We present the methods and data for a detailed structural analysis of over 600 ultracompact dwarf (UCD) galaxy candidates in the Virgo Cluster, based on deep ground-based imaging from the Next Generation Virgo Cluster Survey and archival Hubble Space Telescope (HST) Advanced Camera for Surveys imaging. Our goal is to test two main formation scenarios-whether UCDs are massive globular clusters (GCs) or the remnant nuclei of tidally stripped dwarf galaxies (nucleated dwarf elliptical galaxies, dEs)-by analyzing their morphologies and searching for extended stellar envelopes. We find that over 10% of Virgo UCD candidates exhibit two-component profiles with extended outer structures of Re similar to 30-300 pc that cannot be well fitted by standard King or S & eacute;rsic models, indicating the presence of diffuse envelopes (eUCDs). High-resolution HST imaging is essential for resolving compact eUCDs that are unresolved in ground-based data. We also present results of the Gemini/Gemini Multi-Object Spectrograph spectroscopy follow-up programs to confirm the Virgo Cluster membership for eUCD candidates. The full catalog consists of a near-complete spectroscopic sample of 68 Virgo eUCDs, as well as properties for 51 strongly nucleated dEs that resemble eUCDs. Together, this constitutes the largest and most comprehensive sample thus far of eUCDs and other possible nucleated dE-GC transition objects. This is a companion paper to K. Wang et al., which was based in large part on the data we present here. Future wide-field space-based surveys will offer unprecedented opportunities to identify eUCDs across a range of environments.

The Next Generation Virgo Cluster Survey (NGVS) is a deep, high resolution imaging campaign that used the 1 deg^2 MegaCam instrument on the Canada-France-Hawaii Telescope to carry out a comprehensive optical survey of the Virgo cluster, from its core to its virial radius. The NGVS covers a contiguous area of 104 deg^2 (8.63 Mpc^2 at the 16.5 Mpc distance of Virgo) in the u^*-,g-,i-, and z-band, with additional limited coverage in r. In this paper, we present the final catalog of Virgo galaxies across the entire NGVS area. The catalog includes 3680 galaxies considered to be bona fide members of the cluster, spanning a factor of 2.5 million in luminosity, from g = 8.42 mag to g = 24.41 mag (M_g = -22.67 mag to M_g = -6.68 mag). With 2100 previously uncataloged galaxies, the NGVS catalog augments the number of known Virgo members by a factor 2.3. The catalog is complete down to g = 18.6 mag (M_g=-12.5 mag, corresponding to a stellar mass M_* ∼ 1.6×10^7 M_⊙ for an old stellar population) and 50

On the periphery of galaxy clusters, moderately high galaxy densities and velocity dispersions favour interactions and mergers that influence galaxy evolution prior to cluster infall. Observational studies of this phase in dwarfs remain rare. We present a high-resolution study of the merger remnant VCC 693 in the outskirts of Virgo cluster, using observations from the Atomic gas in Virgo Interacting Dwarf galaxies (AVID) project. We explore the origin of VCC 693 and the consequences of the merger on its star formation and structure through a joint analysis of VLA and FAST HI emission line observations, together with complementary optical imaging and spectroscopy. We employ hydrodynamical simulations to help interpret the observations. Our analysis favours a near-major merger between two dwarfs with a stellar mass ratio of 3:1-4:1, with one likely gas-poor progenitor (i.e., a damp merger). The optical appearance of VCC 693 is dominated by complex tidal structures throughout the system, whereas the HI gas has settled to a regular rotating disk. Compared with similar-mass dwarfs, the central star formation and gas-phase metallicity are moderately enhanced. The global star formation rate, HI gas content, and HI-to-optical size ratio of VCC 693 are broadly consistent with those of typical dwarfs of similar mass, albeit somewhat lower. Decomposition of the HI rotation curve into baryonic and dark matter indicates a high halo concentration, suggesting post-merger relaxation into a more centrally peaked configuration. Together with two recent studies of AVID post-merger systems, these results support the view that even major dwarf mergers can produce remnants with overall stellar structures indistinguishable from ordinary dwarfs, and that the environmental effects in cluster outskirts can promote damp or mixed mergers, constituting an integral part of galactic pre-processing.

We present a kinematical study of the outer halo (rGC similar to 60-160 kpc) of the Milky Way (MW) based on spectroscopy of 55 RR Lyrae stars using the Echelle Spectrograph and Imager instrument on the Keck II telescope. Our spectroscopic targets were selected from three photometric surveys: NGVS, DES, and Pan-STARRS 1. We derive center-of-mass radial velocities with uncertainties of 6-35 km s-1. The halo velocity dispersion measured with our sample is 70 +/- 7 km s-1. The velocity field shows a possible dipole-like structure, with redshifted northern and blueshifted southern hemispheres. Fitting a MW-Large Magellanic Cloud (LMC) dipole perturbation model yields a weak/marginal dipole signal, with an amplitude of -30-20+16 km s-1 and an apex direction (l,b)=(-38.2-31.5+42.4,-41.3-23.8+27.9) deg, along with a bulk compression velocity of -16 +/- 11 km s-1. While limited by sky coverage and sample size, our results are consistent with the presence of LMC-induced disequilibrium in the distant halo beyond 100 kpc. Aside from the 55 RR Lyrae stars, our spectroscopic analysis reveals that 10 additional phometrically selected RR Lyrae candidates are, in fact, quasar/blazar contaminants; this provides a cautionary tale about the presence of such contaminants in sparsely sampled photometric surveys. Our study demonstrates that single-epoch spectroscopy of RR Lyrae stars is a viable method for probing the kinematics of the outer halo, and future surveys like Rubin/LSST and the Dark Energy Spectroscopic Instrument (DESI-II) have the potential to significantly advance this effort.

In the periphery of galaxy clusters, the combination of moderately high galaxy number densities and modest velocity dispersions favors interactions and mergers that affect the galaxy evolution prior to cluster infall. Observational studies of this important phase of galaxy evolution, particularly in the dwarf galaxy regime, are still rare. We present a high-resolution case study of VCC 693 (stellar mass ∼ 2.8 × 108 M⊙), a merger remnant in the outskirts of the Virgo cluster, using observations from the project Atomic gas in Virgo Interacting Dwarf galaxies (AVID). We explored the consequences of the merger on the star formation and structural evolution of VCC 693, based on a joint analysis of high-resolution Karl G. Jansky Very Large Array and high-sensitivity Five-Hundred-meter Aperture Spherical radio Telescope H I emission line observations, optical broadband images, narrowband Hα images, and optical spectra. We also employed hydrodynamical simulations of dwarf-dwarf mergers to aid in interpreting the observations of VCC 693. Our analysis favors a near-major merger between two late-type dwarf galaxies with a stellar mass ratio of approximately 3:1–4:1, in which one of the progenitors is likely relatively gas poor (i.e., a damp merger). The optical appearance of VCC 693 is dominated by complex tidal structures spanning the whole system, whereas the H I gas has settled to a regular rotating disk with a normal surface density profile. Compared to dwarfs of similar mass, the star formation and gas-phase metallicity are moderately enhanced in the center. The global star formation rate, H I gas content, and H I-to-optical size ratio of VCC 693 are broadly consistent with those of typical late-type dwarfs of similar mass, although they fall at the lower side of the distributions. By decomposing the H I rotation curve into the baryonic and dark matter component, we found that the dark matter halo is characterized by an unusually high concentration or core density. This implies that the dark matter halo might have relaxed into a more centrally peaked distribution following the merger event. Together with two other recent studies of AVID post-merger systems, our findings reinforce the emerging view that even major mergers between dwarfs can produce remnants whose overall stellar structures, apart from tidal features, are indistinguishable from those of ordinary dwarfs, and that the diverse environmental effects experienced by galaxies in cluster outskirts can promote damp or mixed mergers, which constitute an integral part of galactic preprocessing.

We present a simulation of the optical efficiency of a DLP7000-series digital micromirror device (DMD) in farto near-ultraviolet (UV) wavelength regime. DMDs are spatial light modulators that enable multi-object spectroscopy for UV astronomy. The optical performance of DMDs in this wavelength regime is not well characterized due to practical difficulties in simulating (significant computing power requirements) and measuring (need for a UV test facility) the performance of the device. This work builds off previous research conducted in the visible to infrared wavelength regime and simulates the performance of DMDs in terms of optical efficiency in the UV. Three-dimensional electromagnetic finite-difference time domain simulations are conducted with the DMD illuminated by an f/20 diffraction-limited point spread function across multiple locations on the face of a micromirror. The electromagnetic near-field output is transformed to the far-field to obtain intensity as a function of solid angle, or radiant intensity. Using a simulated collection aperture, we calculate efficiency from 150 nm to 300 nm.

A recent study of the compact elliptical galaxy NGC 4486B using JWST Near-Infrared Spectrograph integral-field-unit-mode kinematics confirmed a supermassive black hole (SMBH) of mass MBH=3.6 +/- 0.7 & times;108 (similar to 8% of the stellar mass). In addition to its double nucleus, the nuclear kinematics show pronounced asymmetries: a velocity dispersion peak displaced by 6 pc from the galaxy center and a similar to 16 km s-1 offset in the mean stellar line-of-sight velocity near the SMBH. We examine the origin of the 12 pc double nucleus and show that the observations favor an SMBH surrounded by an eccentric nuclear disk (END). One proposed formation pathway for ENDs involves a gravitational-wave (GW) recoil of the SMBH following a binary merger. Our orbit superposition models contain similar to 50% retrograde stars at the edge of the nuclear region, in agreement with END formation simulations. We infer a premerger mass ratio q > 0.15 and a recoil kick of similar to 340 km s(-1). Our N-body simulations show that with such a kick, the SMBH returns to the center within similar to 30 Myr. Its flat central core is also consistent with earlier "binary black hole scouring." We test two alternative mechanisms-buoyancy-driven oscillations and a premerger SMBH binary-but neither reproduces the observed offsets, favoring the GW kick scenario. Our N-body simulations also show that a prograde SMBH binary in a rotating host can become trapped in a corotation resonance, delaying coalescence. Although NGC 4486B is an old galaxy near the Virgo Cluster center, its SMBH appears to have merged only recently, making its nucleus a rare nearby laboratory for postmerger SMBH dynamics.

The origin of extragalactic, almost dark HI clouds with extreme gas-to-stellar mass ratios remains poorly understood. We investigate the nature and fate of the "almost dark" cloud AGC 226178, projected within the Virgo cluster, with an HI-to-stellar mass ratio of 1000. We present deep single-dish HI mapping from the Five-hundred-meter Aperture Spherical Telescope (FAST), complemented by high-resolution interferometric data from the Very Large Array (VLA), as part of the Atomic gas in Virgo Interacting Dwarf galaxies (AVID) project. These observations provide the highest-quality HI analysis to date of such a cloud, combining resolution and sensitivity. FAST data reveal a short, low-velocity tail toward the dwarf galaxy VCC 2034, previously proposed as a possible origin for AGC 226178. However, VCC 2034 shows a line-of-sight asymmetric HI feature and cometary morphology indicating a stripping event unrelated to AGC 226178. VLA data reveal a velocity gradient across AGC 226178 and a clumpy internal structure. The velocity dispersion exceeds the thermal linewidth, implying turbulence or unresolved motions. The cloud cannot be gravitationally bound by atomic gas alone. The resolved HI clumps follow standard HI mass-star formation rate and mass-size relations, with those forming stars reaching surface densities above the threshold for self-shielding. We conclude that AGC 226178 is a free-floating HI cloud of unknown origin. The system appears to be in the process of disintegration. It is likely located well outside the Virgo cluster, as the preservation of its extended HI morphology within the cluster environment would otherwise require a substantial reservoir of unseen molecular gas with a mass exceeding that of the observed HI content. While confinement pressure from the hot intracluster medium may aid its stability, it is unlikely to be the dominant factor preventing its disruption.

We analyze deep (M* greater than or similar to 107 M circle dot) galaxy stellar mass functions (SMFs) of the Virgo cluster using stellar masses derived as part of the Next Generation Virgo Survey. The total SMF has a slope of alpha=-1.35-0.02+0.02 , which is similar to or steeper than typical field values. Using deep H alpha data from the Virgo Environmental Survey Tracing Ionised Gas Emission we separate out star-forming galaxies, quiescent galaxies with no ongoing star formation, and low star formation rate (SFR) galaxies that are intermediate between these two populations. For each of these populations, the shape of the SMF is found to be universal throughout the cluster, from the core to the outskirts. The star-forming and quiescent SMFs show stark differences with values seen in field galaxies. The relative fraction of quiescent galaxies is highest in the core of the cluster, with low-SFR and star-forming galaxies more significant in the outer regions of the cluster. At low stellar masses (M* less than or similar to 109 M circle dot), the quiescent fraction in the main cluster is significantly higher than that of the field and even satellites of massive groups. At high stellar masses, the quiescent fraction is similar to other studies of cluster galaxies. We model the quiescent population in the infall region of the cluster as a combination of backsplash and field quiescent galaxies, and find that the backsplash fractions needed to explain the observed population are unrealistically high. This suggests the existence of a third population of low-mass galaxies that are preprocessed outside the virial radius of the cluster, possibly in groups prior to infall.

The Ultraviolet Near-Infrared Optical Northern Survey (UNIONS) is a "collaboration of collaborations" that is using the Canada-France-Hawai'i Telescope, the Pan-STARRS telescopes, and the Subaru Observatory to obtain $ugriz$ images of a core survey region of 6250 deg$^2$ of the northern sky. The $10\sigma$ point source depth of the data, as measured within a 2-arcsecond diameter aperture, are $[u,g,r,i,z] = [23.7, 24.5, 24.2, 23.8, 23.3]$\ in AB magnitudes. UNIONS is addressing some of the most fundamental questions in astronomy, including the properties of dark matter, the growth of structure in the Universe from the very smallest galaxies to large-scale structure, and the assembly of the Milky Way. It is set to become the major ground-based legacy survey for the northern hemisphere for the next decade and provides an essential northern complement to the static-sky science of the Vera C. Rubin Observatory's Legacy Survey of Space and Time. UNIONS supports the core science mission of the {\it Euclid} space mission by providing the data necessary in the northern hemisphere for the calibration of the wavelength dependence of the {\it Euclid} point-spread function and derivation of photometric redshifts in the North Galactic Cap. This region contains the highest quality sky for {\it Euclid}, with low backgrounds from the zodiacal light, stellar density, extinction, and emission from Galactic cirrus. Here, we describe the UNIONS survey components, science goals, data products, and the current status of the overall program.

Hierarchical galactic evolution models predict that mergers drive galaxy growth, producing low surface brightness (LSB) tidal features that trace galaxies' late assembly. These faint structures encode information about past mergers and are sensitive to the properties and environment of the host galaxy. We investigated the relationships between LSB features and their hosts in a sample of 475 nearby massive galaxies spanning diverse environments (field, groups, Virgo cluster) using deep optical imaging from the Canada-France-Hawaii Telescope (MATLAS, UNIONS/CFIS, VESTIGE, NGVS). Using Jafar, an online annotation tool, we manually annotated tidal features, including 199 tidal tails and 100 streams. Geometric and photometric measurements were extracted to analyse their dependence on galaxy mass, environment, and internal kinematics. At our surface brightness limit of 29 mag arcsec(-2), tidal features contribute 2 per cent of total galaxy luminosity. They are detected in 36 per cent of galaxies, with none fainter than 27.8 mag arcsec(-2). The most massive galaxies are twice as likely to host tidal debris. Although small-scale interactions increase the frequency of tidal features, the large-scale environment (Virgo cluster versus field/group) does not influence it. An anticorrelation between this frequency and rotational support is found, but may reflect the mass-driven effect. We release our data base of annotated features for deep learning applications. Our findings confirm that galaxy mass is the dominant factor influencing tidal feature prevalence, consistent with hierarchical formation models.

Context. Dwarf-dwarf galaxy mergers are among the least explored aspects of dwarf galaxy pre-processing as they fall into clusters. Aims. We present the first case study of a coalesced late-type dwarf major merger (VCC 479; stellar mass similar to 8 x 107 M circle dot) that has undergone significant environmental influence, with the aim of exploring dwarf galaxy evolution under the combined effects of galaxy interactions and environmental processes, and understanding its relevance to the diversity of dwarf galaxies in cluster environments. Methods. Our analysis is based on multifrequency observations, including VLA and FAST HI emission line mapping from the Atomic gas in Virgo Interacting Dwarf galaxies (AVID) survey, and existing ultraviolet and optical images. We also performed idealized hydrodynamical simulations of dwarf-dwarf mergers to help us interpret the observations. Results. We identify symmetric stellar shell structures in VCC 479, indicative of a coalesced major merger of dwarf galaxies. The galaxy features a central starburst, initiated similar to 600 Myr ago, embedded within an exponential disk quenched similar to 1 Gyr ago. The starburst contributes only 2.9 +/- 0.5% of the total stellar mass, and VCC 479's global star formation rate is 0.3 dex lower than typical dwarfs of a similar mass. The galaxy is highly HI-deficient, with most HI gas concentrated within the central 1 kpc and little extended HI envelope. The misalignment of the HI velocity field with the stellar body is best explained by merger-triggered gas inflow, as is seen in our simulations. Conclusions. Our analysis is consistent with a scenario in which the majority of HI gas of the progenitor galaxies was removed by the cluster environment prior to the final coalescence. The merger concentrates the remaining gas toward the galaxy center, triggering a central starburst. The combined effect of environment stripping and galaxy merger has transformed VCC 479 into a blue-core dwarf undergoing a morphological transition from a late-type to an early-type galaxy.

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, approximate to 90% down the globular cluster luminosity function (GCLF). We estimate a total globular cluster population for VCC 615 of N-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 approximate to 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 radii R-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.

The Cosmological Advanced Survey Telescope for Optical and UV Research (CASTOR) is a planned flagship space telescope, covering the blue-optical and UV part of the spectrum. Here, we introduce the CASTOR image simulator, a python GalSim package-based script capable of generating mock CASTOR images from an input catalogue. We generate example images from the CASTOR Wide, Deep, and Ultra-Deep surveys using simulated lightcones from the Santa Cruz semi-analytic model. We make predictions for the performance of these surveys by comparing galaxies that are extracted from each image using Source Extractor to the input catalogue. We find that the Wide, Deep, and Ultra-Deep surveys will be 75 per cent complete for point sources down to similar to 27, 29, and 30 mag, respectively, in the UV, u, and g filters, with the UV-split and u-split filters reaching a shallower depth. With a large area of similar to 2200 deg(2), the Wide survey will detect hundreds of millions of galaxies out to z similar to 4, mostly with M & lowast;greater than or similar to 10(9)M(circle dot). The Ultra-Deep survey will probe to z similar to 5, detecting galaxies with M & lowast;greater than or similar to 10(7)M(circle dot). These galaxy samples will enable precision measurements of the distribution of star formation in the cosmic web, connecting the growth of stellar mass to the assembly of dark matter haloes over two thirds of the history of the Universe, and other core goals of CASTOR's legacy surveys. These image simulations and the tools developed to generate them will be a vital planning tool to estimate CASTOR's performance and iterate the telescope and survey designs prior to launch.

We present a study of morphologies, based on deep u * g ′ i ′ z ′ imaging of the Virgo Cluster from the Next Generation Virgo Cluster Survey, for 3689 Virgo cluster members spanning a mass range of ∼10 11 –10 5 M ⊙ . Our analysis introduces a new, two-component visual classification scheme developed to capture the morphological diversity of galaxies over more than 6 orders of magnitude in stellar mass. Our morphological classifications use two parameters to describe the global structure and star formation activity of each galaxy. Structural subcodes denote features such as spiral arms, bars, disks, shells, and streams, while star formation subcodes indicate the form and location of the current star formation activity (e.g., in cores, clumps, filaments). These visual classifications rely on deep g ′ -band images, supplemented by u * g ′ i ′ ­ color images, as well as unsharp-masked images for a subset of objects. We compare our classifications to previous results for bright member galaxies that used more established schemes, finding good agreement. We also measure quantitative classification statistics (e.g., CASGM 20 ) for a subset of the brighter galaxies, and present catalogs for some galaxy types of special interest, including structurally compact galaxies, ultradiffuse galaxies, candidate ultracompact dwarf transition objects, as well as candidate postmerger systems. These morphological classifications may be useful as a training set in the application of machine learning tools to the next generation of wide-field imaging surveys.

CASTOR is a proposed wide-field (30 ' x30 '=0.25 deg(2)), high-resolution (FWHM similar to 0.15 ''), 1-m-diameter space telescope that is under development by the Canadian Space Agency and the National Research Council of Canada. Optimized for UV/blue-optical wavelengths, the telescope uses dichroics to enable imaging in three channels (and up to five bands) that cover the 0.15 to 0.55 mu m spectral region, simultaneously. CASTOR will also feature low- and low-medium-resolution spectroscopic capabilities through the use of a deployable grism for low-resolution (R less than or similar to 420) slit-less spectroscopy in its UV and u channels, and low-medium-resolution R similar to 1400 multi-object spectroscopy in a parallel field using a digital micro-mirror device. High-speed, precision photometry will be possible using dedicated CMOS detectors in each of its three channels. We present an overview of the mission, including the optical design, instruments and detectors, payload layout, satellite bus, orbit, and ground segment. We describe the mission's scientific capabilities and expected place within the astronomical landscape in the 2030s. The 5-year lifetime is baselined on a combination of legacy surveys, guest observer programs, and target-of-opportunity science. We summarize scientific plans for the mission in each of eight fields: cosmology, time domain and multi-messenger science, active galactic nuclei, galaxies, near-field cosmology, stellar astrophysics, exoplanets, and solar system studies. We conclude by describing ongoing development efforts, highlighting areas of particular relevance for NASA's Habitable Worlds Observatory.