2016A&A...588A..49N

Context. Nuclear star clusters (NSCs) at the dynamical center of galaxies appear to have a complex star formation history. This suggests repeated star formation, even in the influence of the strong tidal field from supermassive black holes. Although the central region of our Galaxy is an ideal target for studies of the star formation history in the NSCs, most studies in the past have concentrated on a projected distance of R_SgrA*∼0.5pc from the supermassive black hole SgrA*.
Aims. In our previous study, we detected 31 so far unknown early-type star candidates throughout the Galactic NSC (at R_SgrA*=0.5-3pc). They were found via near-infrared (NIR) imaging observations with narrow-band filters which are sensitive to CO absorption lines at ∼2.3µm, a prominent feature for old, late-type stars. The aim of this study is to confirm the spectral type for the early-type star candidates.
Methods. We have carried out NIR spectroscopic observations of the early-type star candidates using Subaru/IRCS/AO188 and the laser guide star system. K-band spectra for 20 out of the 31 candidates and reference late-type stars were obtained. By determining an equivalent width, EW(CO), of the ¹²CO absorption feature at ≃2.294µm, we have derived an effective temperature and a bolometric magnitude for each candidate and late-type star, and then constructed an HR diagram.
Results. No young (∼Myr) massive stars are included in the 20 candidates we observed; however, 13 candidates are most likely intermediate-age giants (50-500Myr). Two other sources have ages of ∼1Gyr and the remaining five sources are old (>1Gyr), late-type giants.
Conclusions. Although none of the early-type star candidates from our previous narrow-band imaging observations can be confirmed as a young star, we find that the photometric technique can distinguish old, late-type giants from young and intermediate-age populations. From the 20 spectroscopically observed candidates, 65% of them are confirmed as being younger than 500Myr. The intermediate-age stars could be as yet unknown members of a population formed in a starburst ∼100Myr ago. Finding no young (∼a few Myr) stars at R_SgrA*=0.5-3pc favors the in situ formation scenario for the presence of the young stars at R_SgrA*<0.5pc, although we do not completely exclude the possible existence of unknown young, massive stars in the region from our observations. Furthermore, the different spatial distributions of the young and the intermediate-age stars imply that the Galactic NSC is an aggregate of stars that were born in different places and under a variety of physical conditions.