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&

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&

Compact Radio Sources within 30" of Sgr A*: Proper Motions, Stellar Winds and the Accretion Rate onto Sgr A*

Recent broad-band 34 and 44 GHz radio continuum observations of the Galactic center have revealed 41 massive stars identified with near-IR counterparts, as well as 44 proplyd candidates within 30" of Sgr A*. Radio observations obtained in 2011 and 2014 have been used to derive proper motions of eight young stars near Sgr A*. The accuracy of proper motion estimates based on near-IR observations by Lu et al. and Paumard et al. have been investigated by using their proper motions to predict the 2014 epoch positions of near-IR stars and comparing the predicted positions with those of radio counterparts in the 2014 radio observations. Predicted positions from Lu et al. show an rms scatter of 6 mas relative to the radio positions, while those from Paumard et al. show rms residuals of 20 mas, which is mainly due to uncertainties in the IR-based proper motions. Under the assumption of homogeneous ionized winds, we also determine the mass-loss rates of 11 radio stars, finding rates that are on average $\sim$2 times smaller than those determined from model atmosphere calculations and near-IR data. Clumpiness of ionized winds would reduce the mass loss rate of WR and O stars by additional factors of 3 and 10, respectively. One important implication of this is a reduction in the expected mass accretion rate onto Sgr A* from stellar winds by nearly an order of magnitude to a value of few$\times10^{-7}$ \msol\ yr$^{-1}$. Finally, we present the positions of 318 compact 34.5 GHz radio sources within 30\arcs\ of Sgr A*. At least 45 of these have stellar counterparts in the near-IR $K_s$ (2.18 $\mu$m) and $L'$ (3.8$\mu$m) bands.Comment: 30 pages, 4 figures, ApJ (in press

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 $0.2''$ angular resolution GALACTICNUCLEUS survey (epoch 2015) combined with NICMOS/HST data from the Paschen-$\alpha$ survey (epoch 2008). This study will be the first extensive proper motion study of the central $\sim 36' \times 16'$ of the Galaxy, which is not covered adequately by any of the existing astronomical surveys such as Gaia because of its extreme interstellar extinction ($A_{V} \gtrsim 30$ 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 stellar cusp around the Milky Way’s central black hole

The existence of stellar cusps in dense clusters around massive black holes is a fundamental, decades-old prediction of theoretical stellar dynamics. Yet, observational evidence has been difficult to obtain. With a new, improved analysis of high-angular resolution images of the central parsecs of the Galactic Center, we are finally able to provide the first solid evidence for the existence of a stellar cusp around the Milky Way’s massive black hole. The existence of stellar cusps has a significant impact on predicted event rates of phenomena like tidal disruptions of stars and extreme mass ratio inspirals.EC/FP7/614922/EU/The Fingerprint of a Galactic Nucleus: A Multi-Wavelength, High-Angular Resolution, Near-Infrared Study of the Centre of the Milky Way/GALACTICNUCLEU

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&

Testing the gravitational theory with short-period stars around our Galactic Center

Motion of short-period stars orbiting the supermassive black hole in our Galactic Center has been monitored for more than 20 years. These observations are currently offering a new way to test the gravitational theory in an unexplored regime: in a strong gravitational field, around a supermassive black hole. In this proceeding, we present three results: (i) a constraint on a hypothetical fifth force obtained by using 19 years of observations of the two best measured short-period stars S0-2 and S0-38 ; (ii) an upper limit on the secular advance of the argument of the periastron for the star S0-2 ; (iii) a sensitivity analysis showing that the relativistic redshift of S0-2 will be measured after its closest approach to the black hole in 2018.Comment: 4 pages, 2 figures, proceedings of the 52nd Rencontres de Moriond, Gravitation Sessio