KMOS view of the Galactic centre

I. Young stars are centrally concentrated^{⋆,⋆⋆,⋆⋆⋆}

¹ European Southern Observatory (ESO), Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
e-mail: afeldmei@eso.org
² Max-Planck-Institut für Astronomie, Königsstuhl 17, 69117 Heidelberg, Germany
³ Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁴ Department of Physics and Astronomy, University of Utah, Salt Lake City, UT 84112, USA
⁵ Sterrewacht Leiden, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands
⁶ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁷ ESTEC, Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
⁸ Gemini Observatory, 670 N. A’ohoku Place, Hilo, Hawaii, HI 96720, USA

Received: 16 April 2015
Accepted: 10 September 2015

Abstract

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.