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Abstract We present results from Identifying Dwarfs of MC Analog GalaxiEs (ID-MAGE), a survey aimed at identifying and characterizing unresolved satellite galaxies around 35 nearby LMC- and SMC-mass hosts (D = 4−10 Mpc). We use archival DESI Legacy Survey imaging data and perform an extensive search for dwarf satellites, extending out to a radius of 150 kpc (∼R_vir). We identify 355 candidate satellite galaxies, including 264 new discoveries. Extensive tests with injected galaxies demonstrate that the survey is complete down toM_V ∼ −9.0 (assuming the distance of the host) andμ_0,V ∼ 26 mag arcsec⁻²(assuming ann = 1 Sérsic profile). We perform consistent photometry, via Sérsic profile fitting, on all candidates and have initiated a comprehensive follow-up campaign to confirm and characterize candidates. Through a systematic visual inspection campaign, we classify the top candidates as high-likelihood satellites. On average, we find 4.0 ± 1.4 high-likelihood candidate satellites per LMC-mass host and 2.1 ± 0.6 per SMC-mass host, which is within the range predicted by cosmological models. We use this sample to establish upper and lower estimates on the satellite luminosity function of LMC-/SMC-mass galaxies. ID-MAGE nearly triples the number of low-mass galaxies surveyed for satellites with well-characterized completeness limits, providing a unique data set to explore small-scale structure and dwarf galaxy evolution around low-mass hosts in diverse environments. 

Abstract We present a catalog of 34 new candidate (13 high confidence) isolated, young stellar systems within the Virgo galaxy cluster identified through a citizen science search of public optical and ultraviolet imaging. “Blue blobs” are a class of blue, faint, isolated, extremely low stellar mass, and metal-rich star-forming clouds embedded in the hot intracluster medium of the Virgo cluster. Only six blue blobs were known previously and here we confirm an additional six of our candidates through velocity and metallicity measurements from follow-up optical spectroscopy on the Hobby–Eberly Telescope (HET). Our 13 high confidence candidates (including the six confirmed) have properties consistent with prior known blue blobs and are inconsistent with being low-mass galaxies. Most candidates are concentrated in relatively dense regions, roughly following filamentary structures within the cluster, but avoiding its center. Three of our candidates are likely the stellar counterparts of known “optically dark” clouds of neutral hydrogen in the cluster, while a further four are widely separated extensions to previously known blue blobs. The properties of our new candidates are consistent with previous conclusions that blue blobs likely originated from ram pressure stripping events, however, their locations in velocity–projected cluster-centric radius phase space imply that their parent galaxies are not on their first infall into the cluster. Through our ongoing follow-up program with HET we aim to confirm additional candidates, however, detailed understanding of the stellar populations and star formation histories of blue blobs will require JWST observations. 

Abstract We report the results of the deepest search to date for dwarf galaxies around NGC 3109, a barred spiral galaxy with a mass similar to that of the Small Magellanic Cloud (SMC), using a semiautomated search method. Using the Dark Energy Camera, we survey a region covering a projected distance of ∼70 kpc of NGC 3109 (D= 1.3 Mpc,R_vir∼ 90 kpc,M∼ 10⁸M_*) as part of the MADCASH and DELVE-DEEP programs. We introduce a newly developed semiresolved search method, used alongside a resolved search, to identify crowded dwarf galaxies around NGC 3109. Using both approaches, we successfully recover the known satellites Antlia and Antlia B. We identified a promising candidate, which was later confirmed to be a background dwarf through deep follow-up observations. Our detection limits are well defined, with the sample ∼80% complete down toM_V∼ −8.0, and include detections of dwarf galaxies as faint asM_V∼ −6.0. This is the first comprehensive study of a satellite system through resolved stars around an SMC mass host. Our results show that NGC 3109 has more bright (M_V∼ −9.0) satellites than the mean predictions from cold dark matter models, but well within the host-to-host scatter. A larger sample of LMC/SMC-mass hosts is needed to test whether or not the observations are consistent with current model expectations. 

Abstract We present new follow-up observations of two ultra-diffuse galaxies (UDGs) selected for their distorted morphologies and tidal features, suggestive of tidal influence. Using Hubble Space Telescope Advanced Camera for Surveys F555W and F814W imaging, we identify 8 ± 2 globular clusters in KUG 0203-Dw1 and 6 ± 2 in KDG 013, abundances typical for normal dwarf galaxies of similar stellar mass. Jansky Very Large Array data reveal a clear Hidetection of KUG 0203-Dw1 with a gas mass estimate of $\log M_{\mathrm{H}\mathrm{I}}/M_{\odot }\lesssim 7.4$and evidence of active stripping by the host, while KDG 013 has no clear gas detection. The UDGs likely originated as normal dwarf galaxies that have been subjected to significant stripping and tidal heating, causing them to become more diffuse. These two UDGs complete a sample of five exhibiting tidal features in the full Canada–France–Hawaii Telescope Legacy Survey area (∼150²deg). These tidally influenced UDGs exhibit diverse properties; one stands out as a potential result of a dwarf merger, while the remainder suggest tidal heating origins. We also cannot conclusively rule out that these galaxies became UDGs in the field before processing by the group environment, underscoring the need for broader searches of diffuse galaxies to better understand the impact of galaxy interactions. 

Abstract We present deep optical observations of the stellar halo of NGC 300, an LMC-mass galaxy, acquired with the DEEP subcomponent of the DECam Local Volume Exploration survey using the 4 m Blanco Telescope. Our resolved star analysis reveals a large, low surface brightness stellar stream (M_V ∼ −8.5; [Fe/H] = −1.4 ± 0.15) extending more than 40 kpc north from the galaxy’s center. We also find other halo structures, including potentially an additional stream wrap to the south, which may be associated with the main stream. The morphology and derived low metallicities of the streams and shells discovered surrounding NGC 300 are highly suggestive of a past accretion event. Assuming a single progenitor, the accreted system is approximately Fornax-like in luminosity, with an inferred mass ratio to NGC 300 of approximately 1:15. We also present the discovery of a metal-poor globular cluster (GC) (R_proj = 23.3 kpc;M_V = −8.99 ± 0.16; [Fe/H] ≈ −1.6 ± 0.6) in the halo of NGC 300, the furthest identified GC associated with NGC 300. The stellar structures around NGC 300 represent the richest features observed in a Magellanic Cloud analog to date, strongly supporting the idea that accretion and subsequent disruption is an important mechanism in the assembly of dwarf galaxy stellar halos. 

Abstract We report the discovery of three faint and ultrafaint dwarf galaxies—Sculptor A, Sculptor B, and Sculptor C—in the direction of NGC 300 (D= 2.0 Mpc), a Large Magellanic Cloud–mass galaxy. Deep ground-based imaging with Gemini/GMOS resolves all three dwarf galaxies into stars, each displaying a red giant branch indicative of an old, metal-poor stellar population. No young stars or Higas are apparent, and the lack of a GALEX UV detection suggests that all three systems are quenched. Sculptor C (D= 2.04 ${}_{-0.13}^{+0.10}$Mpc;M_V=  −9.1 ± 0.1 mag orL_V= (3.7 ${}_{-0.3}^{+0.4}$) × 10⁵L_⊙) is consistent with being a satellite of NGC 300. Sculptor A (D= 1.35 ${}_{-0.08}^{+0.22}$Mpc;M_V= −6.9 ± 0.3 mag orL_V= (5 ${}_{-1}^{+1}$) × 10⁴L_⊙) is likely in the foreground of NGC 300 and at the extreme edge of the Local Group, analogous to the recently discovered ultrafaint Tucana B in terms of its physical properties and environment. Sculptor B (D= 2.48 ${}_{-0.24}^{+0.21}$Mpc;M_V= −8.1 ± 0.3 mag orL_V= (1.5 ${}_{-0.4}^{+0.5}$) × 10⁵L_⊙) is likely in the background, but future distance measurements are necessary to solidify this statement. It is also of interest due to its quiescent state and low stellar mass. Both Sculptor A and B are ≳2–4r_virfrom NGC 300 itself. The discovery of three dwarf galaxies in isolated or low-density environments offers an opportunity to study the varying effects of ram-pressure stripping, reionization, and internal feedback in influencing the star formation history of the faintest stellar systems. 

Abstract We report the discovery of Corvus A, a low-mass, gas-rich galaxy at a distance of approximately 3.5 Mpc, identified in DR10 of the Dark Energy Camera Legacy Imaging Survey during the initial phase of our ongoing SEmi-Automated Machine LEarning Search for Semi-resolved galaxies (SEAMLESS). Jansky Very Large Array observations of Corvus A detect Hiline emission at a radial velocity of 523 ± 2 km s⁻¹. Magellan/Megacam imaging reveals an irregular and complex stellar population with both young and old stars. We detect UV emission in Neil Gehrels Swift observations, indicative of recent star formation. However, there are no signs of Hiiregions in Hαimaging from Steward Observatory’s Kuiper telescope. Based on the Megacam color–magnitude diagram we measure the distance to Corvus A via the tip of the red giant branch standard candle as 3.48 ± 0.24 Mpc. This makes Corvus A remarkably isolated, with no known galaxy within ∼1 Mpc. Based on this distance, we estimate the Hiand stellar mass of Corvus A to be $\log M_{\mathrm{H}\mathrm{I}}/M_{\odot }=6.59$and $\log M_{*}/M_{\odot }=6.0$, respectively. Although there are some signs of rotation, the Hidistribution of Corvus A appears to be close to face on, analogous to that of Leo T, and we therefore do not attempt to infer a dynamical mass from its Hiline width. Higher-resolution synthesis imaging is required to confirm this morphology and to draw robust conclusions from its gas kinematics. 

Abstract We investigate the ultradiffuse galaxy (UDG) UGC 9050-Dw1, which was selected because of its disturbed morphology as part of a larger sample of UDGs that display evidence for significant interactions. We use the Hubble Space Telescope’s Advanced Camera for Surveys to identify globular clusters (GCs) associated with UGC 9050-Dw1, and the Jansky Very Large Array to measure its Hicontent. UGC 9050-Dw1, a neighbor to the low surface brightness spiral UGC 9050, exhibits a unique UV-bright central “clump” with clearly associated Higas and an extended stellar tidal plume to the north. We identify 52 ± 12 GCs, implying a specific frequency ofS_N= 122 ± 38, one of the highest reported for a UDG of this luminosity ( $\log L_V/L_{\odot }=7.5\pm 0.1$). Additionally, ∼20% of the total light of the galaxy is contributed by GCs. Nearly uniform GC colors suggest they were formed during a single intense episode of star formation. We posit that UGC 9050-Dw1 represents the initial definitive observational example of UDG formation resulting from a dwarf merger event, where subsequent clumpy star formation has contributed to its present observed characteristics. 

Abstract We have discovered the stellar counterpart to the ALFALFA Virgo 7 cloud complex, which has been thought to be optically dark and nearly star-free since its discovery in 2007. This ∼190 kpc long chain of enormous atomic gas clouds (M_Hi∼ 10⁹M_⊙) is embedded in the hot intracluster medium of the Virgo galaxy cluster but is isolated from any galaxy. Its faint, blue stellar counterpart, BC6, was identified in a visual search of archival optical and UV imaging. Follow-up observations with the Green Bank Telescope, Hobby–Eberly Telescope, and Hubble Space Telescope demonstrate that this faint counterpart is at the same velocity as the atomic gas, actively forming stars, and metal-rich (12 + (O/H) = 8.58 ± 0.25). We estimate its stellar mass to be only $\log \left(M_{*}/M_{\odot }\right)\sim 4.4$, making it one of the most gas-rich stellar systems known. Aside from its extraordinary gas content, the properties of BC6 are entirely consistent with those of a recently identified class of young, low-mass, isolated, and star-forming clouds in Virgo that appear to have formed via extreme ram pressure stripping events. We expand the existing discussion of the origin of this structure and suggest NGC 4522 as a likely candidate; however, the current evidence is not fully consistent with any of our proposed progenitor galaxies. We anticipate that other “dark” gas clouds in Virgo may have similarly faint, star-forming counterparts. We aim to identify these through the help of a citizen science search of the entire cluster. 

Abstract We have imaged the entirety of eight (plus one partial) Milky Way (MW)–like satellite systems, a total of 42 (45) satellites, from the Satellites Around Galactic Analogs II catalog in both Hαand Hiwith the Canada–France–Hawaii Telescope and the Jansky Very Large Array. In these eight systems we have identified four cases where a satellite appears to be currently undergoing ram pressure stripping (RPS) as its Higas collides with the circumgalactic medium (CGM) of its host. We also see a clear suppression of gas fraction (M_HI/M_*) with decreasing (projected) satellite–host separation—to our knowledge, the first time this has been observed in a sample of MW-like systems. Comparisons to the Auriga, A Project Of Simulating The Local Environment, and TNG50 cosmological zoom-in simulations show consistent global behavior, but they systematically underpredict gas fractions across all satellites by roughly 0.5 dex. Using a simplistic RPS model, we estimate the average peak CGM density that satellites in these systems have encountered to be $\log {\rho }_{\mathrm{cgm}}/\mathrm{g}{\mathrm{cm}}^{-3}\approx -27.3$. Furthermore, we see tentative evidence that these satellites are following a specific star formation rate to gas fraction relation that is distinct from field galaxies. Finally, we detect one new gas-rich satellite in the UGC 903 system with an optical size and surface brightness meeting the standard criteria to be considered an ultra-diffuse galaxy.