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

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${}_{-7.0}^{+10.0}$$\%$ fraction to the total stellar halo mass budget than the 2.7${}_{-0.8}^{+1.0}$$\%$ 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${}_{-11.5}^{+15.4}$$\%$ at r = 1.5 kpc and 4.2${}_{-1.3}^{+1.5}$$\%$ 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 $M_{\mathrm{GC},\mathrm{total}}$ = 9.6${}_{-2.6}^{+4.0}$ $\times$ 10${}^7$ M$\odot$.

Paper accepted for Publication in MNRAS, correction of Fig 7

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