A Chemodynamical Census of the Milky Way's Ultra-Faint Compact Satellites. I. A First Population-Level Look at the Internal Kinematics and Metallicities of 19 Extremely-Low-Mass Halo Stellar Systems

TL;DR

This study tackles whether the Milky Way's faint, ultra-compact satellites are galaxies or star clusters by conducting the first population-level spectroscopic census of 19 UFCSs (≈2/3 of the known population) using Magellan/IMACS and Keck/DEIMOS. It yields mean radial velocities for all targets, velocity dispersions for 15 systems, Ca triplet–based metallicities for 17, metallicity dispersions for 8, and Gaia-based mean proper motions for 18, enabling direct comparison of internal dynamics and chemistry across the UFCS population. The results show UFCSs are kinematically colder than UFDs, suggestive of less dense dark matter halos in many cases, and reveal a wide metallicity spread from about $[Fe/H] \u2248 -3.3$ to $-0.8$, including systems below typical GC metallicity floors. Importantly, while some high-metallicity or younger UFCSs appear cluster-like, as many as ~50% of the sample may be the smallest and least-massive galaxies yet identified, highlighting a continuum between star clusters and galaxies at the lowest masses with significant implications for dark matter and galaxy formation in the low-mass regime.

Abstract

Deep, wide-area photometric surveys have uncovered a population of compact ($r_{1/2} \approx$ 1-15 pc), extremely-low-mass ($M_* \approx$ 20-4000 $M_{\odot}$) stellar systems in the Milky Way halo that are smaller in size than known ultra-faint dwarf galaxies (UFDs) and substantially fainter than most classical globular clusters (GCs). Very little is known about the nature and origins of this population of "Ultra-Faint Compact Satellites" (UFCSs) owing to a dearth of spectroscopic measurements. Here, we present the first spectroscopic census of these compact systems based on Magellan/IMACS and Keck/DEIMOS observations of 19 individual UFCSs, representing $\sim$2/3 of the known population. We securely measure mean radial velocities for all 19 systems, velocity dispersions for 15 (predominantly upper limits), metallicities for 17, metallicity dispersions for 8, and $\textit{Gaia}$-based mean proper motions for 18. This large new spectroscopic sample provides the first insights into population-level trends for these extreme satellites. We demonstrate that: (1) the UFCSs are kinematically colder, on average, than the UFDs, disfavoring very dense dark matter halos in most cases, (2) the UFCS population is chemically diverse, spanning a factor of $\sim$300 in mean iron abundance ($\rm -3.3 \lesssim [Fe/H] \lesssim -0.8$), with multiple systems falling beneath the "metallicity floor" proposed for GCs, and (3) while some higher-metallicity and/or younger UFCSs are clearly star clusters, the dynamical and/or chemical evidence allows the possibility that up to $\sim$50% of the UFCSs in our sample (9 of 19) may represent the smallest and least-massive galaxies yet discovered.

Figures (1)

• Figure 1: A visualization of our UFCS definition and the properties of the systems that comprise our spectroscopic sample. (Left) Absolute magnitude ($M_{V}$) vs. azimuthally-averaged half-light radius ($r_{1/2}$) for the UFCSs as well as the MW GCs (grey $\times$ or black $+$, depending on whether they inhabit the disk/bulge or halo) and dwarf satellite galaxies (filled blue triangles if confirmed, unfilled if not). The morphological boundaries of our UFCS selection are demarcated by red lines, and the UFCSs in our spectroscopic sample are shown as colored points with black outlines. The remaining UFCSs are shown as filled red circles. Two classical GCs (Palomar 13 and AM 4) fall within our UFCS selection. (Right) Absolute magnitude vs. heliocentric distance ($D_{\odot}$) for the same satellite samples. At any given distance, the UFCSs represent the faintest known halo systems; the lack of UFCSs at larger distances is likely a selection effect. No errorbars are shown in either panel for visual clarity. In both panels, the righthand y-axis displays the equivalent stellar masses ($M_*$) assuming $M_*/L_V = 2$.