Abstract
<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. <Aim.> In our previous study, we have detected 31 so far unknown early-type star candidates throughout the Galactic NSC (at 0.5 - 3 pc from Sgr A*; Nishiyama and Schoedel 2013). The aim of this study is a confirmation of the spectral type for the candidates. <Method.> We have carried out NIR spectroscopic observations of the candidates using Subaru/IRCS/AO188/LGS. K-band spectra for 20 out of the 31 candidates were obtained. By determining an equivalent width, EW(CO), of the 12CO absorption feature at 2.294 um, we have derived an effective temperature and a bolometric magnitude for each candidate, 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 - 500 Myr). Two other sources have ages of ~1 Gyr, and the remaining five sources are old (> 1 Gyr), 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 is sensitive to distinguish old, late-type giants from young and intermediate-age populations. The intermediate-age stars could be so far unknown members of a population formed in a starburst ~100 Myr ago. Finding no young (~ a few Myr) stars at R = 0.5 - 3 pc favors the in-situ formation scenario for the presence of the young stars at R < 0.5 pc. Furthermore, the different spatial distributions of the young and the intermediate-age stars imply that the Galactic NSC is an aggregate of stars born in different places and under different physical conditions.
Highlights
Most galaxies host luminous nuclear star clusters (NSCs; e.g., Carollo et al 1998; Côté et al 2006; Böker 2010; Georgiev & Böker 2014). Many of these NSCs have been found to coexist with supermassive black holes (SMBHs) at the center of the host galaxies (Seth et al 2008; Graham & Spitler 2009; Neumayer & Walcher 2012)
Unlike SMBHs, it is expected that NSCs provide a visible record of gas accretion and star formation at the center of galaxies; studying stellar populations in NSCs can provide clues as to how stars are formed within the strong tidal field of SMBHs
We have carried out spectroscopic observations of 20 out of 31 early-type star candidates in the NSC at the center of our Galaxy, which were identified in the imaging survey by Nishiyama & Schödel (2013)
Summary
Most galaxies host luminous nuclear star clusters (NSCs; e.g., Carollo et al 1998; Côté et al 2006; Böker 2010; Georgiev & Böker 2014). Most of the past observations of stellar populations in the Galactic NSC have been limited to a region within a projected radius of RSgr A∗ ∼ 0.5 pc from Sgr A* (see, e.g., Genzel et al 2003; Paumard et al 2006; Maness et al 2007; Do et al 2009; Bartko et al 2010; Pfuhl et al 2011; Do et al 2013; Lu et al 2013). Red color (positive value) in [2.25]–[2.34] means a weak CO absorption at 2.34 μm which is an indicator for early spectral type stars. Blue ×s are the early-type star candidates found in Nishiyama & Schödel (2013) and they are distributed at the red side of the RGB, indicating earlier spectral type. We present the results of the observations for 20 of the above mentioned 31 early-type star candidates located outside the central 0.5 pc region
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