arXiv subjects

Yumi Choi

Publications and source records attributed to Yumi Choi.

At least 19 recordsLinked to original sources

arXiv2026

Scylla: Observational Evidence for an Order of Magnitude in Dust Mass Opacity Evolution with ISM Density in the Large Magellanic Cloud

The emissivity of dust is known to vary greatly with radiative environment, density, grain chemistry, and geometry. Discrepancies between dust mass surface densities derived from far-infrared (FIR) emission and visible extinction persist across and within galaxies in the local Universe. Here, we use new extinction and emission measurements towards the LMC to show that this discrepancy is driven by the dust mass opacity evolving with the intrinsic density of the ISM, and that the ratio between FIR and optical dust mass opacity varies with gas surface density. These new findings imply that the dust mass opacity in the FIR could increase by nearly an order of magnitude (e.g., $\kappa_{160} = 0.3 - 6\ m^2\ kg^{-2}$) across over an order of magnitude of total hydrogen surface density $(\Sigma_H = 4 - 100 M_{\odot}\ pc^{-2})$, corroborating previous theoretical models for dust mass opacity evolution in the FIR, and providing new implications for emission-based dust mass estimates.

astro-ph.GA↗

arXiv2026

A Deep Look at the Ultra-Faint Milky Way Satellite Virgo III with Rubin Observatory Data Preview 2

We analyze the ultra-faint Milky Way satellite Virgo III using data from the Vera C. Rubin Observatory Data Preview 2 (DP2). Virgo III was observed in the Rubin "Cosmic Treasure Chest" (M49) First Look field, which contains 924 visits in the u,g,r,i bands comprising ~10.5hrs of exposure time with LSSTCam. These data are considerably deeper than the majority of DP2, with a $5\sigma$ limiting magnitude that approaches the expected 10-year depth of LSST (~25.2-26.5mag, depending on band). We report the morphological and stellar population parameters of Virgo III measured with the maximum-likelihood-based package ugali. The depth of the Rubin imaging yields more than a factor of four increase in the number of candidate member stars ($N_* = 114^{+11}_{-11}$) relative to the Virgo III discovery results ($N_* = 25^{+5}_{-4}$), enabling significantly more precise morphological constraints. Our best-fit parameters broadly agree with previous measurements, further confirming that Virgo III has properties that are consistent with an ultra-faint dwarf galaxy ($M_V = -2.72^{+0.49}_{-0.70}$; $r_{1/2} = 53^{+10}_{-8}$) located at a heliocentric distance of $D_\odot = 151^{+8}_{-8}$. We also demonstrate that the depth and photometric quality of the DP2 data are sufficient to separate metal-poor and metal-rich stars in color-color space. We further present period estimates for the three known RR Lyrae member stars derived from the DP2 forced photometry; we use theoretical Period-Luminosity-Metallicity (PLZ) and Period-Wesenheit-Metallicity (PWZ) relations to obtain independent distance estimates. We find that our period and distance estimates are broadly consistent with previous measurements for these RR Lyrae. These results demonstrate the power of LSST data for the discovery and characterization of ultra-faint dwarf galaxies and motivate future searches for new satellites across the southern sky.

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arXiv2026

Scylla VI: Parsec-Scale Dust Extinction Maps in the SMC and LMC

We present a novel methodology for mapping dust extinction in nearby galaxies at parsec-scale resolution. We apply it to HST 68 fields within the Small and Large Magellanic Clouds (23 fields in the SMC and 45 fields in the LMC) using multi-band HST photometry from the Scylla and METAL surveys. Our technique leverages \textit{kriging}, a geostatistical interpolation method built on the principles of Gaussian Process regression, combined with Gaussian mixture modeling to statistically isolate background stellar sources and account for line-of-sight depth effects. 3D dust simulations demonstrate the method's capability to recover column densities to an accuracy of $A_V \approx 0.1$ mag in fields with at least 1000 sources. The resulting $4^{\prime\prime}$ resolution ($\sim1$-pc) dust maps reveal detailed structure and strong spatial correlation with ancillary ISM tracers, especially in star-forming regions like 30 Doradus. Global extinction of total column densities follows log-normal profiles in both galaxies, with the SMC exhibiting slightly higher mean extinction ($e^{\mu}=0.47$ mag) than the broader LMC ($e^{\mu}=0.43$ mag), likely due to significant line-of-sight depths. We find systematic offsets between dust mass surface densities ($\Sigma_{D}$) derived from extinction versus FIR emission in both galaxies, with $\Sigma_{D, FIR}/\Sigma_{D, A_V}$ ratios ranging from $0.6-1.8$. This work provides the highest-resolution dust extinction maps in SMC and LMC to date, which offer a vital independent benchmark for constraining dust emissivity, $\text{CO}$-dark gas fractions, and the multi-scale structure of the ISM in low-metallicity environments.

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arXiv2026

Scylla at APOGEE: The Impact of Starbursts on the Chemical Evolution of the Magellanic Clouds

Owing to their proximity to the Milky Way, the Large and Small Magellanic Clouds (L/SMC) uniquely probe the evolution of low-mass galaxies undergoing mutual interactions. In this work, we investigate the connection between the star formation histories (SFHs) of the L/SMC measured from HST imaging in the Scylla survey and APOGEE chemical abundances. We model the chemical evolution of the L/SMC in the [Mg/Fe]-[Fe/H] plane within a robust statistical framework to predict chemical abundance signatures resulting directly from starbursts in Scylla SFHs. Both the L/SMC rapidly enrich to high metallicity ([Fe/H] $\gtrsim$ $-1$) within 3 Gyr, followed by slower chemical evolution regulated by sequential starbursts, where the SMC may require higher Fe yields from Type Ia supernovae than the LMC. We also model the [Mg/Fe]-[Fe/H] plane to infer starburst properties across distinct spatial regions in the L/SMC. We identify dominant starbursts in the L/SMC with broadly similar timing, though the SMC may host an earlier burst, and larger burst strength in the LMC. The global starburst properties are nearly uniform across the LMC disk, whereas the dominant SMC population experiences a stronger and later-onset burst in its eastern wing compared to the main body. We also find evidence for a chemically distinct secondary population in the SMC that preferentially traces the foreground and may originate from the LMC. We discuss the implications of these results for the evolutionary history of the L/SMC and for starbursts in interacting low-mass galaxy pairs.

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arXiv2026

The Vera C. Rubin Observatory Data Preview 1

We present Rubin Data Preview 1 DP1, the first data from the NSF DOE 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 Pach\'on Chile in late 2024. DP1 covers $\sim$15 deg$^2$ distributed across seven roughly equal-sized non-contiguous 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 arcseconds, with the sharpest images reaching about 0.58 arcseconds. The 5$\sigma$ 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, $y$ = 23.1. Other fields are no more than 2.2 magnitudes 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 Rubin 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.

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arXiv2026

Unveiling Massive Main-Sequence Stars in Sextans A through Panchromatic Photometry

We present a study of the metal-poor (~6% Z_sun) massive (>8 M_sun) main-sequence star population in the star-forming dwarf galaxy Sextans A. By modeling near-UV to near-IR photometry of individual stars using the Bayesian Extinction and Stellar Tool (BEAST) we infer stellar parameters such as effective temperature, luminosity, and initial mass. We identify 867 massive main-sequence star candidates (present-day mass >8 M_sun and surface gravity >3.7 dex [cgs]) with a plausible spectral energy distribution (SED) fit, 500 of which show a probable SED fit. Comparisons to spectral types of existing observed spectra are consistent with the BEAST-derived stellar parameters, with most discrepancies explained. We identify 292 OBe star candidates through IR photometric signatures and find lower-limit OBe fractions of 15% for M > 8 M_sun, 23% for M > 15 M_sun, and 17% for M > 20 M_sun. We find 57 OB associations and that 24-28% of massive stars are isolated (distance to nearest massive star >28 pc). We discuss six likely runaway candidates (suggested velocities of ~ 50-340 km/s) not clearly associated with any major star-forming complexes. Lastly, we predict Lyman continuum (LyC) escape fractions of f_esc=0.27-0.76 across the star-forming regions and a global value of 0.35-0.71 by assuming low overall extinction and a range of porous geometries, indicating efficient leakage of ionizing photons. Future spectroscopic follow-up and resolved ISM studies will refine these constraints and solidify Sextans A as a benchmark for studying massive-star evolution and feedback at extremely low metallicity.

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arXiv2026

The DECam MAGIC Survey: Uncovering the Tidal Tails of the Crater II Dwarf Galaxy

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 (MAGIC) 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 $7^\circ$ ($\sim 95$ kpc) from the center of CraII, suggesting it has lost $\gtrsim 25$% 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 $\sim 36$ mag arcsec$^{-2}$. We also make the first detection of a metallicity gradient ($-0.34\pm0.17~{\rm dex}~{\rm deg}^{-1}$) in the center of the galaxy and infer a stream width of $w\sim 0.8^\circ$, 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.

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arXiv2025

Warped & Hooked: Mapping the Magellanic Clouds in 3D using Red Clump stars

The Large and Small Magellanic Clouds (LMC and 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 three-dimensional geometry of the 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 $\sim$2.3 million RC stars from \textit{Gaia} DR3 combined with modern reddening maps, we measure median heliocentric distances of $50.62 \pm 2.32$~kpc for the LMC (to $\sim$23$^{\circ}$) and $60.75 \pm 2.85$~kpc for the SMC (to $\sim$12$^{\circ}$). The maps reveal substructures including the LMC Northern Arm, southern hooks, the Magellanic Bridge, and SMC peripheral over-densities, with refreshed distance estimates. Fitting the LMC disk within $7^{\circ}$ yields a global inclination of $i = 25.32^{\circ} \pm 0.10^{\circ}$ and line-of-nodes position angle of $\theta = 142.34^{\circ} \pm 0.21^{\circ}$. Most strikingly, we find the LMC periphery is warped azimuthally into a U-shaped structure reaching vertical amplitudes of $\sim$7~kpc at radii of $\sim$15~kpc. We interpret this warp as a tidal response to the most recent close passage of the SMC $\sim$300 Myr ago, providing new constraints on the orbital history and dynamical evolution of the Magellanic Clouds.

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arXiv2025

Scylla V: Constraints on the spatial and temporal distribution of bursts and the interaction history of the Magellanic Clouds from their resolved stellar populations

We measure the star formation histories (SFHs) from the Scylla survey in approximately 98,000 pc^2 and 75,000 pc^2 of the SMC and LMC, respectively, using deep Hubble Space Telescope imaging (80% complete to more than 1 mag below the ancient main-sequence turnoff, 25.1 and 26.0 mag in F475W and F814W) from 74 pointings. We group the fields into eight sub-regions in the SMC and seven in the LMC. We use the birth rate parameter to identify bursts of star formation and measure their properties in each sub-region. Our methodology provides a standardized framework for burst identification and reveals both broad and fine burst characteristics. We identify global and local bursts, defined as those occurring in at least half or less than half of a galaxy's sub-regions, respectively. In the SMC we find two global (about 5 and 1.5 Gyr ago) and one local burst (about 3 Gyr ago). In the LMC we find one global burst (about 3 Gyr ago). Comparing these findings with dynamical models of the LMC and SMC orbital histories, we find that when models predict a shared dynamical trigger for bursts across both galaxies, the burst begins earlier in the SMC with a greater enhancement in star formation rate than in the LMC. Finally, using age-metallicity relations (AMRs) and cumulative SFHs, we report that the Wing/Bridge region in the SMC resembles the southwestern LMC both chemically and in stellar mass assembly over the last about 7 Gyr, possibly due to stellar material stripped from the LMC during their last interaction.

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arXiv2025

Probing the variations of interstellar dust abundance and properties within and between galaxies with HWO UV spectroscopy in the Local Volume

The cycle of metals between the gas and the dust phases in the neutral interstellar medium (ISM) is an integral part of the baryon cycle in galaxies. The resulting variations in the abundance and properties of interstellar dust have important implications for how accurately we can trace the chemical enrichment of the universe over cosmic time. Multi-object UV spectroscopy with HWO can provide the large samples of abundance and dust depletion measurements needed to understand how the abundance and properties of interstellar dust vary within and between galaxies, thereby observationally addressing important questions about chemical enrichment and galaxy evolution. Medium-resolution (R~50,000) spectroscopy in the full UV range (950-3150 A) toward massive stars in Local Volume galaxies (D < 10 Mpc) will enable gas- and dust-phase abundance measurements of key elements, such as Fe, Si, Mg, S, Zn. These measurements will provide an estimate of how the dust abundance varies with environment, in particular metallicity and gas density. However, measuring the carbon and oxygen contents of dust requires very high resolution (R > 100,000) and high signal-to-noise (S/N > 100) owing to the non-saturated UV transitions for those elements being extremely weak. Since carbon and oxygen in the neutral ISM contribute the largest metal mass reservoir for dust, it is critical that the HWO design include a grating similar to the HST STIS H gratings providing very high resolution, as well as FUV and NUV detectors capable of reaching very high S/N.

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arXiv2025

DELVE-DEEP Survey: The Faint Satellite System of NGC 55

We report the first comprehensive census of the satellite dwarf galaxies around NGC 55 ($2.1$ Mpc) as a part of the DECam Local Volume Exploration DEEP (DELVE-DEEP) survey. NGC 55 is one of four isolated, Magellanic analogs in the Local Volume around which DELVE-DEEP aims to identify faint dwarfs and other substructures. We employ two complementary detection methods: one targets fully resolved dwarf galaxies by identifying them as stellar over-densities, while the other focuses on semi-resolved dwarf galaxies, detecting them through shredded unresolved light components. As shown through extensive tests with injected galaxies, our search is sensitive to candidates down to $M_V \lesssim -6.6$ and surface brightness $μ\lesssim 28.5$ mag arcsec$^{-2}$, and $\sim 80\%$ complete down to $M_V \lesssim -7.8$. We do not report any new confirmed satellites beyond two previously known systems, ESO 294-010 and NGC 55-dw1. We construct the satellite luminosity function of NGC 55 and find it to be consistent with the predictions from cosmological simulations. As one of the first complete luminosity functions for a Magellanic analog, our results provide a glimpse of the constraints on low-mass-host satellite populations that will be further explored by upcoming surveys, such as the Vera C. Rubin Observatory's Legacy Survey of Space and Time.

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arXiv2025

The HST Legacy Archival Uniform Reduction of Local Group Imaging (LAURELIN). I. Photometry and Star Formation Histories for 36 Ultra-faint Dwarf Galaxies

We present uniformly measured resolved stellar photometry and star formation histories (SFHs) for 36 nearby ($\lesssim$ 400 kpc) ultra-faint dwarf galaxies (UFDs; $-7.1 \le M_V \le +0.0$) from new and archival HST imaging. We measure homogeneous distances to all systems via isochrone fitting and find good agreement ($\le$ 2%) for the 18 UFDs that have literature RR Lyrae distances. From the ensemble of SFHs, we find: (i) an average quenching time (here defined as the lookback time by which 80% of the stellar mass formed, $τ_{80}$) of 12.48 $\pm$ 0.18 Gyr ago ($z = 4.6_{-0.5}^{+0.6}$), which is compatible with reionization-based quenching scenarios; and (ii) modest evidence of a delay ($\lesssim$ 800 Myr) in quenching times of UFDs thought to be satellites of the LMC or on their first infall, relative to long-term Galactic satellites, which is consistent with previous findings. We show that robust SFH measurement via the ancient main sequence turnoff (MSTO) requires a minimum effective luminosity (i.e., luminosity within the observed field of view) of $M_V \leq -2.5$, which corresponds to $\sim$100 stars around the MSTO. We also find that increasing the S/N above $\sim$100 at the MSTO does not improve SFH precision, which remains dominated by stochastic effects associated with the number of available stars. A main challenge driving the precision of UFD SFHs is limitations in the accuracy of foreground dust maps. We make all photometry catalogs public as the first data release of a larger HST archival program targeting all dwarf galaxies within $\sim$1.3 Mpc.

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arXiv2025

47 Tuc in Rubin Data Preview 1: Exploring Early LSST Data and Science Potential

We present analyses of the early data from Rubin Observatory's Data Preview 1 (DP1) for the globular cluster 47 Tuc field. The DP1 dataset for 47 Tuc includes four nights of observations from the Rubin Commissioning Camera (LSSTComCam), covering multiple bands ($ugriy$). We address challenges of crowding in the inner region of the cluster and toward the SMC in DP1, and demonstrate improved star-galaxy separation by fitting fifth-degree polynomials to the stellar loci in color-color diagrams and applying multi-dimensional sigma clipping. We compile a catalog of 3,576 probable 47 Tuc member stars selected via a combination of isochrone, Gaia proper-motion, and color-color space matched filtering. We explore the sources of photometric scatter in the 47 Tuc color-color sequence, evaluating contributions from various potential sources, including differential extinction within the cluster. Finally, of the 72 well-characterized variables in the field, we recover five known variable stars, including three RR Lyrae and two eclipsing binaries, in the coadd-based object catalog, and identify 62 in the difference image-based object catalog. Although the DP1 lightcurves have sparse temporal sampling, they appear to follow the patterns of densely-sampled literature lightcurves well. Despite some data limitations for crowded-field stellar analysis, DP1 demonstrates the promising scientific potential for future LSST data releases.

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arXiv2025

The DECam MAGIC Survey: A Wide-field Photometric Metallicity Study of the Sculptor Dwarf Spheroidal Galaxy

The metallicity distribution function and internal chemical variations of a galaxy are fundamental to understand its formation and assembly history. In this work, we analyze photometric metallicities for 3883 stars over seven half-light radii ($\rm r_h$) in the Sculptor dwarf spheroidal (Scl dSph) galaxy, using new narrow-band imaging data from the Mapping the Ancient Galaxy in CaHK (MAGIC) survey conducted with the Dark Energy Camera (DECam) at the 4-m Blanco Telescope. This work demonstrates the scientific potential of MAGIC using the Scl dSph galaxy, one of the most well-studied satellites of the Milky Way. Our sample ranges from $\rm [Fe/H] \approx - 4.0$ to $\rm [Fe/H] \approx - 0.6$, includes six new extremely metal-poor candidates ($\rm [Fe/H] \leq -3.0$), and is almost three times larger than the largest spectroscopic metallicity dataset in the Scl dSph. Our spatially unbiased sample of metallicities provides a more accurate representation of the metallicity distribution function, revealing a more metal-rich peak than observed in the most recent spectroscopic sample. It also reveals a break in the metallicity gradient, with a strong change in the slope: from $-3.26 \pm 0.18 \rm \ dex/deg$ for stars inside $\sim 1\ \rm r_h$ to $-0.55 \pm 0.26 \rm \ dex/deg$ for the outer part of the Scl dSph. Our study demonstrates that combining photometric metallicity analysis with the wide field of view of DECam offers an efficient and unbiased approach for studying the stellar populations of dwarf galaxies in the Local Group.

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arXiv2025

Not-so-heavy metal(s): Chemical Abundances in the Ultra-faint Dwarf Galaxies Eridanus IV and Centaurus I

We present detailed chemical abundances of the brightest star in each of the ultra-faint 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] = $-2.52\pm0.17$ and chemical abundances typical of a star in an ultra-faint dwarf galaxy. We confirm that the star in Eridanus IV, EriIV-9808, is extremely metal-poor ([Fe/H] = $-3.25\pm0.19$) and find that it is carbon-enhanced, with [C/Fe] = $1.07\pm0.34$, as is common for many stars at this metallicity. Both stars are also neutron-capture deficient, which is typical of stars in ultra-faint 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 Pop III progenitor, despite its carbon-enhancement and low metallicity.

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arXiv2025

A second-generation star in a relic dwarf galaxy

Stars that contain only trace amounts of elements heavier than helium, referred to as having low "metallicity", preserve the chemical fingerprints of the first generation of stars and supernovae. In the Milky Way, the lowest metallicity stars show an extreme over-abundance of carbon relative to other elements, which has been hypothesized to be a unique result of the first low-energy supernovae. However, the origin of this signature has remained a mystery, since no such stars have been discovered in the ancient dwarf galaxies where they are thought to have formed. Here, we present observations of a star in the >10 billion year old ultra-faint dwarf galaxy Pictor II, that shows the lowest iron and calcium abundances outside the Milky Way (<1/43,000th solar and ~1/160,000th solar), with a factor of >3000x relative carbon enhancement. As the first unambiguous second-generation star in a relic dwarf galaxy, this object demonstrates that carbon-enhanced second-generation stars can originate in primordial small-scale systems. This star supports the hypothesis that carbon-enhancement is produced by low-energy-supernovae, since the yields of energetic supernovae are harder to retain in small-scale environments. This key local signature of chemical enrichment by the first stars traces a regime inaccessible to current high-redshift observations, which cannot detect the early enrichment of the smallest galaxies.

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arXiv2025

Variability-finding in Rubin Data Preview 1 with LSDB

The Vera C. Rubin Observatory recently released Data Preview 1 (DP1) in advance of the upcoming Legacy Survey of Space and Time (LSST), which will enable boundless discoveries in time-domain astronomy over the next ten years. DP1 provides an ideal sandbox for validating innovative data analysis approaches for the LSST mission, whose scale challenges established software infrastructure paradigms. This note presents a pair of such pipelines for variability-finding using powerful software infrastructure suited to LSST data, namely the HATS (Hierarchical Adaptive Tiling Scheme) format and the LSDB framework, developed by the LSST Interdisciplinary Network for Collaboration and Computing (LINCC) Frameworks team. This article presents a pair of variability-finding pipelines built on LSDB, the HATS catalog of DP1 data, and preliminary results of detected variable objects, two of which are novel discoveries.

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arXiv2025

NSF-DOE Vera C. Rubin Observatory Observations of Interstellar Comet 3I/ATLAS (C/2025 N1)

We report on the observation and measurement of astrometry, photometry, morphology, and activityof the interstellar object 3I/ATLAS, also designated C/2025 N1 (ATLAS) with the NSF-DOE Vera C. Rubin Observatory. Comet 3I/ATLAS, the third known interstellar object, was discovered on UT 2025 July 1. Rubin Observatory had coincidentally collected images of the object's region of the sky during routine commissioning. Facilitated by Rubin's high resolution and large aperture, we successfully recovered object detections from Rubin observations spanning UT 2025 June 21 (10 days before discovery, when 3I/ATLAS was 4.5 au from the Sun) through the date of discovery, and we acquired additional images through UT 2025 July 20 as part of commissioning. We measure on-sky locations of 3I/ATLAS in Rubin ugrizy bands, with a typical precision of about 70 mas, and briefly describe the reason this is coarser than our measured static source astrometric precision of about 3 mas in Rubin images. We measure grizy magnitudes of 3I/ATLAS photometry at about 0.01 mag precision, detecting no short-term photometric variability above 0.01 mag. We derive an estimated near-nucleus dust-to-nucleus scattering cross-section ratio of eta >= 13 on UT 2025 July 2 based on Rubin photometry and an upper limit nucleus size computed from Hubble Space Telescope observations. We find Rubin colors of g - r = (0.657 +/- 0.013) mag, r - i = (0.235 +/- 0.018) mag, i - z = (0.147 +/- 0.042) mag, z - y = (0.047 +/- 0.052) mag. These data represent the earliest observations of this object by a large (>=8-meter class) telescope and illustrate the type of measurements (and discoveries) Rubin's Legacy Survey of Space and Time (LSST) will begin to provide after it begins in early 2026.

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