The nucleus of the Sagittarius dSph galaxy and M54: a window on the process of galaxy nucleation

TL;DR

This study investigates the nucleus of the Sagittarius dwarf galaxy and M54, revealing distinct kinematic profiles that suggest independent formation of the nucleus and a history of dynamical friction bringing M54 to its current position.

Contribution

It provides detailed velocity and metallicity measurements of Sgr and M54, and offers new insights into galaxy nucleation processes through combined observational data and N-body simulations.

Findings

M54 has a steeply decreasing velocity dispersion profile towards the center.

Sgr's nucleus exhibits a flat velocity dispersion profile over a large radius.

Results support independent formation of the nucleus and M54's infall due to dynamical friction.

Abstract

We present the results of a thorough study of the nucleus of the Sgr dwarf spheroidal galaxy and of the bright globular cluster M54 that resides within the same nucleus (Sgr,N). We have obtained accurate radial velocities and metallicity estimates for 1152 candidate Red Giant Branch stars of Sgr and M54 lying within ~ 9 arcmin from the center of the galaxy, from Keck/DEIMOS and VLT/FLAMES spectra of the infrared Calcium II triplet. Using both velocity and metallicity information we selected two samples of 425 and 321 very-likely members of M54 and of Sgr,N, respectively. The two considered systems display significantly different velocity dispersion profiles: M54 has a steeply decreasing profile from r=0, where sigma= 14.2 km/s, to r=3.5 arcmin where it reaches sigma=5.3 km/s, then it appears to rise again to sigma= 10 km/s at r=7 arcmin. In contrast Sgr,N has a uniformly flat profile at sigma=9.6 km/s over the whole 0 < r < 9 arcmin range. Using data from the literature we show that the velocity dispersion of Sgr remains constant at least out to r ~ 100 arcmin and there is no sign of the transition between the outer flat-luminosity-profile core and the inner nucleus in the velocity profile. These results - together with a re-analysis of the surface brightness profile of Sgr,N and a suite of dedicated N-body simulations - provide very strong support for the hypothesis that the nucleus of Sgr formed independently of M54, which probably plunged to its present position, coincident with Sgr,N, because of significant decay of the original orbit due to dynamical friction.