Stirred, not shaken: Dislodging $\omega$ Centauri from the Sausage galaxy through bar resonances

Abstract

We test the idea proposed by Laporte \& Orkney that $\omega$ Centauri ($\omega$Cen) is the central nuclear star cluster of the Gaia-Sausage-Enceladus galaxy (GSE) which got dislodged by the bar. We run a series of simulations in time-evolving bar potentials with present-day pattern speeds within Portail et al's ${\Omega }_b=39\pm 3.5\mathrm{km}{\mathrm{s}}^{-1}\mathrm{kp}{\mathrm{c}}^{-1}$ and rate of slow-down ${\dot{\Omega }}_{\mathrm{b}}=4.5\pm 1.4\mathrm{km}{\mathrm{s}}^{-1}\mathrm{kp}{\mathrm{c}}^{-1}\mathrm{Gy}{\mathrm{r}}^{-1}$ to study how bar resonances modulate the angular momentum and energies of both GSE-debris and $\omega$Cen's location in $(E,L_z)$ space. We find that the retrograde $3:2$ resonance is capable of inflicting changes of $\Delta E\approx 0.4\times 10^5\mathrm{k}{\mathrm{m}}^2{\mathrm{s}}^{-2}$, $\Delta L_z\approx 0.3-0.5\times 10^3\mathrm{km}{\mathrm{s}}^{-1}\mathrm{kp}{\mathrm{c}}^{-1}$ within timescales of $\Delta t\approx 0.5-1\mathrm{Gyr}$, well within the bar's lifespan $t_{\mathrm{bar}}\approx 10-8\mathrm{Gyr}$. These results are independent of the choice of potential or modelling. We uncover a sample of N-rich stars with metallicities $-2.0<[Fe/H]<-1.0$ and aluminium enrichments consistent with $\omega$Cen. In ($E,L_z$) space, the map reveals a corridor of trailing N-rich stars clustered about the region connecting $\omega$Cen to the GSE's debris centroid, matching $\omega$Cen's past predicted track. This provides the best evidence yet that $\omega$Cen is the nuclear star cluster of the GSE. Using M54 and $\omega$Cen's chemistry we show that neither first nor second generations follow general galaxy-wide scale chemical enrichment trends. This is empirical evidence that GC chemical trends are a cluster-scale physics problem, not a galaxy-wide one.

15 pages, 12 figures, submitted to MNRAS. Comments welcome