  <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.
