An Extremely Top-Heavy Initial Mass Function in the Galactic Center Stellar Disks

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 mK ⩾ 16, more than 2 mag 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 Sgr A*, as well as a central compact concentration (the S-star cluster) centered on Sgr A*. The later type B-stars (mK>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. WR/O-stars were formed in situ in a single star formation event ∼6 Myr 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).