OGLE Study of the Sagittarius Dwarf Spheroidal Galaxy and its M54 Globular Cluster

TL;DR

This study uses OGLE-IV data to map the 3D structure of the Sagittarius Dwarf Spheroidal galaxy and its M54 globular cluster, identifying variable stars and analyzing their properties to understand the galaxy's composition and distance.

Contribution

It provides a detailed 3D mapping of the Sgr dSph galaxy using RR Lyrae stars and reports new variable star discoveries within M54, enhancing understanding of its stellar populations.

Findings

Line-of-sight thickness of Sgr dSph stream is 2.42 kpc.

Distance to M54 is 26.7 kpc.

Presence of two old stellar populations confirmed.

Abstract

We use the fundamental-mode RR Lyr-type variable stars (RRab) from OGLE-IV survey to draw a 3D picture of the central part of the tidally disrupted Sagittarius Dwarf Spheroidal (Sgr dSph) galaxy. We estimate the line-of-sight thickness of the Sgr dSph stream to be FWHM_cen=2.42 kpc. Based on OGLE-IV observations collected in seasons 2011-2014 we conduct a comprehensive study of stellar variability in the field of the globular cluster M54 (NGC 6715) residing in the core of this dwarf galaxy. Among the total number of 268 detected variable stars we report the identification of 174 RR Lyr stars, four Type II Cepheids, 51 semi-regular variable red giants, three SX Phe-type stars, 18 eclipsing binary systems. Eighty-three variable stars are new discoveries. The distance to the cluster determined from RRab stars is d_M54 = 26.7 +/-0.03(stat) +/-1.3(sys) kpc. From the location of RRab stars in the period-amplitude (Bailey) diagram we confirm the presence of two old populations, both in the cluster and the Sgr dSph stream.