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.

We present a preview of the faint dwarf galaxy discoveries that will be possible with the Vera C. Rubin Observatory and Subaru Hyper Suprime-Cam in the next decade. In this work, we combine deep ground-based images from the Panoramic Imaging Survey of Centaurus and Sculptor and extensive image simulations to investigate the recovery of faint, resolved dwarf galaxies in the Local Volume with a matched-filter technique. We adopt three fiducial distances (1.5, 3.5, and 5 Mpc) and quantitatively evaluate the effects on dwarf detection of varied stellar backgrounds, ellipticity, and Milky Way foreground contamination and extinction. We show that our matched-filter method is powerful for identifying both compact and extended systems, and near-future surveys will be able to probe at least similar to 4.5 mag below the tip of the red giant branch (TRGB) for a distance of up to 1.5 Mpc and similar to 2 mag below the TRGB at 5 Mpc. This will push the discovery frontier for resolved dwarf galaxies to fainter magnitudes, lower surface brightnesses, and larger distances. Our simulations show the secure census of dwarf galaxies down to M ( V ) approximate to -5, -7, and -8 will soon be within reach, out to 1.5, 3.5, and 5 Mpc, respectively, allowing us to quantify the statistical fluctuations in satellite abundances around hosts and parse environmental effects as a function of host properties.

We present deep Hubble Space Telescope (HST) photometry of the ultra-faint dwarf (UFD) galaxies Pegasus III (Peg III) and Pisces II (Psc II), two of the most distant satellites in the halo of the Milky Way (MW). We measure the structure of both galaxies, derive mass-to-light ratios with newly determined absolute magnitudes, and compare our findings to expectations from UFD-mass simulations. For Peg III, we find an elliptical half-light radius of a(h) = 1.'88(-0.33)(+0.42) (118(-30)(+31) pc) and M-V = -4.17(-0.22)(+0.19); for Psc II, we measure a(h) = 1.'31(-0.09)(+0.10) (69 +/- 8 pc) and M-V = -4.28(-0.16)(+0.19). We do not find any morphological features that indicate a significant interaction between the two has occurred, despite their close separation of only similar to 40 kpc. Using proper motions (PMs) from Gaia early Data Release 3, we investigate the possibility of any past association by integrating orbits for the two UFDs in an MW-only and a combined MW and Large Magellanic Cloud (LMC) potential. We find that including the gravitational influence of the LMC is crucial, even for these outer-halo satellites, and that a possible orbital history exists where Peg III and Psc II experienced a close (similar to 10-20kpc) passage about each other just over similar to 1 Gyr ago, followed by a collective passage around the LMC (similar to 30-60 kpc) just under similar to 1 Gyr ago. Considering the large uncertainties on the PMs and the restrictive priors imposed to derive them, improved PM measurements for Peg III and Psc II will be necessary to clarify their relationship. This would add to the rare findings of confirmed pairs of satellites within the Local Group.

We present observations of the dwarf galaxies GALFA Dw3 and GALFA Dw4 with the Advanced Camera for Surveys on the Hubble Space Telescope. These galaxies were initially discovered as optical counterparts to compact H I clouds in the GALFA survey. Both objects resolve into stellar populations which display old red giant branch (RGB), younger helium-burning, and massive main sequence stars. We use the tip of the RGB method to determine the distance to each galaxy, finding distances of 7.61(-0.29)(+0.28) Mpc and 3.10(-0.17)(+0.16) Mpc, respectively. With these distances we show that both galaxies are extremely isolated, with no other confirmed objects within similar to 1.5 Mpc of either dwarf. GALFA Dw4 is also found to be unusually compact for a galaxy of its luminosity. GALFA Dw3 and Dw4 contain H II regions with young star clusters and an overall irregular morphology; they show evidence of ongoing star formation through both ultraviolet and H alpha observations and are therefore classified as dwarf irregulars (dIrrs). The star formation histories of these two dwarfs show distinct differences: Dw3 shows signs of a recently ceased episode of active star formation across the entire dwarf, while Dw4 shows some evidence for current star formation in spatially limited H II regions. Compact H I sources offer a promising method for identifying isolated field dwarfs in the Local Volume, including GALFA Dw3 and Dw4, with the potential to shed light on the driving mechanisms of dwarf galaxy formation and evolution.

Binary stars can inflate the observed velocity dispersion of stars in dark matter-dominated systems such as ultrafaint dwarf galaxies (UFDs). However, the population of binaries in UFDs is poorly constrained by observations, with preferred binary fractions for individual galaxies ranging from a few percent to near unity. Searching for wide binaries through nearest neighbor (NN) statistics (or the two-point correlation function) has been suggested in the literature, and we apply this method for the first time to detect wide binaries in a UFD. By analyzing the positions of stars in Reticulum II (Ret II) from Hubble Space Telescope images, we search for angularly resolved wide binaries in Ret II. We find that the distribution of their NN distances shows an enhancement at projected separations of less than or similar to 1 '' relative to a model containing no binaries. We show that such an enhancement can be explained by a wide binary fraction of f(b) approximate to 0.007(-0.003)(+0.008) at separations of more than 3000 au. Under the assumption that the binary separation distribution is similar to that in the Milky Way, the total binary fraction in Ret II may be on the order of 50%. We also use the observed magnitude distribution of stars in Ret II to constrain the initial mass function over the mass range 0.34-0.78 M-circle dot, finding that a shallow power-law slope of 1.01 <= alpha <= 1.15 matches the data.

We present deep Hubble Space Telescope (HST) imaging of five faint dwarf galaxies associated with the nearby spiral NGC 253 (D approximate to 3.5 Mpc). Three of these are newly discovered dwarf galaxies, while all five were found in the Panoramic Imaging Survey of Centaurus and Sculptor, a Magellan+Megacam survey to identify faint dwarfs and other substructures in resolved stellar light around massive galaxies outside of the Local Group. Our HST data reach greater than or similar to 3 magnitudes below the tip of the red giant branch for each dwarf, allowing us to derive their distances, structural parameters, and luminosities. All five systems contain mostly old, metal-poor stellar populations (age similar to 12 Gyr, [M/H] less than or similar to -1.5) and have sizes (r (h) similar to 110-3000 pc) and luminosities (M (V) similar to -7 to -12 mag) largely consistent with Local Group dwarfs. The three new NGC 253 satellites are among the faintest systems discovered beyond the Local Group. We also use archival H i data to place limits on the gas content of our discoveries. Deep imaging surveys such as our program around NGC 253 promise to elucidate the faint end of the satellite luminosity function and its scatter across a range of galaxy masses, morphologies, and environments in the decade to come.