1,223 research outputs found
An Upper Limit on the Mass of a Central Black Hole in the Large Magellanic Cloud from the Stellar Rotation Field
We constrain the possible presence of a central black hole (BH) in the center
of the Large Magellanic Cloud (LMC). This requires spectroscopic measurements
over an area of order a square degree, due to the poorly known position of the
kinematic center. Such measurements are now possible with the impressive field
of view of the Multi Unit Spectroscopic Explorer (MUSE) on the ESO Very Large
Telescope. We used the Calcium Triplet (~850nm) spectral lines in many
short-exposure MUSE pointings to create a two-dimensional integrated-light
line-of-sight velocity map from the ~ individual spectra, taking care to
identify and remove Galactic foreground populations. The data reveal a clear
velocity gradient at an unprecedented spatial resolution of 1 arcmin. We
fit kinematic models to arrive at a upper-mass-limit of
M for any central BH - consistent with the known scaling relations for
supermassive black holes and their host systems. This adds to the growing body
of knowledge on the presence of BHs in low-mass and dwarf galaxies, and their
scaling relations with host-galaxy properties, which can shed light on theories
of BH growth and host system interaction.Comment: 12 pages, 11 figures, 1 table, ApJ - in pres
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Suppression of Stimulated Brillouin Scattering in multiple-ion species inertial confinemen fusion Hohlraum Plasmas
A long-standing problem in the field of laser-plasma interactions is to successfully employ multiple-ion species plasmas to reduce stimulated Brillouin scattering (SBS) in inertial confinement fusion (ICF) hohlraum conditions. Multiple-ion species increase significantly the linear Landau damping for acoustic waves. Consequently, recent hohlraum designs for indirect-drive ignition on the National Ignition Facility investigate wall liner material options so that the liner gain for parametric instabilities will be below threshold for the onset SBS. Although the effect of two-ion species plasmas on Landau damping has been directly observed with Thomson scattering, early experiments on SBS in these plasmas have suffered from competing non-linear effects or laser beam filamentation. In this study, a reduction of SBS scattering to below the percent level has been observed in hohlraums at Omega that emulate the plasma conditions in an indirect drive ICF experiments. These experiments have measured the laser-plasma interaction processes in ignition-relevant high-electron temperature regime demonstrating Landau damping as a controlling process for SBS. The hohlraums have been filled with various fractions of CO{sub 2} and C{sub 3}H{sub 8} varying the ratio of the light (H) to heavy (C and O) ion density from 0 to 2.6. They have been heated by 14.5 kJ of 351-nm light, thus increasing progressively Landau damping by an order of magnitude at constant electron density and temperature. A delayed 351-nm interaction beam, spatially smoothed to produce a 200-{micro}m laser spot at best focus, has propagated along the axis of the hohlraum. The backscattered light, both into the lens and outside, the transmitted light through the hohlraum plasma and the radiation temperature of the hohlraum has been measured. For ignition relevant laser intensities (3-9 10{sup 14} Wcm{sup -2}), we find that the SBS reflectivity scales as predicted with Landau damping from >30% to <1%. Simultaneously, the hohlraum radiation temperature increased indicating improved coupling of the heater beams. These observations provide strong justification to pursue employing multiple-ion species plasmas in current target designs for the first attempt at ignition on the National Ignition Facility
Triaxial orbit-based modelling of the Milky Way Nuclear Star Cluster
We construct triaxial dynamical models for the Milky Way nuclear star cluster
using Schwarzschild's orbit superposition technique. We fit the stellar
kinematic maps presented in Feldmeier et al. (2014). The models are used to
constrain the supermassive black hole mass M_BH, dynamical mass-to-light ratio
M/L, and the intrinsic shape of the cluster. Our best-fitting model has M_BH =
(3.0 +1.1 -1.3)x10^6 M_sun, M/L = (0.90 +0.76 -0.08) M_sun/L_{sun,4.5micron},
and a compression of the cluster along the line-of-sight. Our results are in
agreement with the direct measurement of the supermassive black hole mass using
the motion of stars on Keplerian orbits. The mass-to-light ratio is consistent
with stellar population studies of other galaxies in the mid-infrared. It is
possible that we underestimate M_BH and overestimate the cluster's triaxiality
due to observational effects. The spatially semi-resolved kinematic data and
extinction within the nuclear star cluster bias the observations to the near
side of the cluster, and may appear as a compression of the nuclear star
cluster along the line-of-sight. We derive a total dynamical mass for the Milky
Way nuclear star cluster of M_MWNSC = (2.1 +-0.7)x10^7 M_sun within a sphere
with radius r = 2 x r_eff = 8.4 pc. The best-fitting model is tangentially
anisotropic in the central r = 0.5-2 pc of the nuclear star cluster, but close
to isotropic at larger radii. Our triaxial models are able to recover complex
kinematic substructures in the velocity map.Comment: 14 pages, 10 figures. Accepted for publication in MNRA
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
KMOS view of the Galactic Centre - II. Metallicity distribution of late-type stars
Knowing the metallicity distribution of stars in the Galactic Centre has
important implications for the formation history of the Milky Way nuclear star
cluster. However, this distribution is not well known, and is currently based
on a small sample of fewer than 100 stars. We obtained near-infrared K-band
spectra of more than 700 late-type stars in the central 4 pc^2 of the Milky Way
nuclear star cluster with the integral-field spectrograph KMOS (VLT). We
analyse the medium-resolution spectra using a full-spectral fitting method
employing the G\"ottingen Spectral library of synthetic PHOENIX spectra. The
derived stellar metallicities range from metal-rich [M/H]>+0.3 dex to
metal-poor [M/H]<-1.0 dex, with a fraction of 5.2(^{+6.0}+{-3.1}) per cent
metal-poor ([M/H]<-0.5 dex) stars. The metal-poor stars are distributed over
the entire observed field. The origin of metal-poor stars remains unclear. They
could originate from infalling globular clusters. For the metal-rich stellar
population ([M/H]>0 dex) a globular cluster origin can be ruled out. As there
is only a very low fraction of metal-poor stars in the central 4 pc^2 of the
Galactic Centre, we believe that our data can discard a scenario in which the
Milky Way nuclear star cluster is purely formed from infalling globular
clusters.Comment: 18 pages, 9 Figures, accepted for publication in MNRA
Large scale kinematics and dynamical modelling of the Milky Way nuclear star cluster
Within the central 10pc of our Galaxy lies a dense nuclear star cluster
(NSC), and similar NSCs are found in most nearby galaxies. Studying the
structure and kinematics of NSCs reveals the history of mass accretion of
galaxy nuclei. Because the Milky Way (MW) NSC is at a distance of only 8kpc, we
can spatially resolve the MWNSC on sub-pc scales. This makes the MWNSC a
reference object for understanding the formation of all NSCs. We have used the
NIR long-slit spectrograph ISAAC (VLT) in a drift-scan to construct an
integral-field spectroscopic map of the central 9.5 x 8pc of our Galaxy. We use
this data set to extract stellar kinematics both of individual stars and from
the unresolved integrated light spectrum. We present a velocity and dispersion
map from the integrated light and model these kinematics using kinemetry and
axisymmetric Jeans models. We also measure CO bandhead strengths of 1,375
spectra from individual stars. We find kinematic complexity in the NSCs radial
velocity map including a misalignment of the kinematic position angle by 9
degree counterclockwise relative to the Galactic plane, and indications for a
rotating substructure perpendicular to the Galactic plane at a radius of 20" or
0.8pc. We determine the mass of the NSC within r = 4.2pc to 1.4 x 10^7 Msun. We
also show that our kinematic data results in a significant underestimation of
the supermassive black hole (SMBH) mass. The kinematic substructure and
position angle misalignment may hint at distinct accretion events. This
indicates that the MWNSC grew at least partly by the mergers of massive star
clusters. Compared to other NSCs, the MWNSC is on the compact side of the r_eff
- M_NSC relation. The underestimation of the SMBH mass might be caused by the
kinematic misalignment and a stellar population gradient. But it is also
possible that there is a bias in SMBH mass measurements obtained with
integrated light.Comment: 20 pages, 19 Figures, Accepted for publication in A&
KMOS view of the Galactic Centre I. Young stars are centrally concentrated
The Galactic centre hosts a crowded, dense nuclear star cluster with a
half-light radius of 4 pc. Most of the stars in the Galactic centre are cool
late-type stars, but there are also >100 hot early-type stars in the central
parsec of the Milky Way. These stars are only 3-8 Myr old. Our knowledge of the
number and distribution of early-type stars in the Galactic centre is
incomplete. Only a few spectroscopic observations have been made beyond a
projected distance of 0.5 pc of the Galactic centre. The distribution and
kinematics of early-type stars are essential to understand the formation and
growth of the nuclear star cluster. We cover the central >4pc^2 of the Galactic
centre using the integral-field spectrograph KMOS. We extracted more than 1,000
spectra from individual stars and identified early-type stars based on their
spectra. Our data set contains 114 bright early-type stars: 6 have narrow
emission lines, 23 are Wolf-Rayet stars, 9 stars have featureless spectra, and
76 are O/B type stars. Our wide-field spectroscopic data confirm that the
distribution of young stars is compact, with 90% of the young stars identified
within 0.5 pc of the nucleus. We identify 24 new O/B stars primarily at large
radii. We estimate photometric masses of the O/B stars and show that the total
mass in the young population is >12,000M_sun. The O/B stars all appear to be
bound to the Milky Way nuclear star cluster, while less than 30% belong to the
clockwise rotating disk. The central concentration of the early-type stars is a
strong argument that they have formed in situ. A large part of the young O/B
stars is not on the disk, which either means that the early-type stars did not
all form on the same disk or that the disk is dissolving rapidly. [abridged]Comment: 27 pages, 17 figures, matches journal version: Corrected typos,
corrected Notes in Table B.
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&
Cloaked Facebook pages: Exploring fake Islamist propaganda in social media
This research analyses cloaked Facebook pages that are created to spread political propaganda by cloaking a user profile and imitating the identity of a political opponent in order to spark hateful and aggressive reactions. This inquiry is pursued through a multi-sited online ethnographic case study of Danish Facebook pages disguised as radical Islamist pages, which provoked racist and anti-Muslim reactions as well as negative sentiments towards refugees and immigrants in Denmark in general. Drawing on Jessie Daniels’ critical insights into cloaked websites, this research furthermore analyses the epistemological, methodological and conceptual challenges of online propaganda. It enhances our understanding of disinformation and propaganda in an increasingly interactive social media environment and contributes to a critical inquiry into social media and subversive politics
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