The Distance to the Sagittarius Dwarf Galaxy and its Relation to M54 using Infrared Photometry of RR Lyrae Variable Stars

The Sagittarius dwarf galaxy (Sgr) is among the most massive satellites of the Milky Way and is unique due to its extensive tails of tidally stripped stars. Models of these tails provide strong constraints on the orbital history of Sgr, which in turn lend insight into the structure of the Milky Way’s dark matter halo. Fully understanding the dynamics of Sgr, however, relies on a precise distance to the core. Yet past measurements yield a wide range of values (22.0 - 28.4 kpc) or a variation of ∼ 25%. In addition, the line-of-sight separation of Sgr from the globular cluster M 54 is an important test of the “cusp-core problem”, which highlights the discrepancy between expected formation scenarios for dwarf galaxy dark matter haloes and their observed density distributions. Through photometry of 3.6µm Spitzer IRAC data and GLOESS light curve fitting, we have measured the mean apparent magnitudes of 45 RR Lyrae variables in the Sgr core and 25 RR Lyrae stars in M 54. Apparent magnitudes are measured using multiepoch PSF photometry, and corresponding absolute magnitudes are determined via the RR Lyrae period-luminosity relation. We find the mean distances to Sgr and M 54 to be dSgr = 27.10 ± 0.21 (ran) ± 1.11 (sys) kpc and dM54 = 26.42±0.34(ran)±0.97(sys) kpc, respectively. Our result improves the precision with which the distance to Sgr is known. We also show that the separation of M 54 and Sgr indicates that the Sgr dark matter halo is presently cored, suggesting that the dwarf galaxy has undergone a cusp-core transformation.