Eridanus III and DELVE 1: Carbon-rich Primordial Star Clusters or the Smallest Dwarf Galaxies?

Kavli Affiliate: Alexander P. Ji

| First 5 Authors: Joshua D. Simon, Ting S. Li, Alexander P. Ji, Andrew B. Pace, Terese T. Hansen

| Summary:

We present spectroscopy of the ultra-faint Milky Way satellites Eridanus III
(Eri III) and DELVE 1. We identify eight member stars in each satellite and
place non-constraining upper limits on their velocity and metallicity
dispersions. The brightest star in each object is very metal-poor, at [Fe/H] =
-3.1 for Eri III and [Fe/H] = -2.8 for DELVE 1. Both of these stars exhibit
large overabundances of carbon and very low abundances of the neutron-capture
elements Ba and Sr, and we classify them as CEMP-no stars. Because their
metallicities are well below those of the Milky Way globular cluster
population, and because no CEMP-no stars have been identified in globular
clusters, these chemical abundances could suggest that Eri III and DELVE 1 are
dwarf galaxies. On the other hand, the two systems have half-light radii of 8
pc and 6 pc, respectively, which is more compact than any known ultra-faint
dwarfs. We conclude that Eri III and DELVE 1 are either the smallest dwarf
galaxies yet discovered, or they are representatives of a new class of star
clusters that underwent chemical evolution distinct from that of ordinary
globular clusters. In the latter scenario, such objects are likely the most
primordial star clusters surviving today. These possibilities can be
distinguished by future measurements of carbon and/or iron abundances for
larger samples of stars or improved stellar kinematics for the two systems.

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