Program: GN-2023A-Q-142

Title:Uncovering the Nature of the Milky Way’s Ultra-Faint Halo Star Clusters
PI:William Cerny
Co-I(s): Ting Li, Andrew Pace, Alex Drlica-Wagner

Abstract

Large-scale digital sky surveys have recently uncovered a mysterious population of resolved stellar systems in the Halo, falling at smaller sizes and lower luminosities than the known ultra-faint dwarf galaxy population. These systems have typically been referred to as “ultra-faint star clusters” (UFSCs) on the basis of their morphologies. Astonishingly little is known about this class of systems: of the nearly 30 UFSCs discovered to date, just 3 have any published radial velocity measurements and only 2 have published spectroscopic metallicities. The severe lack of information about these systems has entirely precluded us from understanding their orbital and accretion histories, as well as their chemistries, prohibiting us from understanding their nature and origins. By measuring radial velocities and [Fe/H] metallicities for stars in 4 UFSCs, we will answer three key questions: (1) Did the UFSCs accrete during early major mergers or form in-situ? (2) What is the relationship between the UFSCs and the ultra-faint dwarf galaxies? and (3) Can the UFSCs be the progenitors of extremely metal-poor stellar streams? Our program will more than double the number of UFSCs with published spectroscopy, providing the first population-level look into the nature and origins of these extreme stellar systems.