43 research outputs found
The assembly history of the milky way nuclear star cluster
We study the assembly history of the nuclear star cluster in the Milky Way. Dense nuclear star clusters form distinct components in ~75% of nearby galaxies. Because the Milky Way nuclear star cluster is at a distance of only 8 kpc, we can spatially resolve its stellar populations and kinematics much better than possible in external galaxies.
We study the large-scale stellar kinematics using long-slit spectroscopic data in the near-infrared. We extract stellar kinematic maps from the integrated light, and detect the complex kinematic structure of the star cluster. We set up dynamical models to derive the cluster’s mass.
Further, we study stellar populations using integral-field spectroscopic data. From these data we extract more than 1,000 spectra from individual stars. We study the spatial distribution of young and old stars, and the metallicity distribution of cool stars.
We found indications for two different formation mechanisms of the Milky Way nuclear star cluster. On the one hand, gas was accreted to the Galaxy’s centre and stars formed in-situ. On the other hand, stars formed in star clusters outside the centre. These star clusters fell into the Galaxy’s nucleus and contributed to the assembly of the Milky Way nuclear star cluster
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
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
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.
Smooth kinematic and metallicity gradients between the Milky Way's nuclear star cluster and nuclear stellar disc. Different components of the same structure?
The innermost regions of most galaxies are characterised by the presence of
extremely dense nuclear star clusters. Nevertheless, these clusters are not the
only stellar component present in galactic nuclei, where larger stellar
structures known as nuclear stellar discs, have also been found. Understanding
the relation between nuclear star clusters and nuclear stellar discs is
challenging due to the large distance towards other galaxies which limits their
analysis to integrated light. The Milky Way's centre, at only 8 kpc, hosts a
nuclear star cluster and a nuclear stellar disc, constituting a unique template
to understand their relation and formation scenario. We aim to study the
kinematics and stellar metallicity of stars from the Milky Way's nuclear star
cluster and disc to shed light on the relation between these two Galactic
centre components. We used publicly available photometric, proper motions, and
spectroscopic catalogues to analyse a region of centred on
the Milky Way's nuclear star cluster. We built colour magnitude diagrams, and
applied colour cuts to analyse the kinematic and metallicity distributions of
Milky Way's nuclear star cluster and disc stars with different extinction along
the line of sight. We detect kinematics and metallicity gradients for the
analysed stars along the line of sight towards the Milky Way's nuclear star
cluster, suggesting a smooth transition between the nuclear stellar disc and
cluster. We also find a bi-modal metallicity distribution for all the analysed
colour bins, which is compatible with previous work on the bulk population of
the nuclear stellar disc and cluster. Our results suggest that these two
Galactic centre components might be part of the same structure with the Milky
Way's nuclear stellar disc being the grown edge of the nuclear star cluster.Comment: Submitted to A&A. 13 pages, 9 figure
oMEGACat I: MUSE spectroscopy of 300,000 stars within the half-light radius of Centauri
Omega Centauri ( Cen) is the most massive globular cluster of the
Milky Way and has been the focus of many studies that reveal the complexity of
its stellar populations and kinematics. However, most previous studies have
used photometric and spectroscopic datasets with limited spatial or magnitude
coverage, while we aim to investigate it having full spatial coverage out to
its half-light radius and stars ranging from the main sequence to the tip of
the red giant branch. This is the first paper in a new survey of Cen
that combines uniform imaging and spectroscopic data out to its half-light
radius to study its stellar populations, kinematics, and formation history. In
this paper, we present an unprecedented MUSE spectroscopic dataset combining 87
new MUSE pointings with previous observations collected from guaranteed time
observations. We extract spectra of more than 300,000 stars reaching more than
two magnitudes below the main sequence turn-off. We use these spectra to derive
metallicity and line-of-sight velocity measurements and determine robust
uncertainties on these quantities using repeat measurements. Applying quality
cuts we achieve signal-to-noise ratios of 16.47/73.51 and mean metallicity
errors of 0.174/0.031 dex for the main sequence stars (18 mag 22 mag) and red giant branch stars (16 mag 10
mag), respectively. We correct the metallicities for atomic diffusion and
identify foreground stars. This massive spectroscopic dataset will enable
future studies that will transform our understanding of Cen, allowing
us to investigate the stellar populations, ages, and kinematics in great
detail.Comment: 27 pages, 18 figures, 3 tables, accepted for publication in ApJ, the
catalog will be available in the online material of the published articl
ReveaLLAGN 0: First Look at JWST MIRI data of Sombrero and NGC 1052
We present the first results from the Revealing Low-Luminosity Active
Galactic Nuclei (ReveaLLAGN) survey, a JWST survey of seven nearby LLAGN. We
focus on two observations with the Mid-Infrared Instrument's (MIRI) Medium
Resolution Spectrograph (MRS) of the nuclei of NGC 1052 and Sombrero (NGC 4594
/ M104). We also compare these data to public JWST data of a higher-luminosity
AGN, NGC 7319. JWST clearly resolves the AGN component even in Sombrero, the
faintest target in our survey; the AGN components have very red spectra. We
find that the emission-line widths in both NGC 1052 and Sombrero increase with
increasing ionization potential, with FWHM > 1000 km/s for lines with
ionization potential > 50 eV. These lines are also significantly blue-shifted
in both LLAGN. The high ionization potential lines in NGC 7319 show neither
broad widths or significant blue shifts. Many of the lower ionization potential
emission lines in Sombrero show significant blue wings extending > 1000 km/s.
These features and the emission-line maps in both galaxies are consistent with
outflows along the jet direction. Sombrero has the lowest luminosity
high-ionization potential lines ([Ne V] and [O IV]) ever measured in the
mid-IR, but the relative strengths of these lines are consistent with higher
luminosity AGN. On the other hand, the [Ne V] emission is much weaker relative
to the [Ne III}] and [Ne II] lines of higher-luminosity AGN. These initial
results show the great promise that JWST holds for identifying and studying the
physical nature of LLAGN.Comment: Submitted to Ap