Offset of M54 from the Sagittarius Dwarf Spheroidal Galaxy

TL;DR

This study models the positions and motions of stars in the Sagittarius dwarf galaxy and its nucleus M54, revealing a small but significant offset in their centers and motions, which impacts understanding of their orbital history.

Contribution

It provides the first detailed modeling of the positional and kinematic offset between Sgr and M54 using Gaia and APOGEE data, highlighting the importance of offsets in orbital calculations.

Findings

Offset of 0.295 degrees between Sgr and M54 centers

Proper motion offset of approximately 4.9 and -19.7 μas/yr

Line-of-sight velocity offset of 4.1 km/s

Abstract

We present results from simultaneous modeling of 2D (projected along the line of sight) position, proper motion and line-of-sight velocity for \textit{Gaia}- and APOGEE-observed stars near the centre of the Sagittarius (Sgr) dwarf spheroidal galaxy. We use a mixture model that allows for independent sub-populations contributed by the Sgr galaxy, its nuclear star cluster M54, and the Milky Way foreground. We find an offset of $0.295\pm 0.029$ degrees between the inferred centroids of Sgr and M54, corresponding to a (projected) physical separation of $0.135\pm 0.013$ kpc. The detected offset might plausibly be driven by unmodelled asymmetry in Sgr's stellar configuration; however, standard criteria for model selection favour our symmetric model over an alternative that allows for bilateral asymmetry. We infer an offset between the proper motion centres of Sgr and M54 of $[\Delta\mu_{\alpha}\cos\delta,\Delta\mu_{\delta}]=[4.9, -19.7] \pm [6.8, 6.2]$ $\mu$as yr$^{-1}$ ($[0.61, -2.46] \pm [0.85, 0.77] $ km s$^{-1}$), with magnitude similar to the covariance expected due to spatially-correlated systematic error. We infer an offset of $4.1\pm 1.2$ km s$^{-1}$ in line-of-sight velocity. Using inferred values for the systemic positions and motions of Sgr and M54 as initial conditions, we calculate the recent orbital history of a simplified Sgr/M54 system, which we demonstrate to be sensitive to any line-of-sight distance offset between M54 and Sgr, and to the distribution of dark matter within Sgr.