2013A&A...549A..57N

Nuclear star clusters (NSCs) are ubiquitous at the centers of galaxies. They show mixed stellar populations and the spectra of many NSCs indicate recent events of star formation. However, it is impossible to resolve external NSCs in order to examine the relevant processes. The Milky Way NSC, on the other hand, is close enough to be resolved into its individual stars and presents therefore a unique template for NSCs in general. Young, massive stars have been found by systematic spectroscopic studies at projected distances R≲0.5pc from the supermassive black hole, SagittariusA* (SgrA*). In recent years, increasing evidence has been found for the presence of young, massive stars also at R>0.5pc. Our goal in this work is a systematic search for young, massive star candidates throughout the entire region within R∼2.5pc of the black hole. The main criterion for the photometric identification of young, massive early-type stars is the lack of CO-absorption in the spectra. We used narrow-band imaging with the near-infrared camera ISAAC at the ESO VLT under excellent seeing conditions to search for young, massive stars within ∼2.5pc of SgrA*. We have found 63 early-type star candidates at R≲2.5pc, with an estimated erroneous identification rate of only about 20%. Considering their K-band magnitudes and interstellar extinction, they are candidates for Wolf-Rayet stars, supergiants, or early O-type stars. Of these, 31 stars are so far unknown young, massive star candidates, all of which lie at R>0.5pc. The surface number density profile of the young, massive star candidates can be well fit by a single power-law (∝R^–Γ), with Γ=1.6±0.17 at R<2.5pc, which is significantly steeper than that of the late-type giants that make up the bulk of the observable stars in the NSC. Intriguingly, this power-law is consistent with the power-law that describes the surface density of young, massive stars in the same brightness range at R≲0.5pc. The finding of a significant number of newly identified early-type star candidates at the Galactic center suggests that young, massive stars can be found throughout the entire cluster which may require us to modify existing theories for star formation at the Galactic center. Follow-up studies are needed to improve the existing data and lay the foundations for a unified theory of star formation in the Milky Way's NSC.