
galaxy plane both in the SAURON large-scale and in the NIFS nuclear-scale data.
The large-scale [O III] \AA velocity field shows a superposition
of two disk kinematics: one similar to that of the stars and another along the
polar ring, showing non-circular motions that seem to connect with the velocity
field of the nuclear H ring, whose kinematics indicate accelerated inflow
to the nucleus. The estimated mass inflow rate is enough not only to feed an
Active Galactic Nucleus (AGN) but also to trigger circumnuclear star formation
in the near future. We propose a scenario in which gas from the polar ring,
which probably originated from the capture of a dwarf galaxy, is moving inwards
and triggering an AGN, as supported by the local X-ray emission, which seems to
be the source of the H 1-0 S(1) excitation. The fact that we see neither
near-UV nor Br emission suggests that the nascent AGN is still deeply
buried under the optically thick dust of the polar ring.Comment: 18 pages, 21 figure