The contribution of dissolving star clusters to the population of ultra-faint objects in the outer halo of the Milky Way

Abstract

In the last decade, several ultra faint objects (UFOs, $M_V\gtrsim -3.5$) have been discovered in the outer halo of the Milky Way. For some of these objects it is not clear whether they are star clusters or (ultra-faint) dwarf galaxies. In this work we quantify the contribution of star clusters to the population of UFOs. We extrapolated the mass and Galactocentric radius distribution of the globular clusters using a population model, finding that the Milky Way contains about $3.3_{-1.6}^{+7.3}$ star clusters with $M_V\gtrsim -3.5$ and Galactocentric radius $\ge 20$kpc. To understand whether dissolving clusters can appear as UFOs, we run a suite of direct $N$-body models, varying the orbit, the Galactic potential, the binary fraction and the black hole (BH) natal kick velocities. In the analyses, we consider observational biases such as: luminosity limit, field stars, and line-of-sight projection. We find that star clusters contribute to both the compact and the extended population of UFOs: clusters without BHs appear compact with radii $\sim 5$pc, while clusters that retain their BHs after formation have radii $\gtrsim 20$pc. The properties of the extended clusters are remarkably similar to those of dwarf galaxies: high inferred mass-to-light ratios due to binaries; binary properties mildly affected by dynamical evolution; no observable mass segregation; and flattened stellar mass function. We conclude that the slope of the stellar mass function as a function of Galactocentric radius and the presence/absence of cold streams can discriminate between DM free and DM dominated UFOs.

17 pages, 17 figures; added a comment+reference in the discussion; Accepted for publication in MNRAS

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