The lowest luminosity (L < 10(5) L-circle dot) Milky Way satellite galaxies represent the extreme lower limit of the galaxy luminosity function. These ultra-faint dwarfs are the oldest, most dark matter-dominated, most metal-poor, and least chemically evolved stellar systems known. They therefore provide unique windows into the formation of the first galaxies and the behavior of dark matter on small scales. In this review, we summarize the discovery of ultra-faint dwarfs in the Sloan Digital Sky Survey in 2005 and the subsequent observational and theoretical progress in understanding their nature and origin. We describe their stellar kinematics, chemical abundance patterns, structural properties, stellar populations, orbits, and luminosity function, as well as what can be learned from each type of measurement. We conclude the following:

Draco C1 is a known symbiotic binary star system composed of a carbon red giant and a hot, compact companion-likely a white dwarf-belonging to the Draco dwarf spheroidal galaxy. From near-infrared spectroscopic observations taken by the Apache Point Observatory Galactic Evolution Experiment (APOGEE-2), part of Sloan Digital Sky Survey IV, we provide updated stellar parameters for the cool, giant component, and constrain the temperature and mass of the hot, compact companion. Prior measurements of the periodicity of the system, based on only a few epochs of radial velocity data or relatively short baseline photometric observations, were sufficient only to place lower limits on the orbital period (P > 300 days). For the first time, we report precise orbital parameters for the binary system: with 43 radial velocity measurements from APOGEE spanning an observational baseline of more than 3 yr, we definitively derive the period of the system to be 1220.0(-3.5)(+3.7) days. Based on the newly derived orbital period and separation of the system, together with estimates of the radius of the red giant star, we find that the hot companion must be accreting matter from the dense wind of its evolved companion.

We present deep Hubble Space Telescope (HST) photometry of the ultra-faint dwarf galaxy Eridanus II (Eri II). Eri II, which has an absolute magnitude of M-V = -7.1, is located at a distance of 339 kpc, just beyond the virial radius of the Milky Way. We determine the star formation history of Eri II and measure the structure of the galaxy and its star cluster. We find that a star formation history consisting of two bursts, constrained to match the spectroscopic metallicity distribution of the galaxy, accurately describes the Eri II stellar population. The best-fit model implies a rapid truncation of star formation at early times, with >80% of the stellar mass in place before z similar to 6. A small fraction of the stars could be as young as 8 Gyr, but this population is not statistically significant; Monte Carlo simulations recover a component younger than 9 Gyr only 15% of the time, where they represent an average of 7 +/- 4% of the population. These results are consistent with theoretical expectations for quenching by reionization. The HST depth and angular resolution enable us to show that Eri II's cluster is offset from the center of the galaxy by a projected distance of 23 +/- 3 pc. This offset could be an indication of a small (similar to 50-75 pc) dark matter core in Eri II. Moreover, we demonstrate that the cluster has a high ellipticity of 0.31(-0.06)(+0.05) and is aligned with the orientation of Eri II within 3 degrees 6 degrees, likely due to tides. The stellar population of the cluster is indistinguishable from that of Eri II itself.

We present a new catalog of 40,502 globular cluster (GC) candidates in NGC 5128 out to a projected radius of similar to 150 kpc based on data from the Panoramic Imaging Survey of Centaurus and Sculptor, the Gaia Data Release 2, and the NOAO Source Catalog. Ranking these candidates based on the likelihood that they are true GCs, we find that approximately 1900 belong to our top two ranking categories and should be the highest priority for spectroscopic follow-up for confirmation. Taking into account our new data and a vetting of previous GC catalogs, we estimate a total GC population of 1450 +/- 160 GCs. We show that a substantial number of sources previously argued to be low-velocity GCs are instead foreground stars, reducing the inferred GC velocity dispersion. This work showcases the power of Gaia to identify slightly extended sources at the similar to 4 Mpc distance of NGC 5128, enabling accurate identification of GCs throughout the entire extended halo, not just the inner regions that have been the focus of most previous work.

The Milky Way is surrounded by dozens of ultrafaint (<10(5) L-circle dot) dwarf satellite galaxies(1-3). They are the remnants of the earliest galaxies(4), as confirmed by their ancient5 and chemically primitive(6,7) stars. Simulations(8-10) suggest that these systems formed within extended dark matter halos and experienced early galaxy mergers and feedback. However, the signatures of these events would lie outside their core regions(11), where spectroscopic studies are challenging(12). Here we identify members of the Tucana II ultrafaint dwarf galaxy out to nine half-light radii, demonstrating the system to be markedly more spatially extended and chemically primitive than previously found. The distant stars in this galaxy are, on average, extremely metal poor (less than or similar to 1/1000 of the solar iron abundance), affirming Tucana II as the most metal-poor known galaxy. We observationally establish an extended dark matter halo surrounding an ultrafaint dwarf galaxy out to 1 kpc, with a total mass of > 10(7) M-circle dot, consistent with a generalized Navarro- Frenk-White density profile. The extended nature of Tucana II suggests that it may have undergone strong bursty feedback or been the product of an early galactic merger(10,11). We demonstrate that spatially extended stellar populations in ultrafaint dwarf galaxies(13,14) are observable, opening up the possibility for detailed studies of the stellar halos of relic galaxies.

We present the color-magnitude diagrams and star formation histories (SFHs) of seven ultra-faint dwarf galaxies: Horologium 1, Hydra 2, Phoenix 2, Reticulum 2, Sagittarius 2, Triangulum 2, and Tucana 2, derived from high-precision Hubble Space Telescope photometry. We find that the SFH of each galaxy is consistent with them having created at least 80% of the stellar mass by z similar to 6. For all galaxies, we find quenching times older than 11.5 Gyr ago, compatible with the scenario in which reionization suppresses the star formation of small dark matter halos. However, our analysis also reveals some differences in the SFHs of candidate Magellanic Cloud satellites, i.e., galaxies that are likely satellites of the Large Magellanic Cloud and that entered the Milky Way potential only recently. Indeed, Magellanic satellites show quenching times about 600 Myr more recent with respect to those of other Milky Way satellites, on average, even though the respective timings are still compatible within the errors. This finding is consistent with theoretical models that suggest that satellites' SFHs may depend on their host environment at early times, although we caution that within the error bars all galaxies in our sample are consistent with being quenched at a single epoch.

We present new spectroscopic observations of the diffuse Milky Way satellite galaxies Antlia 2 and Crater 2, taken as part of the Southern Stellar Stream Spectroscopic Survey (S (5)). The new observations approximately double the number of confirmed member stars in each galaxy and more than double the spatial extent of spectroscopic observations in Antlia 2. A full kinematic analysis, including Gaia EDR3 proper motions, detects a clear velocity gradient in Antlia 2 and a tentative velocity gradient in Crater 2. The velocity gradient magnitudes and directions are consistent with particle stream simulations of tidal disruption. Furthermore, the orbit and kinematics of Antlia 2 require a model that includes the reflex motion of the Milky Way induced by the Large Magellanic Cloud. We also find that Antlia 2's metallicity was previously overestimated, so it lies on the empirical luminosity-metallicity relation and is likely only now experiencing substantial stellar mass loss. Current dynamical models of Antlia 2 require it to have lost over 90% of its stars to tides, in tension with the low stellar mass loss implied by the updated metallicity. Overall, the new kinematic measurements support a tidal disruption scenario for the origin of these large and extended dwarf spheroidal galaxies.

We present deep g- and r-band Magellan/Megacam photometry of two dwarf galaxy candidates discovered in the Dark Energy Survey (DES), Grus I and Indus II (DES J2038-4609). For the case of Grus I, we resolved the main sequence turn-off (MSTO) and similar to 2 mags below it. The MSTO can be seen at g(0) similar to 24 with a photometric uncertainty of 0.03 mag. We show Grus I to be consistent with an old, metal-poor (similar to 13.3 Gyr, [Fe/H] similar to -1.9) dwarf galaxy. We derive updated distance and structural parameters for Grus I using this deep, uniform, wide-field data set. We find an azimuthally-averaged halflight radius more than two times larger (similar to 151(-31)(+21) pc; similar to 4'. 16(-0.74)(+0.54)) and an absolute V-band magnitude similar to-4.1 that is similar to 1 magnitude brighter than previous studies. We obtain updated distance, ellipticity, and centroid parameters that are in agreement with other studies within uncertainties. Although our photometry of Indus II is similar to 2-3 magnitudes deeper than the DES Y1 public release, we find no coherent stellar population at its reported location. The original detection was located in an incomplete region of sky in the DES Y2Q1 data set and was flagged due to potential blue horizontal branch member stars. The best-fit isochrone parameters are physically inconsistent with both dwarf galaxies and globular clusters. We conclude that Indus II is likely a false positive, flagged due to a chance alignment of stars along the line of sight.