An Extremely Top-Heavy IMF in the Galactic Center Stellar Disks

TL;DR

This study reveals an extremely top-heavy initial mass function in the stellar disks near the Galactic Center, indicating a unique star formation environment compared to typical IMFs.

Contribution

It provides the first detailed spectroscopic analysis showing a top-heavy IMF in the Galactic Center stellar disks, based on deep observations of 177 early-type stars.

Findings

Most early-type stars are in two warped disks and a central cluster.

The stellar mass function in the disks is extremely top-heavy, with a power law of ~ m^(-0.45).

The IMF of stars outside the disks and in the S-star cluster is consistent with a standard Salpeter/Kroupa IMF.

Abstract

We present new observations of the nuclear star cluster in the central parsec of the Galaxy with the adaptive optics assisted, integral field spectrograph SINFONI on the ESO/VLT. Our work allows the spectroscopic detection of early and late type stars to m_K >= 16, more than 2 magnitudes deeper than our previous data sets. Our observations result in a total sample of 177 bona fide early-type stars. We find that most of these Wolf Rayet (WR), O- and B- stars reside in two strongly warped disks between 0.8" and 12" from SgrA*, as well as a central compact concentration (the S-star cluster) centered on SgrA*. The later type B stars (m_K>15) in the radial interval between 0.8" and 12" seem to be in a more isotropic distribution outside the disks. The observed dearth of late type stars in the central few arcseconds is puzzling, even when allowing for stellar collisions. The stellar mass function of the disk stars is extremely top heavy with a best fit power law of dN/dm ~ m^(-0.45+/-0.3). Since at least the WR/O-stars were formed in situ in a single star formation event ~6 Myrs ago, this mass function probably reflects the initial mass function (IMF). The mass functions of the S-stars inside 0.8" and of the early-type stars at distances beyond 12" are compatible with a standard Salpeter/Kroupa IMF (best fit power law of dN/dm ~ m^(-2.15+/-0.3)).