27 research outputs found
The distribution of old stars around the Milky Way's central black hole I: Star counts
(abridged) In this paper we revisit the problem of inferring the innermost
structure of the Milky Way's nuclear star cluster via star counts, to clarify
whether it displays a core or a cusp around the central black hole. Through
image stacking and improved PSF fitting we push the completeness limit about
one magnitude deeper than in previous, comparable work. Contrary to previous
work, we analyse the stellar density in well-defined magnitude ranges in order
to be able to constrain stellar masses and ages. The RC and brighter giant
stars display a core-like surface density profile within a projected radius
R<0.3 pc of the central black hole, in agreement with previous studies, but
show a cusp-like surface density distribution at larger R. The surface density
of the fainter stars can be described well by a single power-law at R<2 pc. The
cusp-like profile of the faint stars persists even if we take into account the
possible contamination of stars in this brightness range by young pre-main
sequence stars. The data are inconsistent with a core-profile for the faint
stars.Finally, we show that a 3D Nuker law provides a very good description of
the cluster structure. We conclude that the observed stellar density at the
Galactic Centre, as it can be inferred with current instruments, is consistent
with the existence of a stellar cusp around the Milky Way's central black hole,
Sgr A*. This cusp is well developed inside the influence radius of about 3 pc
of Sgr A* and can be described by a single three-dimensional power-law with an
exponent gamma=1.23+-0.05. The apparent lack of RC stars and brighter giants at
projected distances of R < 0.3 pc (R<8") of the massive black hole may indicate
that some mechanism has altered their distribution or intrinsic luminosity.Comment: Accepted for publication A&
The distribution of stars around the Milky Way's central black hole II: Diffuse light from sub-giants and dwarfs
This is the second of three papers that search for the predicted stellar cusp
around the Milky Way's central black hole, Sagittarius A*, with new data and
methods. We aim to infer the distribution of the faintest stellar population
currently accessible through observations around Sagittarius A*. We use
adaptive optics assisted high angular resolution images obtained with the NACO
instrument at the ESO VLT. Through optimised PSF fitting we remove the light
from all detected stars above a given magnitude limit. Subsequently we analyse
the remaining, diffuse light density. The analysed diffuse light arises from
sub-giant and main-sequence stars with KS ~ 19 - 20 with masses of 1 - 2 Msol .
These stars can be old enough to be dynamically relaxed. The observed power-law
profile and its slope are consistent with the existence of a relaxed stellar
cusp around the Milky Way's central black hole. We find that a Nuker law
provides an adequate description of the nuclear cluster's intrinsic shape
(assuming spherical symmetry). The 3D power-law slope near Sgr A* is \gamma =
1.23 +- 0.05. At a distance of 0.01 pc from the black hole, we estimate a
stellar mass density of 2.3 +- 0.3 x 10^7 Msol pc^-3 and a total enclosed
stellar mass of 180 +- 20 Msol. These estimates assume a constant mass-to-light
ratio and do not take stellar remnants into account. The fact that no cusp is
observed for bright (Ks 16) giant stars at projected distances of roughly
0.1-0.3 pc implies that some mechanism has altered their appearance or
distribution.Comment: Accepted for publication A&
First results from a large-scale proper motion study of the Galactic Centre
Proper motion studies of stars in the centre of the Milky Way have been
typically limited to the Arches and Quintuplet clusters and to the central
parsec. Here, we present the first results of a large-scale proper motion study
of stars within several tens of parsecs of Sagittarius A* based on our
angular resolution GALACTICNUCLEUS survey (epoch 2015) combined with NICMOS/HST
data from the Paschen- survey (epoch 2008). This study will be the
first extensive proper motion study of the central of the
Galaxy, which is not covered adequately by any of the existing astronomical
surveys such as Gaia because of its extreme interstellar extinction ( mag). Proper motions can help us to disentangle the different
stellar populations along the line-of-sight and interpret their properties in
combination with multi-wavelength photometry from GALACTICNUCLEUS and other
sources. It also allows us to infer the dynamics and interrelationship between
the different stellar components of the Galactic Centre (GC). In particular, we
use proper motions to detect co-moving groups of stars which can trace low mass
or partially dissolved young clusters in the GC that can hardly be discovered
by any other means. Our pilot study in this work is on a field in the nuclear
bulge associated by HII regions that show the presence of young stars. We
detect the first group of co-moving stars coincident with an HII region. Using
colour-magnitude diagrams, we infer that the co-moving stars are consistent
with being the post-main sequence stars with ages of few Myrs. Simulations show
that this group of stars is a real group that can indicate the existence of a
dissolving or low to intermediate mass young cluster. A census of these
undiscovered clusters will ultimately help us to constrain star formation at
the GC in the past few ten Myrs.Comment: Accepted for publication in A&A. 13 pages, 17 figure
The formation history of our Galaxy's nuclear stellar disc constrained from HST observations of the Quintuplet field
Until recently it was thought that the nuclear stellar disc at the centre of
our Galaxy was formed via quasi-continuous star formation over billions of
years. However, an analysis of GALACTICNUCLEUS survey data indicates that >80%
of the mass of the stellar disc formed at least 8 Gyr ago and about 5% roughly
1 Gyr ago. Our aim is to derive new constraints on the formation history of the
nuclear stellar disc. We analysed a catalogue of HST/WFC3-IR observations of
the Quintuplet cluster field. From this catalogue, we selected about 24000
field stars that probably belong to the nuclear stellar disc. We used red clump
giants to deredden the sample and fit the resulting F153M luminosity function
with a linear combination of theoretical luminosity functions created from
different stellar evolutionary models. We find that >70% of the stellar
population in the nuclear disc probably formed more than 10 Gyr ago, while ~15%
formed in an event (or series of events) ~1Gyr ago. Up to 10% of the stars
appear to have formed in the past tens to hundreds of Myr. These results do not
change significantly for reasonable variations in the assumed mean metallicity,
sample selection, reddening correction, or stellar evolutionary models. We
confirm previous work that changed the formation paradigm for stars in the
Galactic Centre. The nuclear stellar disc is indeed a very old structure. There
seems to have been little star formation activity between its formation and
about 1 Gyr ago.Comment: Accepted for publication in A&
A tactile model of the night summer northern sky for the teaching of astronomy to the BVI
Astroaccesible is an outreach project hosted by the Instituto de
Astrof\'{\i}sica de Andaluc\'{\i}a - CSIC aimed at the teaching and
popularisation of the astronomy among all publics independently of their
capabilities and abilities, paying special attention to the collective of blind
and visually impaired (BVI). Among the different strategies and resources using
in our project, we have developed new 3D models representing in relief some of
the stars, constellations and deep sky objects that can be observed during
night from the Northern hemisphere in spring and summer. These models can be
used by BVI to transmit to them the spatial configuration of the sky during
night, but can be also used as an additional resource for all kind of publics
to complement their sensorial experience. We also describe additional resources
based on sounds that can also be employed to get deeper into this
multisensorial experience. Finally, we summarize some of the activities and the
context in which this new material has been used in the last 2 years.Comment: 6 pages, 2 figures. To appear as proceeding of the 15th Scientific
Meeting of the Spanish Astronomical Societ
New constraints on the structure of the nuclear stellar cluster of the Milky Way from star counts and MIR imaging
The Milky Way nuclear star cluster (MWNSC) is a crucial laboratory for
studying the galactic nuclei of other galaxies, but its properties have not
been determined unambiguously until now. Aims. We aim to study the size and
spatial structure of the MWNSC. Methods. This study uses data and methods that
address potential shortcomings of previous studies on the topic. We use
angular resolution data to create a stellar density map in the central
pc x pc at the Galactic center. We include data from selected
adaptive-optics-assisted images obtained for the inner parsecs. In addition, we
use IRAC/Spitzer mid-infrared (MIR) images. We model the Galactic bulge and the
nuclear stellar disk in order to subtract them from the MWNSC. Finally, we fit
a S\'ersic model to the MWNSC and investigate its symmetry. Results. Our
results are consistent with previous work. The MWNSC is flattened with an axis
ratio of , an effective radius of pc, and
a S\'ersic index of . Its major axis may be tilted out of the
Galactic plane by up to degrees. The distribution of the giants brighter
than the Red Clump (RC) is found to be significantly flatter than the
distribution of the faint stars. We investigate the 3D structure of the central
stellar cusp using our results on the MWNSC structure on large scales to
constrain the deprojection of the measured stellar surface number density,
obtaining a value of the 3D inner power law of
. Conclusions. The MWNSC shares its
main properties with other extragalactic NSCs found in spiral galaxies. The
differences in the structure between bright giants and RC stars might be
related to the existence of not completely mixed populations of different ages.
This may hint at recent growth of the MWNSC through star formation or cluster
accretion.Comment: 20 page