Andrew B. Pace

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 deep, wide-field Magellan/Megacam imaging of the ultrafaint Milky Way (MW) satellites Grus II (Gru II) and Horologium II (Hor II), with the aim of deriving improved constraints on their distances, luminosities, and structural parameters, while also searching for possible signs of tidal disturbance. Our photometry reaches ∼3 mag deeper than the discovery data, enabling robust measurements of these quantities. Both systems exhibit color–magnitude diagrams consistent with old (∼12.5 Gyr), very metal-poor stellar populations. We find Gru II to be at a distance of 52.3 ± 1.9 kpc, with absolute magnitude M _V = −4.07 ± 0.50 mag, while fitting an exponential profile to the stellar distribution yields a half-light radius of 6 . ′ ⁡ 8 ± 0 . ′ ⁡ 5 (103 ± 9 pc), and ellipticity ϵ = 0.25 ± 0.07. Hor II is further away at a distance of 72 . 4 − 5.5 + 5.9 kpc, with M _V = −2.10 ± 0.44 mag, and is more compact with r _h = 2 . ′ ⁡ 1 ± 0 . ′ ⁡ 2 ( 4 4 − 5 + 6 pc) and ϵ = 0.3 2 − 0.16 + 0.20 , adopting an exponential profile. Both galaxies lie within the typical size–luminosity locus of MW ultrafaint dwarfs. Gru II shows an asymmetric morphology including multidirectional clumpy features, some of which could be suggestive of tidal disturbance. We further identify and spectroscopically confirm a new distant member just outside 3 r _h in Gru II, providing independent evidence for member stars at large projected radii. In contrast to Gru II, Hor II appears regular, with no significant extended structure detected to the surface-brightness limits of our data.

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.

We analyze the properties of satellite galaxies around 1024 Milky Way–mass hosts from the DREAMS Project, simulated within a ΛCDM cosmology. Utilizing the TNG galaxy-formation model, the DREAMS simulations incorporate both baryonic physics and cosmological uncertainties for a large sample of galaxies with diverse environments and formation histories. We investigate the relative impact of the physical uncertainty from the galaxy-formation model on predicted satellite properties using four metrics: the satellite stellar mass function, radial distribution, inner slope of the dark matter density profile, and stellar half-light radius. We compare these predictions to observations from the SAGA survey and the DREAMS N -body simulations and find that uncertainties from baryonic physics modeling are subdominant to the scatter arising from halo-to-halo variance. Where baryonic modeling does affect satellites, the supernova wind energy has the largest effect on the satellite properties that we investigate. Specifically, increased supernova wind energy suppresses the stellar mass of satellites and results in more extended stellar half-light radii. The adopted wind speed has only a minor impact, and other astrophysical and cosmological parameters show no measurable effect. Our findings highlight the robustness of satellite properties against uncertainties in baryonic physics modeling.

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.

The Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) are the Milky Way’s nearest interacting galaxy pair, offering a unique laboratory for studying tidal effects on galactic disks. Despite extensive survey efforts, the 3D geometry of the Magellanic Clouds, particularly the putative warp of the LMC, remains poorly constrained due to incompleteness in their crowded centers and the low stellar density of their peripheries, which demand wide-field coverage. Using red clump (RC) stars as standard candles, corrected for age- and metallicity-dependent population effects with empirically calibrated color–magnitude relations and spatially resolved star formation histories, we construct the most detailed distance map of the Magellanic system to date. Based on ∼2.3 million RC stars from Gaia Data Release 3 combined with modern reddening maps, we measure median heliocentric distances of 50.62 ± 2.32 kpc for the LMC (to ∼23°) and 60.75 ± 2.85 kpc for the SMC (to ∼12°). The maps reveal substructures including the LMC Northern Arm, southern hooks, the Magellanic Bridge, and SMC peripheral overdensities, with refreshed distance estimates. Fitting the LMC disk within 7° yields a global inclination of i = 25 . ° 32 ± 0 . ° 10 and a line-of-nodes position angle of θ = 142 . ° 34 ± 0 . ° 21 . Most strikingly, we find the LMC periphery is warped azimuthally into a U-shaped structure reaching a vertical amplitude of ∼7 kpc at a radius of ∼15 kpc. In future work, we will perform detailed comparisons with live N -body simulations to assess possible formation scenarios for the LMC warp.

Dark matter direct detection experiments require information about the local dark matter speed distribution to produce constraints on dark matter candidates, or infer their properties in the event of a discovery. In this paper, we analyze how the uncertainty in the dark matter speed distribution near the Sun is affected by baryonic feedback, halo-to-halo variance, and halo mass. To do so, we harness the statistical power of the new DREAMS cold dark matter simulation suite, which is comprised of 1024 zoom-in Milky Way–mass halos with varied initial conditions as well as cosmological and astrophysical parameters. Applying a normalizing flows emulator to these simulations, we find that the uncertainty in the local dark matter speed distribution is dominated by halo-to-halo variance and, to a lesser extent, uncertainty in host halo mass. Uncertainties in supernova and black hole feedback (from the IllustrisTNG model in this case) are negligible in comparison. Using the DREAMS suite, we present a state-of-the-art prediction for the dark matter speed distribution in the Milky Way. Although the standard halo model is contained within the uncertainty of this prediction, individual galaxies may have distributions that differ from it. Lastly, we apply our DREAMS results to the XENON1T experiment and demonstrate that the astrophysical uncertainties are comparable to the experimental ones, solidifying previous results in the literature obtained with a smaller sample of simulated Milky Way–mass halos.

We present detailed chemical abundances of the brightest star in each of the ultrafaint dwarf galaxies Eridanus IV and Centaurus I using high-resolution Magellan/MIKE spectroscopy. The brightest star in Centaurus I, CenI-5136, is a very metal-poor star with metallicity [Fe/H] = −2.52 ± 0.17 and chemical abundances typical of a star in an ultrafaint dwarf galaxy. We confirm that the star in Eridanus IV, EriIV-9808, is extremely metal poor ([Fe/H] = −3.25 ± 0.19) and find that it is carbon enhanced ([C/Fe] = 1.07 ± 0.34), as is common for many stars at this metallicity. Both stars are also neutron-capture deficient, which is typical of stars in ultrafaint dwarf galaxies, but less common in other environments. We consider possible enrichment scenarios for EriIV-9808 and tentatively conclude that it is unlikely to be the descendant of a single Population III progenitor, despite its carbon enhancement and low metallicity.