The contribution of N-rich stars to the Galactic stellar halo using APOGEE red giants

TL;DR

This study quantifies the contribution of dissolved globular clusters to the Milky Way's stellar halo using APOGEE data, revealing a higher fraction of former GC stars near the Galactic center compared to outer regions.

Contribution

It provides the first detailed radial profile of N-rich stars in the halo, estimating the total mass contribution of disrupted GCs within 15 kpc.

Findings

N-rich stars contribute up to 16.8% of the halo mass at 1.5 kpc.

Disrupted GCs account for approximately 27.5% of the inner halo stellar mass.

Total stellar mass from dissolved GCs within 15 kpc is about 9.6 x 10^7 solar masses.

Abstract

The contribution of dissolved globular clusters (GCs) to the stellar content of the Galactic halo is a key constraint on models for GC formation and destruction, and the mass assembly history of the Milky Way. Earlier results from APOGEE pointed to a large contribution of destroyed GCs to the stellar content of the inner halo, by as much as 25$\%$, which is an order of magnitude larger than previous estimates for more distant regions of the halo. We set out to measure the ratio between N-rich and normal halo field stars, as a function of distance, by performing density modelling of halo field populations in APOGEE DR16. Our results show that at 1.5 kpc from the Galactic Centre, N-rich stars contribute a much higher 16.8$^{+10.0}_{-7.0}$$\%$ fraction to the total stellar halo mass budget than the 2.7$^{+1.0}_{-0.8}$$\%$ ratio contributed at 10 kpc. Under the assumption that N-rich stars are former GC members that now reside in the stellar halo field, and assuming the ratio between first-and second-population GC stars being 1:2, we estimate a total contribution from disrupted GC stars of the order of 27.5$^{+15.4}_{-11.5}$$\%$ at r = 1.5 kpc and 4.2$^{+1.5}_{-1.3}$$\%$ at r = 10 kpc. Furthermore, since our methodology requires fitting a density model to the stellar halo, we integrate such density within a spherical shell from 1.5-15 kpc in radius, and find a total stellar mass arising from dissolved and/or evaporated GCs of $M_{\mathrm{GC,total}}$ = 9.6$^{+4.0}_{-2.6}$ $\times$ 10$^{7}$ M$\odot$.