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  1. Abstract

    We present our photometric search for potential nuclear star clusters (NSCs) in ultra-diffuse galaxies (UDGs) as an extension of the SMUDGes catalog. We identify 325 SMUDGes galaxies with NSCs and, from the 144 with existing distance estimates, identify 33 NSC hosts as UDGs (μ0,g≥ 24 mag arcsec−2,re≥ 1.5 kpc). The SMUDGes with NSCs lie on the galaxy red sequence, satisfy the relationship between NSC and host galaxy stellar masses, have a mean NSC stellar mass fraction of 0.02 but reach as high as 0.1, have NSCs that are displaced from the host center with a standard deviation of 0.10re, and weakly favor higher-density environments. All of these properties are consistent with previous results from higher surface brightness galaxy samples, allowing for at most a relatively weak dependence of NSC behavior on host galaxy surface brightness.

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  2. Abstract

    To better understand the formation of large, low-surface-brightness galaxies, we measure the correlation function between ultradiffuse galaxy (UDG) candidates and Milky Way analogs (MWAs). We find that: (1) the projected radial distribution of UDG satellites (projected surface density ∝r−0.84±0.06) is consistent with that of normal satellite galaxies; (2) the number of UDG satellites per MWA (SUDG) is ∼0.5 ± 0.1 over projected radii from 20 to 250 kpc and −17 <Mr< −13.5; (3)SUDGis consistent with a linear extrapolation of the relationship between the number of UDGs per halo versus halo mass obtained over galaxy group and cluster scales; (4) red UDG satellites dominate the population of UDG satellites (∼80%); (5) over the range of satellite magnitudes studied, UDG satellites comprise ∼10% of the satellite galaxy population of MWAs; and (6) a significant fraction of these (∼13%) have estimated total masses >1010.9Mor, equivalently, at least half the halo mass of the LMC, and populate a large fraction (∼18%) of the expected subhalos down to these masses. All of these results suggest a close association between the overall low-mass galaxy population and UDGs, which we interpret as favoring models where UDG formation principally occurs within the general context of low-mass galaxy formation over models invoking more exotic physical processes specifically invoked to form UDGs.

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  3. Abstract

    The Magellanic Stream (MS)—an enormous ribbon of gas spanning 140° of the southern sky trailing the Magellanic Clouds—has been exquisitely mapped in the five decades since its discovery. However, despite concerted efforts, no stellar counterpart to the MS has been conclusively identified. This stellar stream would reveal the distance and 6D kinematics of the MS, constraining its formation and the past orbital history of the Clouds. We have been conducting a spectroscopic survey of the most distant and luminous red giant stars in the Galactic outskirts. From this data set, we have discovered a prominent population of 13 stars matching the extreme angular momentum of the Clouds, spanning up to 100° along the MS at distances of 60–120 kpc. Furthermore, these kinematically selected stars lie along an [α/Fe]-deficient track in chemical space from −2.5 < [Fe/H] <− 0.5, consistent with their formation in the Clouds themselves. We identify these stars as high-confidence members of the Magellanic Stellar Stream. Half of these stars are metal-rich and closely follow the gaseous MS, whereas the other half are more scattered and metal-poor. We argue that the metal-rich stream is the recently formed tidal counterpart to the MS, and we speculate that the metal-poor population was thrown out of the SMC outskirts during an earlier interaction between the Clouds. The Magellanic Stellar Stream provides a strong set of constraints—distances, 6D kinematics, and birth locations—that will guide future simulations toward unveiling the detailed history of the Clouds.

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  4. Abstract

    We report the discovery of Pavo, a faint (MV= −10.0), star-forming, irregular, and extremely isolated dwarf galaxy atD≈ 2 Mpc. Pavo was identified in Dark Energy Camera Legacy Survey imaging via a novel approach that combines low surface brightness galaxy search algorithms and machine-learning candidate classifications. Follow-up imaging with the Inamori-Magellan Areal Camera and Spectrograph on the 6.5 m Magellan Baade telescope revealed a color–magnitude diagram (CMD) with an old stellar population, in addition to the young population that dominates the integrated light, and a tip of the red giant branch distance estimate of1.990.22+0.20Mpc. The blue population of stars in the CMD is consistent with the youngest stars having formed no later than 150 Myr ago. We also detected no Hαemission with SOAR telescope imaging, suggesting that we may be witnessing a temporary low in Pavo’s star formation. We estimate the total stellar mass of Pavo to belogM*/M=5.6±0.2and measure an upper limit on its Higas mass of 1.0 × 106Mbased on the HIPASS survey. Given these properties, Pavo’s closest analog is Leo P (D= 1.6 Mpc), previously the only known isolated, star-forming, Local Volume dwarf galaxy in this mass range. However, Pavo appears to be even more isolated, with no other known galaxy residing within over 600 kpc. As surveys and search techniques continue to improve, we anticipate an entire population of analogous objects being detected just outside the Local Group.

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    We study the relative fractions of quenched and star-forming satellite galaxies in the Satellites Around Galactic Analogs (SAGA) survey and Exploration of Local VolumE Satellites (ELVES) program, two nearby and complementary samples of Milky Way-like galaxies that take different approaches to identify faint satellite galaxy populations. We cross-check and validate sample cuts and selection criteria, as well as explore the effects of different star-formation definitions when determining the quenched satellite fraction of Milky Way analogues. We find the mean ELVES quenched fraction (〈QF〉), derived using a specific star formation rate (sSFR) threshold, decreases from ∼50 per cent to ∼27 per cent after applying a cut in absolute magnitude to match that of the SAGA survey (〈QF〉SAGA ∼9 per cent). We show these results are consistent for alternative star-formation definitions. Furthermore, these quenched fractions remain virtually unchanged after applying an additional cut in surface brightness. Using a consistently derived sSFR and absolute magnitude limit for both samples, we show that the quenched fraction and the cumulative number of satellites in the ELVES and SAGA samples broadly agree. We briefly explore radial trends in the ELVES and SAGA samples, finding general agreement in the number of star-forming satellites per host as a function of radius. Despite the broad agreement between the ELVES and SAGA samples, some tension remains with these quenched fractions in comparison to the Local Group and simulations of Milky Way analogues.

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  6. Abstract

    We present the completed catalog of ultradiffuse galaxy (UDG) candidates (7070 objects) from our search of the DR9 Legacy Survey images, including distance and total mass estimates for 1529 and 1436 galaxies, respectively, that we provide and describe in detail. From the sample with estimated distances, we obtain a sample of 585 UDGs (μ0,g≥ 24 mag arcsec−2andre≥ 1.5 kpc) over 20,000 square degrees of sky in various environments. We conclude that UDGs in our sample are limited to 1010Mh/M≲ 1011.5and are on average a factor of 1.5–7 deficient in stars relative to the general population of galaxies of the same total mass. That factor increases with increasing galaxy size and mass up to a factor of ∼10 when the total mass of the UDG increases beyondMh= 1011M. We do not find evidence that this factor has a dependence on the UDGs large-scale environment.

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    We present a photometric halo mass estimation technique for local galaxies that enables us to establish the stellar mass–halo mass (SMHM) relation down to stellar masses of 105 M⊙. We find no detectable differences among the SMHM relations of four local galaxy clusters or between the cluster and field relations and we find agreement with extrapolations of previous SMHM relations derived using abundance matching approaches. We fit a power law to our empirical SMHM relation and find that for adopted NFW dark matter profiles and for M* < 109 M⊙, the halo mass is Mh = 1010.35 ± 0.02(M*/108 M⊙)0.63 ± 0.02. The normalization of this relation is susceptible to systematic modelling errors that depend on the adopted dark matter potential and the quoted uncertainties refer to the uncertainties in the median relation. For galaxies with M* < 109 M⊙ that satisfy our selection criteria, the scatter about the fit in Mh, including uncertainties arising from our methodology, is 0.3 dex. Finally, we place lower luminosity Local Group galaxies on the SMHM relationship using the same technique, extending it to M* ∼ 103 M⊙ and suggest that some of these galaxies show evidence for additional mass interior to the effective radius beyond that provided by the standard dark matter profile. If this mass is in the form of a central black hole, the black hole masses are in the range of intermediate mass black holes, 10(5.7 ± 0.6) M⊙, which corresponds to masses of a few percent of Mh, well above values extrapolated from the relationships describing more massive galaxies.

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    We extend the Ultra-Diffuse Galaxy (UDG) abundance relation, NUDG − M200, to lower halo mass hosts $(M_{200}\sim 10^{11.6-12.2}\, \mathrm{M}_{\odot })$. We select UDG satellites from published catalogues of dwarf satellite galaxies around Milky Way analogues, namely the Exploration of Local Volume Satellites (ELVES) survey, the Satellite Around Galactic Analogs (SAGA) survey, and a survey of Milky Way-like systems conducted using the Hyper-Suprime Cam. Of the 516 satellites around a total of 75 Milky Way-like hosts, we find that 41 satellites around 33 hosts satisfy the UDG criteria. The distributions of host halo masses peak around $M_{200}\sim 10^{12}\, \mathrm{M}_{\odot }$, independent of whether the host has a UDG satellite or not. We use literature UDG abundances and those derived here to trace the NUDG − M200 relation over three orders of magnitude down to $M_{200}=10^{11.6}\, \mathrm{M}_{\odot }$ and find the best-fitting linear relation of $N_{\mathrm{ UDG}} = (38\pm 5) (\frac{M_{200}}{10^{14}})^{0.89\,\,\pm ~ 0.04}$. This sub-linear slope is consistent with earlier studies of UDG abundances as well as abundance relations for brighter dwarf galaxies, excluding UDG-formation mechanisms that require high-density environments. However, we highlight the need for further homogeneous characterization of UDGs across a wide range of environments to properly understand the NUDG − M200 relation.

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    We explore models of massive (>1010 M⊙) satellite quenching in massive clusters at z ≳ 1 using an MCMC framework, focusing on two primary parameters: Rquench (the host-centric radius at which quenching begins) and τquench (the time-scale upon which a satellite quenches after crossing Rquench). Our MCMC analysis shows two local maxima in the 1D posterior probability distribution of Rquench at approximately 0.25 and 1.0 R200. Analysing four distinct solutions in the τquench–Rquench parameter space, nearly all of which yield quiescent fractions consistent with observational data from the GOGREEN survey, we investigate whether these solutions represent distinct quenching pathways and find that they can be separated between ‘starvation’ and ‘core quenching’ scenarios. The starvation pathway is characterized by quenching time-scales that are roughly consistent with the total cold gas (H2 + H i) depletion time-scale at intermediate z, while core quenching is characterized by satellites with relatively high line-of-sight velocities that quench on short time-scales (∼0.25 Gyr) after reaching the inner region of the cluster (<0.30 R200). Lastly, we break the degeneracy between these solutions by comparing the observed properties of transition galaxies from the GOGREEN survey. We conclude that only the ‘starvation’ pathway is consistent with the projected phase-space distribution and relative abundance of transition galaxies at z ∼ 1. However, we acknowledge that ram pressure might contribute as a secondary quenching mechanism.

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    We investigate the role of dense environments in suppressing star formation by studying $\rm \log _{10}(M_\star /M_\odot) \gt 9.7$ star-forming galaxies in nine clusters from the Local Cluster Survey (0.0137 < z < 0.0433) and a large comparison field sample drawn from the Sloan Digital Sky Survey. We compare the star formation rate (SFR) with stellar mass relation as a function of environment and morphology. After carefully controlling for mass, we find that in all environments, the degree of SFR suppression increases with increasing bulge-to-total (B/T) ratio. In addition, the SFRs of cluster and infall galaxies at a fixed mass are more suppressed than their field counterparts at all values of B/T. These results suggest a quenching mechanism that is linked to bulge growth that operates in all environments and an additional mechanism that further reduces the SFRs of galaxies in dense environments. We limit the sample to B/T ≤ 0.3 galaxies to control for the trends with morphology and find that the excess population of cluster galaxies with suppressed SFRs persists. We model the time-scale associated with the decline of SFRs in dense environments and find that the observed SFRs of the cluster core galaxies are consistent with a range of models including a mechanism that acts slowly and continuously over a long (2–5 Gyr) time-scale, and a more rapid (<1 Gyr) quenching event that occurs after a delay period of 1–6 Gyr. Quenching may therefore start immediately after galaxies enter clusters.

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