Discovery of low-metallicity stars in the central parsec of the Milky Way

• Main Authors: Do, Tuan, Kerzendorf, Wolfgang, Winsor, Nathan, Støstad, Morten, Morris, Mark R, Lu, Jessica R, Ghez, Andrea M
• Format: Journal Article
• Language: English
• Published: 25-06-2015
• Subjects: Physics - Astrophysics of Galaxies
• DOI: 10.48550/arxiv.1506.07891

We present a metallicity analysis of 83 late-type giants within the central 1
pc of the Milky Way. K-band spectroscopy of these stars were obtained with the
medium-spectral resolution integral-field spectrograph NIFS on Gemini North
using laser-guide star adaptive optics. Using spectral template fitting with
the MARCS synthetic spectral grid, we find that there is large variation in
metallicity, with stars ranging from [M/H] $<$ -1.0 to above solar metallicity.
About 6\% of the stars have [M/H] $<$ -0.5. This result is in contrast to
previous observations, with smaller samples, that show stars at the Galactic
center have approximately solar metallicity with only small variations. Our
current measurement uncertainties are dominated by systematics in the model,
especially at [M/H] $>$ 0, where there are stellar lines not represented in the
model. However, the conclusion that there are low metallicity stars, as well as
large variations in metallicity is robust. The metallicity may be an indicator
of the origin of these stars. The low-metallicity population is consistent with
that of globular clusters in the Milky Way, but their small fraction likely
means that globular cluster infall is not the dominant mechanism for forming
the Milky Way nuclear star cluster. The majority of stars are at or above solar
metallicity, which suggests they were formed closer to the Galactic center or
from the disk. In addition, our results indicate that it will be important for
star formation history analyses using red giants at the Galactic center to
consider the effect of varying metallicity.