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We present a new approach to measuring the thickness of a partially face-on stellar disk, using dust geometry. In a moderately-inclined disk galaxy, the fraction of reddened stars is expected to be 50% everywhere, assuming that dust lies in a thin midplane. In a thickened disk, however, a wide range of radii project onto the line of sight. Assuming stellar density declines with radius, this geometrical projection leads to differences in the numbers of stars on the near and far sides of the thin dust layer. The fraction of reddened stars will thus differ from the 50% prediction, with a deviation that becomes larger for puffier disks. We map the fraction of reddened red giant branch (RGB) stars across M31, which shows prominent dust lanes on only one side of the major axis. The fraction of reddened stars varies systematically from 20% to 80%, which requires that these stars have an exponential scale height h z that is 0.14 ± 0.015 times the exponential scale length (h r ≈ 5.5 kpc). M31's RGB stars must therefore have h z = 770 ± 80 pc, which is far thicker than the Milky Way’s thin disk, but comparable to its thick disk. The lack of a significant thin disk in M31 is unexpected, but consistent with its interaction history and high disk velocity dispersion. We suggest that asymmetric reddening be used as a generic criterion for identifying “thick disk”-dominated systems, and discuss prospects for future 3D tomographic mapping of the gas and stars in M31.

The typical galaxy in the local Universe is expected to be in a self-regulated quasi-equilibrium, displaying a settled morphology that falls within the Hubble sequence. The Arp and Arp–Madore catalogs are filled with striking examples of galaxies that defy these expectations, making them useful targets for studying the astrophysics that controls dramatic, but short-lived, episodes of disequilibrium that mark galaxies’ evolution. In this paper, we greatly expand the available Hubble Space Telescope imaging of galaxies drawn from the Arp and Arp–Madore catalogs. We present new optical F606W images and point-source photometry for 216 systems, whose sizes are well matched to the Advanced Camera for Surveys’ wide field of view. Essentially, none of the samples had been previously observed with Hubble. The resulting images display rich morphologies, revealing a variety of massive stars, H ii regions, stellar clusters, dust lanes, tidal tails, backlit galaxies, and occasional chance superpositions. We provide a pedagogical guide for interpreting highly resolved optical galaxy images, which has general application beyond this atlas. The atlas images also provide a superb starting point for more detailed studies with high-resolution imaging in other wavelengths, and spectroscopy to track kinematics and the interstellar medium (ISM). Areas of obvious scientific relevance include feedback and star formation in merging and interacting galaxies, resolved stellar populations at the extremes of stellar density, the properties of young massive stars and stellar clusters, the physics of the cold ISM and dust, and stellar and gas dynamics.

We present a low-dispersion period–luminosity relation (PL) based on 154 Cepheids in Messier 33 (M33) with Hubble Space Telescope (HST) photometry from the PHATTER survey. Using high-quality ground-based light curves, we recover Cepheid phases and amplitudes for multi-epoch HST data and we perform template fitting to derive intensity-averaged mean magnitudes. HST observations in the SH0ES near-infrared Wesenheit system significantly reduce the effect of crowding relative to ground-based data, as seen in the final PL scatter of σ = 0.11 mag. We adopt the absolute calibration of the PL based on HST observations in the Large Magellanic Cloud and a distance derived using late-type detached eclipsing binaries to obtain a distance modulus for M33 of μ = 24.622 ± 0.030 mag (d = 840 ± 11 kpc), a best-to-date precision of 1.3%. We find very good agreement with past Cepheid-based measurements. Several tip of the red giant branch estimates bracket our result while disagreeing with each other. Finally, we show that the flux contribution from star clusters hosting Cepheids in M33 does not impact the distance measurement and we find only ∼3.7% of the sample is located in (or nearby) young clusters. M33 offers one of the best sites for the cross-calibration of many primary distance indicators. Thus, a precise independent geometric determination of its distance would provide a valuable new anchor to measure the Hubble constant.

Stellar streams from globular clusters (GCs) offer constraints on the nature of dark matter and have been used to explore the dark matter halo structure and substructure of our Galaxy. Detection of GC streams in other galaxies would broaden this endeavor to a cosmological context, yet no such streams have been detected to date. To enable such exploration, we develop the Hough Stream Spotter (HSS), and apply it to the Pan-Andromeda Archaeological Survey (PAndAS) photometric data of resolved stars in M31's stellar halo. We first demonstrate that our code can re-discover known dwarf streams in M31. We then use the HSS to blindly identify 27 linear GC stream-like structures in the PAndAS data. For each HSS GC stream candidate, we investigate the morphologies of the streams and the colors and magnitudes of all stars in the candidate streams. We find that the five most significant detections show a stronger signal along the red giant branch in color–magnitude diagrams than spurious non-stream detections. Lastly, we demonstrate that the HSS will easily detect globular cluster streams in future Nancy Grace Roman Space Telescope data of nearby galaxies. This has the potential to open up a new discovery space for GC stream studies, GC stream gap searches, and for GC stream-based constraints on the nature of dark matter.

Current population synthesis modeling suggests that 30%–50% of Type II supernovae originate from binary progenitors; however, the identification of a binary progenitor is challenging. One indicator of a binary progenitor is that the surrounding stellar population is too old to contain a massive single star. Measurements of the progenitor mass of SN 2017eaw are starkly divided between observations made temporally close to core collapse, which show a progenitor mass of 13–15 M ⊙ (final helium-core mass MHe,core=4.4–6.0M⊙ —which is a more informative property than initial mass) and those from the stellar population surrounding the SN, which find M ≤ 10.8 M ⊙ ( MHe,core≤3.4M⊙ ). In this paper, we reanalyze the surrounding stellar population with improved astrometry and photometry, finding a median age of 16.8−1.0+3.2 Myr for all stars younger than 50 Myr ( MHe,core=4.7M⊙ ) and 85.9−6.5+3.2 Myr for stars younger than 150 Myr. 16.8 Myr is now consistent with the helium-core mass range derived from the temporally near-explosion observations for single stars. Applying the combined constraints to population synthesis models, we determine that the probability of the progenitor of SN 2017eaw being an initially single star is 65% compared to 35% for prior binary interaction. 85.9 Myr is inconsistent with any formation scenarios. We demonstrate that combining progenitor age constraints with helium-core mass estimates from red supergiant SED modeling, late-time spectra, and indirectly from light-curve modeling can help to differentiate single and binary progenitor scenarios and provide a framework for the application of this technique to future observations.

The population-wide properties and demographics of extragalactic X-ray binaries (XRBs) correlate with the star formation rates (SFRs), stellar masses (M ⋆), and environmental factors (such as metallicity, Z) of their host galaxy. Although there is evidence that XRB scaling relations (L X/SFR for high-mass XRBs (HMXBs) and L X/M ⋆ for low-mass XRBs) may depend on metallicity and stellar age across large samples of XRB-hosting galaxies, disentangling the effects of metallicity and stellar age from stochastic effects, particularly on subgalactic scales, remains a challenge. We use archival X-ray through IR observations of the nearby galaxy NGC 300 to self-consistently model the broadband spectral energy distribution and examine radial trends in its XRB population. We measure a current (<100 Myr) SFR of 0.18 ± 0.08 M⊙ yr−1 and stellar mass M ⋆ = (2.15−0.14+0.26)×109 M⊙. Although we measure a metallicity gradient and radially resolved star formation histories that are consistent with the literature, there is a clear excess in the number of X-ray sources below ∼1037 erg s−1 that are likely a mix of variable XRBs and additional background active galactic nuclei. When we compare the subgalactic L X/SFR ratios as a function of Z to the galaxy-integrated L X-SFR-Z relationships from the literature, we find that only the regions hosting the youngest (≲30 Myr) HMXBs agree with predictions, hinting at time evolution of the L X–SFR–Z relationship.

The association of the Vela Pulsar with the Vela Supernova Remnant has long supported the hypothesis that core-collapse supernovae yield neutron stars, but its surrounding stellar population now offers new insights into progenitor evolution. By age-dating stars within 150 pc of the Vela Pulsar, we infer properties of its progenitor. These stars belong to the Vela-Puppis complex, revealing the region’s star formation history. While stellar population models with standard assumptions suggest a likely progenitor age and mass, these predictions are internally inconsistent with the observed population, indicating that something is missing in the standard modeling approach. With those assumptions, there is very weak support for a ≲10 Myr old population, moderate support for a 40 Myr old population, and strong support for an intermediate-age population around 65–100 Myr old. The ≲10 Myr signal hinges on two peculiar O stars, which are unlike any others in the Vela-Puppis complex and imply nearly three times more main-sequence (MS) stars than are observed. The 40 Myr old population is supported by six red supergiants and several Be stars, but this population is again marginally inconsistent with the observed distribution of MS stars. The red giant and MS distributions are consistent with a 65–100 Myr old population. We discuss several possible resolutions, emphasizing how binary evolution and/or very rapid rotation could resolve these discrepancies. Gaia parallaxes and Stellar Ages enable these results; Stellar Ages is a novel stellar population modeling algorithm that combines individual and population-level age inferences.

We present a novel statistical algorithm, Stellar Ages, which currently infers the age, metallicity, and extinction posterior distributions of stellar populations from their magnitudes. While this paper focuses on these parameters, the framework is readily adaptable to include additional properties, such as rotation, in future work. Historical age-dating techniques either model individual stars or populations of stars, often sacrificing population context or precision for individual estimates. Stellar Ages does both, combining the strengths of these approaches to provide precise individual ages for stars while leveraging population-level constraints. We verify the algorithm’s capabilities by determining the age of synthetic stellar populations and actual stellar populations surrounding a nearby supernova, SN 2004dj. In addition to inferring an age, we infer a progenitor mass consistent with direct observations of the precursor star. The median age inferred from the brightest nearby stars is log10 (Age yr−1) = 7.19−0.13+0.10 , and its corresponding progenitor mass is 13.95−1.96+3.33 M⊙.

We construct a catalog of star clusters from Hubble Space Telescope images of the inner disk of the Triangulum Galaxy (M33) using image classifications collected by the Local Group Cluster Search, a citizen science project hosted on the Zooniverse platform. We identify 1214 star clusters within the Hubble Space Telescope imaging footprint of the Panchromatic Hubble Andromeda Treasury: Triangulum Extended Region (PHATTER) survey. Comparing this catalog to existing compilations in the literature, 68% of the clusters are newly identified. The final catalog includes multiband aperture photometry and fits for cluster properties via integrated light spectral energy distribution fitting. The cluster catalog’s 50% completeness limit is ∼1500 M ☉ at an age of 100 Myr, as derived from comprehensive synthetic cluster tests.

deepCR is a deep-learning-based cosmic-ray (CR) rejection framework originally presented by Zhang & Bloom. The original approach requires a dedicated training set that consists of multiple frames of the same fields, enabling automatic CR labeling through comparison with their median coadds. Here, we present a novel training approach that circumvents the need for a dedicated training set, but instead utilizes dark frames and the science images requiring CR removal themselves. During training, CRs present in dark frames are added to the science images, which the network is then trained to identify. In turn, the trained deepCR model can then be applied to identify CRs originally present in the science images. Using this approach, we present a new deepCR model trained on a diverse set of Hubble Space Telescope images taken from resolved galaxies in the Local Group, which is universally applicable across all WFC3/UVIS filters. We introduce a robust approach to determining the threshold for generating binary cosmic-ray masks from predictions from deepCR probability maps. When applied to the Panchromatic Hubble Andromeda Southern Treasury survey, our new deepCR model added ∼7% of good-quality stars that exhibit distinct features in their color–magnitude diagrams.