Ultra-diffuse Galaxies Research Articles

Low surface brightness galaxies in z > 1 galaxy clusters: HST approaching the progenitors of local ultra diffuse galaxies

Ultra diffuse galaxies (UDGs) are a type of large low surface brightness (LSB) galaxies with particularly large effective radii (reff > 1.5 kpc) that are now routinely studied in the Local (z < 0.1) Universe. While they are found to be abundant in clusters, groups, and in the field, their formation mechanisms remain elusive and comprise an active topic of debate. New insights may be found by studying their counterparts at higher redshifts (z > 1.0), even though cosmological surface brightness dimming makes them particularly difficult to detect and study in this channel. In this work, we use the deepest Hubble Space Telescope (HST) imaging stacks of z > 1 clusters, namely, SPT-CL J2106−5844 and MOO J1014+0038. These two clusters, at z = 1.13 and z = 1.23, respectively, were monitored as part of the HST See-Change programme. In making a comparison with the Hubble Extreme Deep Field as the reference field, we find statistical over-densities of large LSB galaxies in both clusters. Based on stellar-population modelling and assuming no size evolution, we find that the faintest sources we can detect are about as bright as expected for the progenitors of the brightest local UDGs. We find that the LSBs we detect in SPT-CL J2106−5844 and MOO J1014−5844 already have old stellar populations that place them on the red sequence. In correcting for incompleteness and based on an extrapolation of local scaling relations, we estimate that distant UDGs are relatively under-abundant, as compared to local UDGs, by a factor ∼3. A plausible explanation for the implied increase over time would be the significant growth of these galaxies over the last ∼8 Gyr, as also suggested by hydrodynamical simulations.

Shadows in the Dark: Low-surface-brightness Galaxies Discovered in the Dark Energy Survey

Abstract We present a catalog of 23,790 extended low-surface-brightness galaxies (LSBGs) identified in from the first three years of imaging data from the Dark Energy Survey (DES). Based on a single-component Sérsic model fit, we define extended LSBGs as galaxies with g-band effective radii and mean surface brightness . We find that the distribution of LSBGs is strongly bimodal in (g − r) versus (g − i) color space. We divide our sample into red (g − i ≥ 0.60) and blue (g − i < 0.60) galaxies and study the properties of the two populations. Redder LSBGs are more clustered than their blue counterparts and are correlated with the distribution of nearby (z < 0.10) bright galaxies. Red LSBGs constitute ∼33% of our LSBG sample, and of these are located within 1° of low-redshift galaxy groups and clusters (compared to ∼8% of the blue LSBGs). For nine of the most prominent galaxy groups and clusters, we calculate the physical properties of associated LSBGs assuming a redshift derived from the host system. In these systems, we identify 41 objects that can be classified as ultradiffuse galaxies, defined as LSBGs with projected physical effective radii and central surface brightness . The wide-area sample of LSBGs in DES can be used to test the role of environment on models of LSBG formation and evolution.

The formation of isolated ultradiffuse galaxies in romulus25

ABSTRACT We use the romulus25 cosmological simulation volume to identify the largest-ever simulated sample of field ultradiffuse galaxies (UDGs). At z = 0, we find that isolated UDGs have average star formation rates (SFRs), colours, and virial masses for their stellar masses and environment. UDGs have moderately elevated H i masses, being 70 per cent (300 per cent) more H i rich than typical isolated dwarf galaxies at luminosities brighter (fainter) than MB = −14. However, UDGs are consistent with the general isolated dwarf galaxy population and make up ∼20 per cent of all field galaxies with 107 < M⋆/M⊙ < 109. The H i masses, effective radii, and overall appearances of our UDGs are consistent with existing observations of field UDGs, but we predict that many isolated UDGs have been missed by current surveys. Despite their isolation at z = 0, the UDGs in our sample are the products of major mergers. Mergers are no more common in UDG than non-UDG progenitors, but mergers that create UDGs tend to happen earlier – almost never occurring after z = 1, produce a temporary boost in spin, and cause star formation to be redistributed to the outskirts of galaxies, resulting in lower central SFRs. The centres of the galaxies fade as their central stellar populations age, but their global SFRs are maintained through bursts of star formation at larger radii, producing steeper negative g −r colour gradients. This formation channel is unique relative to other proposals for UDG formation in isolated galaxies, demonstrating that UDGs can potentially be formed through multiple mechanisms.

The origin of low-surface-brightness galaxies in the dwarf regime

ABSTRACT Low-surface-brightness galaxies (LSBGs) – defined as systems that are fainter than the surface-brightness limits of past wide-area surveys – form the overwhelming majority of galaxies in the dwarf regime (M⋆ < 109 M⊙). Using NewHorizon, a high-resolution cosmological simulation, we study the origin of LSBGs and explain why LSBGs at similar stellar mass show the large observed spread in surface brightness. NewHorizon galaxies populate a well-defined locus in the surface brightness–stellar mass plane, with a spread of ∼3 mag arcsec−2, in agreement with deep Sloan Digital Sky Survey (SDSS) Stripe 82 data. Galaxies with fainter surface brightnesses today are born in regions of higher dark matter density. This results in faster gas accretion and more intense star formation at early epochs. The stronger resultant supernova feedback flattens gas profiles at a faster rate, which, in turn, creates shallower stellar profiles (i.e. more diffuse systems) more rapidly. As star formation declines towards late epochs ( z < 1), the larger tidal perturbations and ram pressure experienced by these systems (due to their denser local environments) accelerate the divergence in surface brightness, by increasing their effective radii and reducing star formation, respectively. A small minority of dwarfs depart from the main locus towards high surface brightnesses, making them detectable in past wide surveys (e.g. standard-depth SDSS images). These systems have anomalously high star formation rates, triggered by recent fly-by or merger-driven starbursts. We note that objects considered extreme or anomalous at the depth of current data sets, e.g. ‘ultra-diffuse galaxies’, actually dominate the predicted dwarf population and will be routinely visible in future surveys like the Legacy Survey of Space and Time (LSST).

An excess of globular clusters in Ultra-Diffuse Galaxies formed through tidal heating

ABSTRACT To investigate the origin of elevated globular cluster (GC) abundances observed around Ultra-Diffuse Galaxies (UDGs), we simulate GC populations hosted by UDGs formed through tidal heating. Specifically, GC formation is modelled as occurring in regions of dense star formation. Because star formation-rate densities are higher at high redshift, dwarf galaxies in massive galaxy clusters, which formed most of their stars at high redshift, form a large fraction of their stars in GCs. Given that UDGs formed through environmental processes are more likely to be accreted at high redshift, these systems have more GCs than non-UDGs. In particular, our model predicts that massive UDGs have twice the GC mass of non-UDGs of similar stellar mass, in rough agreement with observations. Although this effect is somewhat diminished by GC disruption, we find that the relationship between GC mass fraction and cluster-centric distance, and the relationship between GC mass fraction and galaxy half-light radius are remarkably similar to observations. Among our model objects, both UDGs and non-UDGs present a correlation between halo mass and GC mass, although UDGs have lower dynamical masses at a given GC mass. Furthermore, because of the effectiveness of GC disruption, we predict that GCs around UDGs should have a more top heavy mass function than GCs around non-UDGs. This analysis suggests that dwarfs with older stellar populations, such as UDGs, should have higher GC mass fractions than objects with young stellar populations, such as isolated dwarfs.

The properties of dwarf spheroidal galaxies in the Cen A group

Dwarf spheroidal galaxies (dSphs) have been extensively investigated in the Local Group, but their low luminosity and surface brightness make similar work in more distant galaxy groups challenging. Modern instrumentation unlocks the possibility of scrutinizing these faint systems in other environments, expanding the parameter space of group properties. We use MUSE spectroscopy to study the properties of 14 known or suspected dSph satellites of Cen A. Twelve targets are confirmed to be group members based on their radial velocities. Two targets are background galaxies at ∼50 Mpc: KK 198 is a face-on spiral galaxy, and dw1315−45 is an ultra-diffuse galaxy with an effective radius of ∼2300 pc. The 12 confirmed dSph members of the Cen A group have old and metal-poor stellar populations and follow the stellar metallicity-luminosity relation defined by the dwarf galaxies in the Local Group. In the three brightest dwarf galaxies (KK 197, KKs 55, and KKs 58), we identify globular clusters, as well as a planetary nebula in KK 197, although its association with this galaxy and/or the extended halo of Cen A is uncertain. Using four discrete tracers, we measure the velocity dispersion and dynamical mass of KK 197. This dSph appears dark matter dominated and lies on the radial acceleration relation of star-forming galaxies within the uncertainties. It also is consistent with predictions stemming from modified Newtonian dynamics. Surprisingly, in the dwarf KK 203 we find an extended Hα ring. Careful examination of Hubble Space Telescope photometry reveals a very low level of star formation at ages between 30 and 300 Myr. The Hα emission is most likely linked to a ∼40 Myr old supernova remnant, although other possibilities for its origin cannot be entirely ruled out.

Abundance of dwarf galaxies around low-mass spiral galaxies in the Local Volume

The abundance of satellite dwarf galaxies has long been considered a crucial test for the current model of cosmology leading to the well-known missing satellite problem. Recent advances in simulations and observations have allowed the study of dwarf galaxies around host galaxies in more detail. Using the Dark Energy Camera we surveyed a 72 deg2 area of the nearby Sculptor group, also encompassing the two low-mass Local Volume galaxies NGC 24 and NGC 45 residing behind the group, to search for as yet undetected dwarf galaxies. Apart from the previously known dwarf galaxies we found only two new candidates down to a 3σ surface brightness detection limit of 27.4 r mag arcsec−2. Both systems are in projection close to NGC 24. However, one of these candidates could be an ultra-diffuse galaxy associated with a background galaxy. We compared the number of known dwarf galaxy candidates around NGC 24, NGC 45, and five other well-studied low-mass spiral galaxies (NGC 1156, NGC 2403, NGC 5023, M 33, and the LMC) with predictions from cosmological simulations, and found that for the stellar-to-halo mass models considered, the observed satellite numbers tend to be on the lower end of the expected range. This could mean either that there is an overprediction of luminous subhalos in ΛCDM or that we are missing some of the satellite members due to observational biases.

On the Evolution of the Globular Cluster System in NGC 1052-DF2: Dynamical Friction, Globular–Globular Interactions, and Galactic Tides

Abstract The ultradiffuse galaxy NGC 1052-DF2 has an overabundance of luminous globular clusters (GCs), and its kinematics is consistent with the presence of little to no dark matter. As the velocity dispersion among the GCs is comparable to the expected internal dispersions of the individual GCs, the galaxy might be highly conducive to GC–GC merging. If true, this could explain the puzzling luminosity function of its GCs. Here we examine this possibility by resimulating three of our earlier simulations of the GC system, where the GCs were modeled as single particles, with live GCs. Somewhat surprisingly, we infer a low merger rate of ∼0.03 Gyr−1. The main reason is that the GCs are too dense for tidal shock capture, caused by impulsive encounters among them, to operate efficiently (we infer a tidal capture rate of only ∼0.002 Gyr−1). Therefore, whatever mergers occur are driven by other mechanisms, which we find to be captures induced by dynamical friction and compressive tides from other GCs. The low merger rate inferred here makes it unlikely that the unusually large luminosities of the GCs can be explained as a result of past GC–GC mergers. Our simulations also indicate that, if NGC 1052-DF2 is indeed largely devoid of dark matter, its tidal field is too weak to induce any significant mass loss from the GCs. Therefore, in such a scenario, we predict that it is improbable for the GCs to reveal tidal features, something that can be tested with future deep observations.

The quiescent fraction of isolated low surface brightness galaxies: observational constraints

ABSTRACT Understanding the formation and evolution of low surface brightness galaxies (LSBGs) is critical for explaining their wide-ranging properties. However, studies of LSBGs in deep photometric surveys are often hindered by a lack of distance estimates. In this work, we present a new catalogue of 479 LSBGs, identified in deep optical imaging data from the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). These galaxies are found across a range of environments, from the field to groups. Many are likely to be ultra-diffuse galaxies (UDGs). We see clear evidence for a bimodal population in colour–Sérsic index space, and split our sample into red and blue LSBG populations. We estimate environmental densities for a sub-sample of 215 sources by statistically associating them with nearby spectroscopic galaxies from the overlapping GAMA spectroscopic survey. We find that the blue LSBGs are statistically consistent with being spatially randomized with respect to local spectroscopic galaxies, implying they exist predominantly in low-density environments. However, the red LSBG population is significantly spatially correlated with local structure. We find that $26\pm 5{{\ \rm per\ cent}}$ of isolated, local LSBGs belong to the red population, which we interpret as quiescent. This indicates that high environmental density plays a dominant, but not exclusive, role in producing quiescent LSBGs. Our analysis method may prove to be very useful, given the large samples of LSB galaxies without distance information expected from e.g. the Vera C. Rubin observatory (aka LSST), especially in combination with upcoming comprehensive wide-field spectroscopic surveys.

Long tidal tails in merging galaxies and their implications

ABSTRACT We investigate the properties of long tidal tails using the largest to date sample of 461 merging galaxies with $\log (M_\ast /\rm M_\odot)\ge 9.5$ within 0.2 ≤ z ≤ 1 from the COSMOS survey in combination with Hubble Space Telescope imaging data. Long tidal tails can be briefly divided into three shape types: straight (41 per cent), curved (47 per cent), and plume (12 per cent). Their host galaxies are mostly at late stages of merging, although 31 per cent are galaxy pairs with projected separations d > 20 kpc. The high formation rate of straight tidal tails needs to be understood as the projection of curved tidal tails accounts for only a small fraction of the straight tails. We identify 165 tidal dwarf galaxies (TDGs), yielding a TDG production rate of 0.36 per merger. Combined with a galaxy merger fraction and a TDG survival rate from the literature, we estimate that ∼5 per cent of local dwarf galaxies (DGs) are of tidal origin, suggesting the tidal formation is not an important formation channel for the DGs. About half of TDGs are located at the tip of their host tails. These TDGs have stellar masses in the range of $7.5\le \log (M_\ast /\rm M_\odot)\le 9.5$ and appear compact with half-light radii following the M*–Re relation of low-mass elliptical galaxies. However, their surface brightness profiles are generally flatter than those of local disc galaxies. Only 10 out of 165 TDGs have effective radii larger than 1.5 kpc and would qualify as unusually bright ultradiffuse galaxies.

Stellar velocity dispersion and dynamical mass of the ultra diffuse galaxy NGC 5846_UDG1 from the keck cosmic web imager

ABSTRACTThe ultra diffuse galaxy in the NGC 5846 group (NGC 5846_UDG1) was shown to have a large number of globular cluster (GC) candidates from deep imaging as part of the VEGAS survey. Recently, Müller et al. published a velocity dispersion, based on a dozen of its GCs. Within their quoted uncertainties, the resulting dynamical mass allowed for either a dark matter free or a dark-matter-dominated galaxy. Here, we present spectra from KCWI that reconfirms membership of the NGC 5846 group and reveals a stellar velocity dispersion for UDG1 of σGC = 17 ± 2 km s−1. Our dynamical mass, with a reduced uncertainty, indicates a very high contribution of dark matter within the effective radius. We also derive an enclosed mass from the locations and motions of the GCs using the tracer mass estimator, finding a similar mass inferred from our stellar velocity dispersion. We find no evidence that the galaxy is rotating and is thus likely pressure supported. The number of confirmed GCs, and the total number inferred for the system (∼45), suggests a total halo mass of ∼2 × 1011 M⊙. A cored mass profile is favoured when compared to our dynamical mass. Given its stellar mass of 1.1 × 108 M⊙, NGC 5846_UDG1 appears to be an ultra diffuse galaxy with a dwarf-like stellar mass and an overly massive halo.

Unravelling the enigmatic ISM conditions in Minkowski’s object

ABSTRACT Local examples of jet-induced star formation lend valuable insight into its significance in galaxy evolution and can provide important observational constraints for theoretical models of positive feedback. Using optical integral field spectroscopy, we present an analysis of the ISM conditions in Minkowski’s object (z = 0.0189), a peculiar star-forming dwarf galaxy located in the path of a radio jet from the galaxy NGC 541. Full spectral fitting with ppxf indicates that Minkowski’s object primarily consists of a young stellar population $\sim \! 10\, \rm Myr$ old, confirming that the bulk of the object’s stellar mass formed during a recent jet interaction. Minkowski’s object exhibits line ratios largely consistent with star formation, although there is evidence for a low level ($\lesssim \! 15 \, \rm per \, cent$) of contamination from a non-stellar ionizing source. Strong-line diagnostics reveal a significant variation in the gas-phase metallicity within the object, with $\log \left(\rm O / H \right) + 12$ varying by $\sim \! 0.5\, \rm dex$, which cannot be explained by in-situ star formation, an enriched outflow from the jet, or enrichment of gas in the stellar bridge between NGC 541 and NGC 545/547. We hypothesize that Minkowski’s object either (i) was formed as a result of jet-induced star formation in pre-existing gas clumps in the stellar bridge, or (ii) is a gas-rich dwarf galaxy that is experiencing an elevation in its star formation rate due to a jet interaction, and will eventually redden and fade, becoming an ultradiffuse galaxy as it is processed by the cluster.

The number of globular clusters around the iconic UDG DF44 is as expected for dwarf galaxies

ABSTRACT There is a growing consensus that the vast majority of ultradiffuse galaxies (UDGs) are dwarf galaxies. However, there remain a few UDGs that seem to be special in terms of their globular cluster (GC) systems. In particular, according to some authors, certain UDGs exhibit large GC populations when compared to expectations from their stellar (or total) mass. Among these special UDGs, DF44 in the Coma cluster is one of the better-known examples. DF44 has been claimed to have a relatively high number of GCs, $N_{\mathrm{ GC}}=74^{+18}_{-18}$, for a stellar mass of only $3\times 10^8\, \mathrm{ M}_{ \odot}$ which would indicate a much larger dark halo mass than dwarfs of similar stellar mass. In this paper, we revisit this number and, contrary to previous results, find $N_{\mathrm{ GC}}=21^{+7}_{-9}$ assuming that the distribution of the GCs follows the same geometry as the galaxy. If we assume that the GCs around DF44 are distributed in a (projected) circularly symmetric way and, if we use a less strict criterion for the selection of the GCs, we find $N_{\mathrm{ GC}}=18^{+23}_{-12}$. Making use of the MGC–Mhalo relation, this number of GCs suggests a dark matter halo mass of $M_{\mathrm{ halo}}=1.1^{+0.4}_{-0.5} \times 10^{11} \mathrm{ M}_{\odot}$, a value which is consistent with the expected total mass for DF44 based on its velocity dispersion, $\sigma =33^{+3}_{-3}$ km s−1. We conclude that the number of GCs around DF44 is as expected for regular dwarf galaxies of similar stellar mass and DF44 is not extraordinary in this respect.

Hyper Suprime-Cam Low Surface Brightness Galaxies. II. A Hubble Space Telescope Study of the Globular Cluster Systems of Ultradiffuse Galaxies in Groups*

Abstract We increase the sample of ultradiffuse galaxies (UDGs) in lower-density environments with characterized globular cluster (GC) populations using new Hubble Space Telescope observations of nine UDGs in group environments. While the bulk of our UDGs have GC abundances consistent with normal dwarf galaxies, two of these UDGs have excess GC populations. These two UDGs both have GC luminosity functions consistent with higher surface brightness galaxies and cluster UDGs. We then combine our nine objects with previous studies to create a catalog of UDGs with analyzed GC populations that spans a uniquely diverse range of environments. We use this catalog to examine broader trends in the GC populations of low stellar mass galaxies. The highest GC abundances are found in cluster UDGs, but whether cluster UDGs are actually more extreme requires the study of many more UDGs in groups. We find a possible positive correlation between GC abundance and stellar mass, and between GC abundance and galaxy size at fixed stellar mass. However, we see no significant relation between stellar mass and galaxy size, over our limited stellar mass range. We consider possible origins of the correlation between GC abundance and galaxy size, including the possibility that these two galaxy properties are both dependent on the galaxy dark matter halo, or that they are related through baryonic processes like internal feedback.

The first detection of ultra-diffuse galaxies in the Hydra I cluster from the VEGAS survey

In this paper, we report the discovery of 27 low-surface brightness galaxies, of which 12 are candidates for ultra-diffuse galaxies (UDG) in the Hydra I cluster, based on deep observations taken as part of the VST Early-type Galaxy Survey (VEGAS). This first sample of UDG candidates in the Hydra I cluster represents an important step in our project that aims to enlarge the number of confirmed UDGs and, through study of statistically relevant samples, constrain the nature and formation of UDGs. This study presents the main properties of this class of galaxies in the Hydra I cluster. For all UDGs, we analysed the light and colour distribution, and we provide a census of the globular cluster (GC) systems around them. Given the limitations of a reliable GC selection based on two relatively close optical bands only, we find that half of the UDG candidates have a total GC population consistent with zero. Of the other half, two galaxies have a total population larger than zero at 2σ level. We estimate the stellar mass, the total number of GCs, and the GC specific frequency (SN). Most of the candidates span a range of stellar masses of 107 − 108 M⊙. Based on the GC population of these newly discovered UDGs, we conclude that most of these galaxies have a standard or low dark matter content, with a halo mass of ≤1010 M⊙.

Systematically Measuring Ultradiffuse Galaxies in H i: Results from the Pilot Survey

Abstract We present neutral hydrogen (H i) observations using the Robert C. Byrd Green Bank Telescope (GBT) of 70 optically detected UDG candidates in the Coma region from the Systematically Measuring Ultra-Diffuse Galaxies survey (SMUDGes). We detect H i in 18 targets, confirming nine to be gas-rich UDGs and the remainder to be foreground dwarfs. None of our H i-detected UDGs are Coma Cluster members and all but one are in low-density environments. The H i-detected UDGs are bluer and have more irregular morphologies than the redder, smoother candidates not detected in H i, with the combination of optical color and morphology being a better predictor of gas richness than either parameter alone. There is little visual difference between the gas-rich UDGs and the foreground dwarfs in the SMUDGes imaging, and distances are needed to distinguish between them. We find that the gas richnesses of our H i-confirmed UDGs and those from other samples scale with their effective radii in two stellar mass bins, possibly providing clues to their formation. We attempt to place our UDGs on the baryonic Tully–Fisher relation (BTFR) using optical ellipticities and turbulence-corrected H i line widths to estimate rotation velocities, but the potential systematics associated with fitting smooth Sérsic profiles to clumpy, low-inclination disks of low surface brightness precludes a meaningful analysis of potential BTFR offsets. These observations are a pilot for a large campaign now under way at the GBT to use the H i properties of gas-rich UDGs to quantitatively constrain how these galaxies form and evolve.

The Archetypal Ultra-diffuse Galaxy, Dragonfly 44, Is not a Dark Milky Way

Abstract Due to the peculiar properties of ultra-diffuse galaxies (UDGs), understanding their origin presents a major challenge. Previous X-ray studies demonstrated that the bulk of UDGs lack substantial X-ray emission, implying that they reside in low-mass dark matter halos. This result, in concert with other observational and theoretical studies, pointed out that most UDGs belong to the class of dwarf galaxies. However, a subset of UDGs is believed to host a large population of globular clusters (GCs), which is indicative of massive dark matter halos. This, in turn, hints that some UDGs may be failed L ⋆ galaxies. In this work, I present Chandra and XMM-Newton observations of two archetypal UDGs, Dragonfly 44 and DF X1, and I constrain their dark matter halo mass based on the X-ray emission originating from hot gaseous emission and from the population of low-mass X-ray binaries residing in GCs. Both Dragonfly 44 and DF X1 remain undetected in X-rays. The upper limits on the X-ray emission exclude the possibility that these galaxies reside in massive (M vir ≳ 5 × 1011 M ⊙) dark matter halos, suggesting that they are not failed L ⋆ galaxies. These results demonstrate that even these iconic UDGs resemble to dwarf galaxies with M vir ≲ 1011 M ⊙, implying that UDGs represent a single galaxy population.

Self-Interacting Dark Matter and the Origin of Ultradiffuse Galaxies NGC1052-DF2 and -DF4.

Observations of ultradiffuse galaxies NGC 1052-DF2 and -DF4 show they may contain little dark matter, challenging our understanding of galaxy formation. Using controlled N-body simulations, we explore the possibility that their properties can be reproduced through tidal stripping from the elliptical galaxy NGC 1052, in both cold dark matter (CDM) and self-interacting dark matter (SIDM) scenarios. To explain the dark matter deficiency, we find that a CDM halo must have a very low concentration so that it can lose sufficient inner mass in the tidal field. In contrast, SIDM favors a higher and more reasonable concentration as core formation enhances tidal mass loss. Final stellar distributions in our SIDM benchmarks are more diffuse than the CDM one, and hence the former are in better agreement with the data. We further show that a cored CDM halo model modified by strong baryonic feedback is unlikely to reproduce the observations. Our results indicate that SIDM is more favorable for the formation of dark-matter-deficient galaxies.

CO observations towards H i-rich Ultradiffuse Galaxies

ABSTRACT We present CO observations towards a sample of six H i-rich Ultradiffuse galaxies (UDGs) as well as one UDG (VLSB-A) in the Virgo Cluster with the Institut de RadioAstronomie Millimétrique (IRAM) 30-m telescope. CO J = 1–0 is marginally detected at 4σ level in AGC 122966, as the first detection of CO emission in UDGs. We estimate upper limits of molecular mass in other galaxies from the non-detection of CO lines. These upper limits and the marginal CO detection in AGC 122966 indicate low mass ratios between molecular and atomic gas masses. With the star formation efficiency derived from the molecular gas, we suggest that the inefficiency of star formation in such H i-rich UDGs is likely caused by the low efficiency in converting molecules from atomic gas, instead of low efficiency in forming stars from molecular gas.

Ultraviolet and X-ray properties of Coma’s ultra-diffuse galaxies

ABSTRACT Many ultra-diffuse galaxies (UDGs) have been discovered in the Coma cluster, and there is evidence that some, notably Dragonfly 44, have Milky Way-like dynamical masses despite dwarf-like stellar masses. We used X-ray, ultraviolet (UV), and optical data to investigate the star formation and nuclear activity in the Coma UDGs, and we obtained deep UV and X-ray data (Swift and XMM–Newton) for Dragonfly 44 to search for low-level star formation, hot circumgalactic gas, and the integrated emission from X-ray binaries. Among the Coma UDGs, we find UV luminosities consistent with quiescence but NUV − r colours indicating star formation in the past Gyr. This indicates that the UDGs were recently quenched. The r-band luminosity declines with projected distance from the Coma core. The Dragonfly 44 UV luminosity is also consistent with quiescence, with SFR$\lt 6\times 10^{-4} \, \mathrm{M}_{\odot }$ yr−1, and no X-rays are detected down to a sensitivity of 1038 erg s−1. This rules out a hot corona with a $M \gt 10^8 \, \mathrm{M}_{\odot }$ within the virial radius, which would normally be expected for a dynamically massive galaxy. The absence of bright, low-mass X-ray binaries is consistent with the expectation from the Galaxy total stellar mass, but it is unlikely if most low-mass X-ray binaries form in globular clusters, as Dragonfly 44 has a very large population. Based on the UV and X-ray analysis, the Coma UDGs are consistent with quenched dwarf galaxies, although we cannot rule out a dynamically massive population.