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

Abstract

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 ${{\ \rm per\ cent}}$, 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 nitrogen-rich (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$^{+10.0}_{-7.0}\, {{\ \rm per\ cent}}$ fraction to the total stellar halo mass budget than the 2.7$^{+1.0}_{-0.8}\, {{\ \rm per\ cent}}$ 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$^{+15.4}_{-11.5}\, {{\ \rm per\ cent}}$ at r = 1.5 kpc and 4.2$^{+1.5}_{-1.3}\, {{\ \rm per\ cent}}$ 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 to 15 kpc in radius, and find a total stellar mass arising from dissolved and/or evaporated GCs of MGC,total = 9.6$^{+4.0}_{-2.6}\, \times$ 107 M⊙.