KMOS view of the Galactic centre. I. Young stars are centrally concentrated

20 Jan

[2015A&A…584A…2F]  Feldmeier-Krause, A.; Neumayer, N.; Schödel, R.; Seth, A.; Hilker, M.; de Zeeuw, P. T.; Kuntschner, H.; Walcher, C. J.; Lützgendorf, N.; Kissler-Patig, M.

Context. 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.
Aims: 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. Methods: We cover the central >4 pc2 (0.75 sq. arcmin) of the Galactic centre using the integral-field spectrograph KMOS (VLT). We extracted more than 1000 spectra from individual stars and identified early-type stars based on their spectra.
Results: 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 ≳12 000 M. 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. We add one new star to the sample of stars affiliated with this disk.
Conclusions: The central concentration of the early-type stars is a strong argument that they have formed in situ. An alternative scenario, in which the stars formed outside the Galactic centre in a cluster that migrated to the centre, is refuted. 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.