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1. The Disturbed and Globular-cluster-rich Ultradiffuse Galaxy UGC 9050-Dw1

   https://doi.org/10.3847/2041-8213/acf0c3  

   Fielder, Catherine E; Jones, Michael G; Sand, David J; Bennet, Paul; Crnojević, Denija; Karunakaran, Ananthan; Mutlu-Pakdil, Burçin; Spekkens, Kristine (September 2023, The Astrophysical Journal Letters)

   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. 

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2. The Globular Cluster System of the Virgo Cluster Ultradiffuse Galaxy VCC 615

   https://doi.org/10.3847/1538-4357/ad99cd  

   Mihos, J Christopher; Durrell, Patrick R; Toloba, Elisa; Peng, Eric W; Lim, Sungsoon; Côté, Patrick; Guhathakurta, Puragra; Ferrarese, Laura (January 2025, The Astrophysical Journal)

   Abstract We use Hubble Space Telescope imaging to study the globular cluster system of the Virgo Cluster ultradiffuse galaxy (UDG) VCC 615. We select globular cluster candidates through a combination of size and color, while simultaneously rejecting contamination from background galaxies that would be unresolved in ground-based imaging. Our sample of globular cluster candidates is essentially complete down to a limiting magnitude of F814W = 24.0, ≈90% down the globular cluster luminosity function (GCLF). We estimate a total globular cluster population for VCC 615 of $N_{\mathrm{GC}}=25.1_{-5.4}^{+6.5}$, resulting in a specific frequency of $S_N=55.5_{-12.0}^{+14.5}$, quite high compared to normal galaxies of similar luminosity, but consistent with the large specific frequencies found in some other UDGs. The abundant cluster population suggests the galaxy is enshrouded by a massive dark halo, consistent with previous dynamical mass estimates using globular cluster kinematics. While the peak of the GCLF appears slightly brighter than expected (by ≈0.3–0.5 mag), this difference is comparable to the 0.3 mag uncertainty in the measurement, and we see no sign of an extremely luminous population of clusters similar to those detected in the UDGs NGC1054-DF2 and -DF4. However, we do find a relatively high fraction ( $32_{-4}^{+5}$%) of large clusters with half-light radiiR_h > 9 pc. The galaxy's offset nucleus appears photometrically distinct from the globular clusters, and is more akin to ultracompact dwarfs (UCDs) in Virgo. Over time, VCC615’s already diffuse stellar body may be further stripped by cluster tides, leaving the nucleus intact to form a new Virgo UCD. 

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3. Stellar Metallicities and Gradients in the Isolated, Quenched Low-mass Galaxy Tucana

   https://doi.org/10.3847/1538-4357/ad25ed  

   Fu, Sal_Wanying; Weisz, Daniel_R; Starkenburg, Else; Martin, Nicolas; Mercado, Francisco_J; Savino, Alessandro; Boylan-Kolchin, Michael; Côté, Patrick; Dolphin, Andrew_E; Longeard, Nicolas; et al (April 2024, The Astrophysical Journal)

   Abstract We measure the metallicities of 374 red giant branch (RGB) stars in the isolated, quenched dwarf galaxy Tucana using Hubble Space Telescope narrowband (F395N) calcium H and K imaging. Our sample is a factor of ∼7 larger than what is available from previous studies. Our main findings are as follows. (i) A global metallicity distribution function (MDF) with $〈\mathrm{[Fe/H]}〉=-{1.55}_{-0.04}^{+0.04}$and ${\sigma }_{\mathrm{[Fe/H]}}={0.54}_{-0.03}^{+0.03}$. (ii) A metallicity gradient of −0.54 ± 0.07 dex $R_e^{-1}$(−2.1 ± 0.3 dex kpc⁻¹) over the extent of our imaging (∼2.5R_e), which is steeper than literature measurements. Our finding is consistent with predicted gradients from the publicly available FIRE-2 simulations, in which bursty star formation creates stellar population gradients and dark matter cores. (iii) Tucana’s bifurcated RGB has distinct metallicities: a blue RGB with $〈\mathrm{[Fe/H]}〉=-{1.78}_{-0.06}^{+0.06}$and ${\sigma }_{\mathrm{[Fe/H]}}={0.44}_{-0.06}^{+0.07}$and a red RGB with $〈\mathrm{[Fe/H]}〉=-{1.08}_{-0.07}^{+0.07}$and ${\sigma }_{\mathrm{[Fe/H]}}={0.42}_{-0.06}^{+0.06}$. (iv) At fixed stellar mass, Tucana is more metal-rich than Milky Way satellites by ∼0.4 dex, but its blue RGB is chemically comparable to the satellites. Tucana’s MDF appears consistent with star-forming isolated dwarfs, though MDFs of the latter are not as well populated. (v) About 2% of Tucana’s stars have [Fe/H] < −3% and 20% have [Fe/H] > −1. We provide a catalog for community spectroscopic follow-up. 

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4. Forecasts for Galaxy Formation and Dark Matter Constraints from Dwarf Galaxy Surveys

   https://doi.org/10.3847/1538-4357/ad3bb1  

   Nadler, Ethan_O; Gluscevic, Vera; Driskell, Trey; Wechsler, Risa_H; Moustakas, Leonidas_A; Benson, Andrew; Mao, Yao-Yuan (May 2024, The Astrophysical Journal)

   Abstract The abundance of faint dwarf galaxies is determined by the underlying population of low-mass dark matter (DM) halos and the efficiency of galaxy formation in these systems. Here, we quantify potential galaxy formation and DM constraints from future dwarf satellite galaxy surveys. We generate satellite populations using a suite of Milky Way (MW)–mass cosmological zoom-in simulations and an empirical galaxy–halo connection model, and assess sensitivity to galaxy formation and DM signals when marginalizing over galaxy–halo connection uncertainties. We find that a survey of all satellites around one MW-mass host can constrain a galaxy formation cutoff at peak virial masses of ${\mathcal{M}}_{50}={10}^8{M}_{\odot }$at the 1σlevel; however, a tail toward low ${\mathcal{M}}_{50}$prevents a 2σmeasurement. In this scenario, combining hosts with differing bright satellite abundances significantly reduces uncertainties on ${\mathcal{M}}_{50}$at the 1σlevel, but the 2σtail toward low ${\mathcal{M}}_{50}$persists. We project that observations of one (two) complete satellite populations can constrain warm DM models withm_WDM≈ 10 keV (20 keV). Subhalo mass function (SHMF) suppression can be constrained to ≈70%, 60%, and 50% that in cold dark matter (CDM) at peak virial masses of 10⁸, 10⁹, and 10¹⁰M_⊙, respectively; SHMF enhancement constraints are weaker (≈20, 4, and 2 times that in CDM, respectively) due to galaxy–halo connection degeneracies. These results motivate searches for faint dwarf galaxies beyond the MW and indicate that ongoing missions like Euclid and upcoming facilities including the Vera C. Rubin Observatory and Nancy Grace Roman Space Telescope will probe new galaxy formation and DM physics. 

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5. Corvus A: A Low-mass, Isolated Galaxy at 3.5 Mpc

   https://doi.org/10.3847/2041-8213/ad676e  

   Jones, Michael_G; Sand, David_J; Mutlu-Pakdil, Burçin; Fielder, Catherine_E; Crnojević, Denija; Bennet, Paul; Spekkens, Kristine; Donnerstein, Richard; Doliva-Dolinsky, Amandine; Karunakaran, Ananthan; et al (August 2024, The Astrophysical Journal Letters)

   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. 

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