A deep view into the nucleus of the Sagittarius Dwarf Spheroidal Galaxy with MUSE. I. Data and stellar population characterization

TL;DR

This study uses MUSE data to analyze the stellar populations of the nucleus of the Sagittarius Dwarf Spheroidal Galaxy, revealing multiple populations with distinct ages and metallicities, and insights into their formation history.

Contribution

It provides a detailed characterization of the stellar populations in Sgr dSph's nucleus using combined spectroscopic and photometric data, revealing their age-metallicity relation and spatial distribution.

Findings

Identified three distinct stellar populations with different ages and metallicities.

Found that the young metal-rich population is likely formed in situ in the nucleus.

Old and metal-poor populations may result from accreted globular clusters.

Abstract

The center of the Sagittarius dwarf spheroidal galaxy (Sgr dSph) hosts a nuclear star cluster, M54, which is the only galaxy nucleus that can be resolved into individual stars at optical wavelengths. It is thus a key target for understanding the formation of nuclear star clusters and their relation to globular clusters. We present a large Multi-Unit Spectroscopic Explorer (MUSE) data set that covers M54 out to $\sim$2.5 half-light radius, from which we extracted the spectra of $\sim$6600 cluster member stars. We use these data in combination with HST photometry to derive age and metallicity for each star. The stellar populations show a well defined age-metallicity relation, implying an extended formation history for the central region of Sgr dSph. We classify these populations into three groups, all with the same systemic velocity: young metal-rich (YMR; 2.2\,Gyr, \mbox{[Fe/H]$=-0.04$}); intermediate-age metal-rich (IMR; 4.3\,Gyr, \mbox{[Fe/H]$=-0.29$}); and old metal-poor (OMP; 12.2\,Gyr, \mbox{[Fe/H]$=-1.41$}). The YMR and OMP populations are more centrally concentrated than the IMR population, which are likely stars of the Sgr dSph. We suggest the OMP population is the result of accretion and merging of two or more old and metal-poor globular clusters dragged to the center by dynamical friction. The YMR is consistent with being formed by in situ star formation in the nucleus. The ages of the YMR population suggest that it may have been triggered into forming when the Sgr dSph began losing its gas during the most recent interaction with the Milky Way, $\sim$3\,Gyr ago.