Variable stars in the nuclear star cluster of the Milky Way

We have analysed four-year baseline HST/WFC3 observations of the Milky Way nuclear star cluster and identified almost 4000 variable stars in a field of 2.3 arcmin x 2.3 arcmin size (correponding to about 5.4 pc x 5.4 pc). Besides previously known bright Miras, we also identified many dim variable stars, such as delta Scuti, RR Lyrae, and eclipsing binaries. In particular, we identified 4 new RR Lyrae ab stars with their typical sawtooth light curves. From the small number of these stars, we estimate that not more than about 18% of the nuclear star cluster can be made up of old, metal-poor stars. This poses strong limits on the possible contribution of dissolved globular clusters to the mass of the nuclear cluster.

Dong et al. 2017,  MNRAS: Near-infrared variability study of the central 2.3 arcmin × 2.3 arcmin of the Galactic Centre - I. Catalogue of variable sources

Dong et al. 2017,  MNRAS: Near-infrared variability study of the central 2.3 × 2.3 arcmin2 of the Galactic Centre - II. Identification of RR Lyrae stars in the Milky Way nuclear star cluster

Massive young stars in the Galactic Centre

We present new [Ne II] (12.8 μm) IRTF/TEXES observations of the Galactic Center H II regions H1 and H2, which are at a projected distance of ∼11 pc from the centre of the Galaxy. The new observations allow us to map the radial velocity distributions of ionized gas. Our new results confirm what we had previously suggested: The O supergiant P114 in H1 is a runaway star, moving towards us through the −30 to 0 km s−1 molecular cloud, whereas the O If star P35 in H2 formed in situ, and may mark the position of a so-far unknown small star cluster formed within the central 30 pc of the Galaxy.

Dong et al. 2017,  MNRAS: IRTF/TEXES observations of the H II regions H1 and H2 in the Galactic Centre

The stellar cusp at the Galactic Centre

In a series of three papers we revisit the so-called missing cusp problem. With advanced analysis techniques we analyse deeper star counts than before and also study the diffuse light density from unresolved stars. We find that the stellar density increases as a power law toward Sagittarius A* inside the black hole’s radius of influence. The data are compared to new Nbody simulations. Observations and modelling agree very well.

We conclude that a stellar cusp exists around the Milky Way’s massive black hole (MBH). By analogy, this lets us expect that such structures exist at the centres of other galaxies, too. The implications of our findings are twofold: (1) We confirm that our basic understanding of stellar dynamics is correct. (2) The existence of stellar cusps around MBHs implies that future space based gravitational wave observatories, such as LISA (Laser Interferometer Space Antenna), will observe a significant number of so-called EMRI events (Extreme Mass Ratio Inspirals), where stellar remnants, e.g. stellar mass black holes, merge with MBH through the repeated emission of bursts of gravitational waves. EMRIs are considered the most exquisite probes of the validity of General Relativity and will serve to obtain ultra-precise measurements of MBHs’ masses and angular momenta (see Amaro-Seoane et al. 2007).

Gallego-Cano et al. 2017,   A&A, accepted for publication: The distribution of old stars around the Milky Way's central black hole I: Star counts

Schödel et al. 2017, A&A, accepted for publication: The distribution of stars around the Milky Way's central black hole II: Diffuse light from sub-giants and dwarfs

Baumgardt et al. 2017, A&A, accepted for publication: The distribution of stars around the Milky Way's black hole III: Comparison with simulations

Metallicity of late type stars in the Galactic Centre

We obtained KMOS/VLT spectroscopic observations of the GC and analysed the spectra of more than 700 stars. Only about 5% of the stars are found to be metal-poor with [M/H] ≤ -0.5 dex. This has important implications for the formation history of the NSC because it rules out any significant contribution to the NSC stellar population from the inspiral and merging of metal poor globular clusters.

Feldmeier-Krause et al. 2017, MNRAS: KMOS view of the Galactic Centre - II. Metallicity distribution of late-type stars

Structure and mass of the Milky Way’s nuclear star cluster (NSC)

We determined the mass and large-scale structure of the Milky Ways nuclear cluster from mid-infrared images obtained by the Spitzer Space Telescope. We provide accurate numbers for its global properties, the first detailed update since a work by Launhardt et al. (2002). We find that the NSC has a half-light radius of about 4 pc and a total mass of 25 million solar masses. The NSC is not spherically symmetric, but flattened along the Galactic Plane. This may give clues to its formation: In agreement with other recent studies it appears that the NSC formed from gas and stars falling into the centre from the inner Milky Way’s Disk or Bar.

Schödel et al. 2014, A & A: Surface brightness profile of the Milky Way's nuclear star cluster

A new distance and mass estimate for Sagittarius A*

With new and improved astrometric and spectroscopic measurements we derive fits for multiple orbits of stars around Sagittarius A*. The new best estimates for its mass and distance are 4.02 ± (0.16 ± 0.04) × 106 M and 7.86 ± (0.14 ± 0.04) kpc.

Boehle et al. 2016, ApJ: An Improved Distance and Mass Estimate for Sgr A* from a Multistar Orbit Analysis