Alex Drlica-Wagner

We present the discovery of the Milky Way satellite Rubin-GC1 (Rubin J1628−1735) using data from the NSF-DOE Vera C. Rubin Observatory’s Early Data Preview 2. The system was discovered in the direction of the Galactic bulge with ( l , b ) ∼ (359°, 21°) in a region of high stellar density and a foreground reddening of E ( B − V ) ≈ 0.7. We simultaneously fit the morphology and isochrone parameters of Rubin-GC1, determining that it is a faint ( M V = − 2 . 4 − 0.7 + 0.5 ) and compact ( r 1 / 2 = 5 − 1 + 1 pc) Milky Way satellite at a distance of D ⊙ = 3 1 − 3 + 3 kpc; its stellar population is intermediate-age ( τ = 9 . 2 − 0.7 + 1.2 Gyr) and moderately metal-rich ( [ Fe / H ] iso = − 0.7 3 − 0.04 + 0.04 ). Together, these properties suggest Rubin-GC1 is most likely a star cluster in the faint tail of the luminosity function of globular clusters surrounding the Milky Way. We identify a distinct proper motion signature from Rubin-GC1 in Gaia DR3, and from a comparison of the system’s 5D phase space information to N -body models of the Sagittarius dwarf spheroidal galaxy’s tidal stream, we determine that Rubin-GC1 is likely an accreted cluster associated with the Sagittarius leading arm. The discovery of Rubin-GC1 highlights the potential of Rubin to uncover faint, resolved satellites in dust-shrouded regions of the Milky Way, including remnants of the accretion events that have shaped the Galaxy’s hierarchical assembly.

Erratum: “A Pride of Satellites in the Constellation Leo? Discovery of the Leo VI Milky Way Satellite Ultra-faint Dwarf Galaxy with DELVE Early Data Release 3” (2025, ApJ, 979, 176), Tan, C. Y., Cerny, W., Drlica-Wagner, A., Pace, A. B., Geha, M., Ji, A. P., Li, T. S., Adamów, M., Anbajagane, D., Bom, C. R., Carballo-Bello, J. A., Carlin, J. L., Chang, C., Chaturvedi, A., Chiti, A., Choi, Y., Collins, M. L. M., Doliva-Dolinsky, A., Ferguson, P. S., Gruendl, R. A., James, D. J., Limberg, G., Navabi, M., Martínez-Delgado, D., Martínez-Vázquez, C. E., Medina, G. E., Mutlu-Pakdil, B., Nidever, D. L., Noël, N. E. D., Riley, A. H., Sakowska, J. D., Sand, D. J., Sharp, J., Stringfellow, G. S., Tolley, C., Tucker, D., Vivas, A. K., (DELVE Collaboration) Erratum: “A Pride of Satellites in the Constellation Leo? Discovery of the Leo VI Milky Way Satellite Ultra-faint Dwarf Galaxy with DELVE Early Data Release 3” (2025, ApJ, 979, 176), Tan, C. Y., Cerny, W., Drlica-Wagner, A., Pace, A. B., Geha, M., Ji, A. P., Li, T. S., Adamów, M., Anbajagane, D., Bom, C. R., Carballo-Bello, J. A., Carlin, J. L., Chang, C., Chaturvedi, A., Chiti, A., Choi, Y., Collins, M. L. M., Doliva-Dolinsky, A., Ferguson, P. S., Gruendl, R. A., James, D. J., Limberg, G., Navabi, M., Martínez-Delgado, D., Martínez-Vázquez, C. E., Medina, G. E., Mutlu-Pakdil, B., Nidever, D. L., Noël, N. E. D., Riley, A. H., Sakowska, J. D., Sand, D. J., Sharp, J., Stringfellow, G. S., Tolley, C., Tucker, D., Vivas, A. K., (DELVE Collaboration)

We present results of a spectroscopic campaign around the diffuse dwarf galaxy Crater 2 (Cra2) and its tidal tails as part of the Southern Stellar Stream Spectroscopic Survey ( S ⁵ ). Cra2 is a Milky Way dwarf spheroidal satellite with extremely cold kinematics, but a huge size similar to the Small Magellanic Cloud, which may be difficult to explain within collisionless cold dark matter. We identify 143 Cra2 members, of which 114 belong to the galaxy’s main body, and 29 are deemed part of its stellar stream. We confirm that Cra2 is dynamically cold (central velocity dispersion 2.5 1 − 0.30 + 0.33 km s − 1 ) and also discover a ≈7 σ velocity gradient consistent with its tidal debris track. We separately estimate the stream’s internal velocity dispersion to be 5.7 4 − 0.83 + 0.98 km s − 1 . We develop a suite of N -body simulations with both cuspy and cored density profiles on a realistic Cra2 orbit to compare with S ⁵ observations. We find that the velocity dispersion ratio between the Cra2 stream and galaxy ( 2.3 0 − 0.35 + 0.41 ) is difficult to reconcile with a cuspy halo with fiducial concentration and an initial mass predicted by standard stellar mass–halo mass relationships. Instead, either a cored halo with a relatively small core radius or a low-concentration cuspy model can reproduce this ratio. Despite tidal mass loss, Cra2 is metal-poor (〈[Fe/H]〉 = −2.16 ± 0.04) compared to the stellar mass–metallicity relation for its luminosity. Other diffuse dwarf galaxies similar to Cra2 in the Local Group (Antlia 2 and Andromeda 19) also challenge galaxy formation models. Finally, we discuss possible formation scenarios for Cra2, including ram-pressure stripping of a gas-rich progenitor combined with tides.

We present the discovery of Aquarius IV (Rubin J2201−0234)—the first ultra-faint Milky Way (MW) satellite to be identified using data from the Vera C. Rubin Observatory. This system was detected at ∼8 σ significance using Rubin Early Data Preview 2 (EDP2) photometry and independently confirmed at ∼6 σ significance in archival Dark Energy Camera imaging. Jointly fitting its morphology and distance, we find that Aquarius IV is a low-luminosity ( M V = − 1 . 9 − 1.0 + 0.6 ), compact ( r 1 / 2 = 1 9 − 6 + 4 pc; r h = 0.6 0 − 0.17 + 0.14 arcmin) stellar system in the outer Galactic halo ( D _⊙ = 10 9 − 8 + 6 kpc ). Its stellar population is consistent with an ancient, metal-poor stellar isochrone ( τ = 13 Gyr, Z = 0.0001). These properties closely resemble those of the smallest and faintest confirmed ultra-faint dwarf galaxies (UFDs), though a globular cluster classification is not ruled out. Given the small number of detected member stars in Rubin EDP2, deeper imaging and spectroscopy will be critical for determining the properties and classification of Aquarius IV at higher confidence.

Crater II (CraII), a large and low-density dwarf spheroidal galaxy, has unusual observed properties that are difficult to reproduce in cold dark matter simulations. Ongoing tidal disruption may help explain the discrepancies, as evidenced by the recent discovery of tidal tails. Here we present metallicity-sensitive narrowband photometry of the Ca II H and K lines from the Dark Energy Camera, covering 128 deg ² across the center and identified tidal tails of CraII as part of the Mapping the Ancient Galaxy in CaHK survey. Our combined photometric metallicity, color–magnitude, proper motion, and parallax selections identify 162 CraII candidates. Of these, 37 candidates are located in the tidal tails, which extend at least 7° (∼95 kpc) from the center of CraII, suggesting it has lost ≳25% of its initial stellar mass. We confirm low contamination rates with dedicated control fields and highlight the extremely low surface brightness stellar features that can be uncovered with CaHK data, as faint as ∼36 mag arcsec ⁻² . We also make the first detection of a metallicity gradient (−0.34 ± 0.17 dex deg ⁻¹ ) in the center of the galaxy and infer a stream width of w ∼ 0 . ° 8, roughly 50% larger than the CraII half-light radius. The detection of candidates in the most distant CraII pointings from its center implies that the tidal tails extend beyond our footprint. We compare the CraII stream to N -body models with “cored” and “cuspy” dark matter halo progenitors, determining that CraII’s density profile is still ambiguous and warrants further modeling.

Studies of Fermi Large Area Telescope (LAT) data coincident with dwarf spheroidal satellite galaxies (dSphs) of the Milky Way have put the most stringent constraints on models of annihilating dark matter (DM) with candidate masses in the GeV–TeV range. Recent results found the presence of small, local significance excesses from these targets, at the 2 σ –3 σ level. However, these excesses disagree on the predicted properties of the DM candidate, and their significance vanishes when considering correction factors for the number of trials. In this work, we apply key improvements to the analysis of dSphs. We use stricter cuts on the data, implement a method to adaptively model the background, and assume an updated framework for DM annihilation. We find that our improved background modeling leads to a better agreement between the model and the data. This produces an increase in the local and global significance of the dSph excess compared to previous studies. Finally, we find that the DM properties obtained in this work are less dependent on the sample of dSphs being considered compared to previous studies, while remaining in agreement with the predictions from the Galactic center excess observed by Fermi/LAT and the antiproton excess observed by the Alpha Magnetic Spectrometer (AMS-02). Considering our improvements, a future significant increase in the number of dwarfs may lead to a definitive confirmation or exclusion of the DM interpretation of the Galactic center excess.

We present Rubin Data Preview 1 (DP1), the first data from the National Science Foundation–Department of Energy Vera C. Rubin Observatory, comprising raw and calibrated single-epoch images, coadds, difference images, detection catalogs, and ancillary data products. DP1 is based on 1792 optical–near-infrared exposures acquired over 48 distinct nights by the Rubin Commissioning Camera (LSSTComCam) on the Simonyi Survey Telescope at the Summit Facility on Cerro Pachón, Chile in late 2024. DP1 covers ∼15 deg ² distributed across seven roughly equal-sized noncontiguous fields, each independently observed in six broad photometric bands, ugrizy . The median FWHM of the point-spread function across all bands is approximately 1 . ″ 14, with the sharpest images reaching about 0 . ″ 58. The 5 σ point-source depths for coadded images in the deepest field, the Extended Chandra Deep Field South, are u = 24.55, g = 26.18, r = 25.96, i = 25.71, z = 25.07, and y = 23.1. Other fields are no more than 2.2 mag shallower in any band, where they have nonzero coverage. DP1 contains approximately 2.3 million distinct astrophysical objects, of which 1.6 million are extended in at least one band in coadds, and 431 solar system objects, of which 93 are new discoveries. DP1 is approximately 3.5 TB in size and is available to Vera C. Rubin Observatory data rights holders via the Rubin Science Platform, a cloud-based environment for the analysis of petascale astronomical data. While small compared to future LSST releases, its high quality and diversity of data support a broad range of early science investigations ahead of full operations in 2026.

Precise measurements of Type Ia supernovae (SNe Ia) at low redshifts ( z ) serve as one of the most viable keys to unlocking our understanding of cosmic expansion, isotropy, and growth of structure. The Dark Energy Bedrock All-Sky Supernovae (DEBASS) program will deliver a uniformly calibrated low- z dataset of more than 400 spectroscopically confirmed SNe Ia in the Southern Hemisphere. DEBASS utilizes the Dark Energy Camera to image supernovae in conjunction with the Wide-Field Spectrograph to gather comprehensive host-galaxy information. By using the same photometric instrument as both the Dark Energy Survey (DES) and the DECam Local Volume Exploration Survey, DEBASS not only benefits from a robust photometric pipeline and well-calibrated images across the Southern sky, but can replace the historic and external low- z samples that were used in the final DES supernova analysis. In this paper, along with a companion paper, we present an early data release of 77 DEBASS SNe within the DES footprint. We introduce the DEBASS program, discuss its scientific goals and the advantages it offers for supernova cosmology, and present our initial results demonstrating data quality. With this early data release, we find a robust median absolute standard deviation of Hubble diagram residuals of ∼0.10 mag and an initial measurement of the host-galaxy mass step of 0.06 ± 0.04 mag, both before performing bias corrections. This low scatter shows the promise of a low- z SN Ia program with a well-calibrated telescope and high signal-to-noise ratio across multiple bands.

We conduct a search for strong gravitational lenses in the Dark Energy Survey (DES) Year 6 imaging data. We implement a pre-trained Vision Transformer (ViT) for our machine learning (ML) architecture and adopt interactive machine learning to construct a training sample with multiple classes to address common types of false positives. Our ML model reduces ∼236 million DES cutout images to 22,564 targets of interest, including ∼85% of previously reported galaxy–galaxy lens candidates discovered in DES. These targets were visually inspected by citizen scientists, who ruled out ∼90% as false positives. Of the remaining 2618 candidates, 149 were expert-classified as “definite” lenses and 516 as “probable” lenses, for a total of 665 systems, with 147 of these candidates being newly identified. Additionally, we trained a second ViT to find double-source plane lens systems, finding at least one double-source system. Our main ViT excels at identifying galaxy–galaxy lenses, consistently assigning high scores to candidates with high expert assessments. The top 800 ViT-scored images include ∼100 of our “definite” lens candidates. This selection is an order of magnitude higher in purity than previous convolutional neural-network-based lens searches and demonstrates the feasibility of applying our methodology for discovering large samples of lenses in future surveys.

Stellar streams are sensitive laboratories for understanding the small-scale structure in our Galaxy’s gravitational field. Here, we analyze the morphology of the 300 S stellar stream, which has an eccentric, retrograde orbit and thus could be an especially powerful probe of both baryonic and dark substructures within the Milky Way. Due to extensive background contamination from the Sagittarius stream (Sgr), we perform an analysis combining Dark Energy Camera Legacy Survey photometry, ???????????????? DR3 proper motions, and spectroscopy from the Southern Stellar Stream Spectroscopic Survey ( ???? 5 ). We redetermine the stream coordinate system and distance gradient, then apply two approaches to describe 300 S ’s morphology. In the first, we analyze stars from ???????????????? using proper motions to remove Sgr. In the second, we generate a simultaneous model of 300 S and Sgr based purely on photometric information. Both approaches agree within their respective domains and describe the stream over a region spanning 33 ∘ . Overall, 300 S has three well-defined density peaks and smooth variations in stream width. Furthermore, 300 S has a possible gap of ∼ 4.7 ∘ and a kink. Dynamical modeling of the kink implies that 300 S was dramatically influenced by the Large Magellanic Cloud. This is the first model of 300 S ’s morphology across its entire known footprint, opening the door for deeper analysis to constrain the structures of the Milky Way.