of \( \sim \, 1 0 ^ { 8 - 9 } M _ { \odot } \) do host stars. These halos can be observed as ultrafaint galaxies. These systems are currently the most DM-dominated systems with a ratio of DM to stellar mass significantly exceeding a factor of a hundred. DM halos below this threshold are expected to be depleted of stars and are effectively invisible. Halos at and below the galaxy formation threshold offer a pristine environment in which baryonic physics is subdominant to the dark matter behavior, and hence are substantially less affected by baryonic systematics.

# 5.2 Observational Probes

In the following we list some of the promising current and future probes that can probe the abundance and inner composition of halos in the mass range of \( 1 0 ^ { 6 - 9 } M _ { \odot } \).

# 5.2.1 Ultra-faint galaxies

Ultra-faint dwarf (UFD) galaxies, defined as dark matter-dominated stellar systems with luminosity \( L < 1 0 ^ { 5 } \)\( ; L _ { \odot } \) [435], occupy the smallest dark matter halos that form galaxies, with \( 1 0 ^ { 8 } ~ M _ { \odot } \lesssim M _ { \mathrm { h a l o } } \lesssim 1 0 ^ { 1 0 } ~ M _ { \odot } \)[436, 437]. These systems therefore provide crucial information about the astrophysical processes that govern the faint end of galaxy formation and the dark matter physics that impacts the abundance and properties of halos near the galaxy formation threshold.

To date, ultra-faints have exclusively been detected within distances of ∼ 300 kpc as satellites of the Milky Way. Deep photometric surveys including the Dark Energy Survey have significantly increased the number of known Milky Way satellites in recent years, which now exceeds ∼ 60 systems [438]. Nevertheless, both empirical and theoretical arguments suggest that our census of ultra-faints—even within the Milky Way’s virial radius—is highly incomplete [439–441]. Forthcoming observational facilities including the Vera C. Rubin Ob-servatory and the Nancy Grace Roman Space Telescope are expected to drastically improve upon current ultra-faint detection capabilities. Specifically, Rubin is forecasted to discover the entire population of UFD Milky Way satellites in the Southern hemisphere outside the Galactic disk and with surface brightness down to \( \sim 3 2 \mathrm { m a g ~ a r c s e c } ^ { - 2 } \)[157, 314]. Excitingly, these detections are expected to extend well beyond distances of 300 kpc, allowing UFD populations to be characterized throughout much of the Local Volume [442].

Our understanding of the dark matter halos that host ultra-faints has significantly im-proved over the last decade. In particular, hydrodynamical simulations have started to re-solve the formation and evolution of UFDs in a cosmological context, including as satellites of larger hosts [443–448]. In parallel, semi-analytic and empirical galaxy–halo connection models have been combined with observations to infer ultra-faints’ halo properties; in these models, the number of observable ultra-faints suggests that these systems occupy halos with masses down to \( \sim 1 0 ^ { 8 } \)\( : M _ { \odot } \) (near the “atomic cooling limit”; [436, 437, 449, 450]) or even lower [451, 452]. The fraction of low-mass halos that host UFDs and the scatter in the stellar mass–halo mass relation for these systems are also being studied in order to precisely characterize the properties of halos near the galaxy formation threshold [453–456].

Ultra-faints provide stringent constraints on dark matter particle properties. Specifically, the number of observable UFD Milky Way satellites informs dark matter physics that sup-presses the present-day abundance of dark matter halos with masses of \( \sim 1 0 ^ { 8 } ~ M _ { \odot } \), yielding