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.

This paper documents the seventeenth data release (DR17) from the Sloan Digital Sky Surveys; the fifth and final release from the fourth phase (SDSS-IV). DR17 contains the complete release of the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey, which reached its goal of surveying over 10,000 nearby galaxies. The complete release of the MaNGA Stellar Library accompanies this data, providing observations of almost 30,000 stars through the MaNGA instrument during bright time. DR17 also contains the complete release of the Apache Point Observatory Galactic Evolution Experiment 2 survey that publicly releases infrared spectra of over 650,000 stars. The main sample from the Extended Baryon Oscillation Spectroscopic Survey (eBOSS), as well as the subsurvey Time Domain Spectroscopic Survey data were fully released in DR16. New single-fiber optical spectroscopy released in DR17 is from the SPectroscipic IDentification of ERosita Survey subsurvey and the eBOSS-RM program. Along with the primary data sets, DR17 includes 25 new or updated value-added catalogs. This paper concludes the release of SDSS-IV survey data. SDSS continues into its fifth phase with observations already underway for the Milky Way Mapper, Local Volume Mapper, and Black Hole Mapper surveys.

We report the kinematic, orbital, and chemical properties of 12 stellar streams with no evident progenitors using line-of-sight velocities and metallicities from the Southern Stellar Stream Spectroscopic Survey (S (5)), proper motions from Gaia EDR3, and distances derived from distance tracers or the literature. This data set provides the largest homogeneously analyzed set of streams with full 6D kinematics and metallicities. All streams have heliocentric distances between similar to 10 and 50 kpc. The velocity and metallicity dispersions show that half of the stream progenitors were disrupted dwarf galaxies (DGs), while the other half originated from disrupted globular clusters (GCs), hereafter referred to as DG and GC streams. Based on the mean metallicities of the streams and the mass-metallicity relation, the luminosities of the progenitors of the DG streams range between those of Carina and Ursa Major I (-9.5 less than or similar to M ( V ) less than or similar to -5.5). Four of the six GC streams have mean metallicities of [Fe/H] < -2, more metal poor than typical Milky Way (MW) GCs at similar distances. Interestingly, the 300S and Jet GC streams are the only streams on retrograde orbits in our dozen-stream sample. Finally, we compare the orbital properties of the streams with known DGs and GCs in the MW, finding several possible associations. Some streams appear to have been accreted with the recently discovered Gaia-Enceladus-Sausage system, and others suggest that GCs were formed in and accreted together with the progenitors of DG streams whose stellar masses are similar to those of Draco to Carina (similar to 10(5)-10(6) M (circle dot)).

We measure homogeneous distances to M31 and 38 associated stellar systems (-16.8 <= M ( V ) <= -6.0), using time-series observations of RR Lyrae stars taken as part of the Hubble Space Telescope Treasury Survey of M31 Satellites. From >700 orbits of new/archival Advanced Camera for Surveys imaging, we identify >4700 RR Lyrae stars and determine their periods and mean magnitudes to a typical precision of 0.01 day and 0.04 mag. Based on period-Wesenheit-metallicity relationships consistent with the Gaia eDR3 distance scale, we uniformly measure heliocentric and M31-centric distances to a typical precision of similar to 20 kpc (3%) and similar to 10 kpc (8%), respectively. We revise the 3D structure of the M31 galactic ecosystem and: (i) confirm a highly anisotropic spatial distribution such that similar to 80% of M31's satellites reside on the near side of M31; this feature is not easily explained by observational effects; (ii) affirm the thin (rms 7-23 kpc) planar "arc" of satellites that comprises roughly half (15) of the galaxies within 300 kpc from M31; (iii) reassess the physical proximity of notable associations such as the NGC 147/185 pair and M33/AND xxii; and (iv) illustrate challenges in tip-of-the-red-giant branch distances for galaxies with M ( V ) > - 9.5, which can be biased by up to 35%. We emphasize the importance of RR Lyrae for accurate distances to faint galaxies that should be discovered by upcoming facilities (e.g., Rubin Observatory). We provide updated luminosities and sizes for our sample. Our distances will serve as the basis for future investigation of the star formation and orbital histories of the entire known M31 satellite system.

MegaMapper is a 6.5m Magellan-like telescope fitted with a wide-field-corrector (WFC) and atmospheric-dispersion-corrector (ADC) that delivers a 3. diameter corrected field-of-view. The telescope's focal surface is populated by similar to 25, 000 robotic fiber-positioners feeding a cluster of 36 DESI-like medium resolution spectrographs. We present the facility concept for MegaMapper including: conceptual optical and opto-mechanical designs for the telescope and WFC/ADC that deliver less than or similar to 0.4 '' image quality over the full FOV for zenith distances <= 50 degrees; the development of a new and modular robotic fiber-positioner focal plane design that can populate the focal surface at high densities (6.2 mm pitch or similar to 1 per arcmin(2)); and concepts for hosting the MegaMapper spectrograph cluster under environmentally controlled conditions inside the telescope enclosure. Building on existing and proven designs and technologies, MegaMapper aims to minimize the project's technical risk and cost while delivering a competitive next-generation massively multiplexed spectroscopic facility. MegaMapper will lead the study of inflation, dark energy, dark matter, and time-domain astronomy over the next decades by carrying out wide-field cosmological galaxy-redshift surveys, massive spectroscopic surveys of stars in the Milky Way halo and satellites, and by providing a spectroscopic follow-up counterpart to wide field imaging facilities like the Vera C. Rubin Observatory and the Nancy Grace Roman space telescope.

As the Milky Way and its satellite system become more entrenched in near field cosmology efforts, the need for an accurate mass estimate of the Milky Way's dark matter halo is increasingly critical. With the second and early third data releases of stellar proper motions from Gaia, several groups calculated full 6D phase-space information for the population of Milky Way satellite galaxies. Utilizing these data in comparison to subhalo properties drawn from the Phat ELVIS simulations, we constrain the Milky Way dark matter halo mass to be similar to 1-1.2 x 10(12) M-circle dot. We find that the kinematics of subhaloes drawn from more- or less-massive hosts (i.e. >1.2 x 10(12) M-circle dot or <10(12) M-circle dot) are inconsistent, at the 3 sigma confidence level, with the observed velocities of the Milky Way satellites. The preferred host halo mass for the Milky Way is largely insensitive to the exclusion of systems associated with the Large Magellanic Cloud, changes in galaxy formation thresholds, and variations in observational completeness. As more Milky Way satellites are discovered, their velocities (radial, tangential, and total) plus Galactocentric distances will provide further insight into the mass of the Milky Way dark matter halo.