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1. The PHANGS-HST Survey: Physics at High Angular Resolution in Nearby Galaxies with the Hubble Space Telescope

   https://doi.org/10.3847/1538-4365/ac1fe5  

   Lee, Janice C.; Whitmore, Bradley C.; Thilker, David A.; Deger, Sinan; Larson, Kirsten L.; Ubeda, Leonardo; Anand, Gagandeep S.; Boquien, Médéric; Chandar, Rupali; Dale, Daniel A.; et al (January 2022, The Astrophysical Journal Supplement Series)

   Abstract The PHANGS program is building the first data set to enable the multiphase, multiscale study of star formation across the nearby spiral galaxy population. This effort is enabled by large survey programs with the Atacama Large Millimeter/submillimeter Array (ALMA), MUSE on the Very Large Telescope, and the Hubble Space Telescope (HST), with which we have obtained CO(2–1) imaging, optical spectroscopic mapping, and high-resolution UV–optical imaging, respectively. Here, we present PHANGS-HST, which has obtained NUV– U – B – V – I imaging of the disks of 38 spiral galaxies at distances of 4–23 Mpc, and parallel V - and I -band imaging of their halos, to provide a census of tens of thousands of compact star clusters and multiscale stellar associations. The combination of HST, ALMA, and VLT/MUSE observations will yield an unprecedented joint catalog of the observed and physical properties of ∼100,000 star clusters, associations, H ii regions, and molecular clouds. With these basic units of star formation, PHANGS will systematically chart the evolutionary cycling between gas and stars across a diversity of galactic environments found in nearby galaxies. We discuss the design of the PHANGS-HST survey and provide an overview of the HST data processing pipeline and first results. We highlight new methods for selecting star cluster candidates, morphological classification of candidates with convolutional neural networks, and identification of stellar associations over a range of physical scales with a watershed algorithm. We describe the cross-observatory imaging, catalogs, and software products to be released. The PHANGS high-level science products will seed a broad range of investigations, in particular, the study of embedded stellar populations and dust with the James Webb Space Telescope, for which a PHANGS Cycle 1 Treasury program to obtain eight-band 2–21 μ m imaging has been approved. 

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2. Tip of the Red Giant Branch Distances with JWST: An Absolute Calibration in NGC 4258 and First Applications to Type Ia Supernova Hosts

   https://doi.org/10.3847/1538-4357/ad2e0a  

   Anand, Gagandeep S; Riess, Adam G; Yuan, Wenlong; Beaton, Rachael; Casertano, Stefano; Li, Siyang; Makarov, Dmitry I; Makarova, Lidia N; Tully, R Brent; Anderson, Richard I; et al (April 2024, The Astrophysical Journal)

   Abstract The tip of the red giant branch (TRGB) allows for the measurement of precise and accurate distances to nearby galaxies based on the brightest ascent of low-mass red giant branch stars before they undergo the helium flash. With the advent of JWST, there is great promise to utilize the technique to measure galaxy distances out to at least 50 Mpc, significantly further than the Hubble Space Telescope's (HST's) reach of 20 Mpc. However, with any standard candle, it is first necessary to provide an absolute reference. Here, we use Cycle 1 data to provide an absolute calibration in the F090W filter. F090W is most similar to the F814W filter commonly used for TRGB measurements with HST, which had been adopted by the community due to its minimal dependence on the underlying metallicities and ages of stars. The imaging we use was taken in the outskirts of NGC 4258, which has a direct geometrical distance measurement from the Keplerian motion of its water megamaser. Utilizing several measurement techniques, we find $M_{\mathrm{TRGB}}^{\mathrm{F}090\mathrm{W}}$ = −4.362 ± 0.033 (stat) ± 0.045 (sys) mag (Vega) for the metal-poor TRGB. We also perform measurements of the TRGB in two Type Ia supernova hosts, NGC 1559 and NGC 5584. We find good agreement between our TRGB distances and previous determinations of distances to these galaxies from Cepheids (Δ = 0.01 ± 0.06 mag), with these differences being too small to explain the Hubble tension (∼0.17 mag). In addition, we showcase the serendipitous discovery of a faint dwarf galaxy near NGC 5584. 

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3. New Measurements of Distances to Galaxies in the NGC 1052 Field with the Hubble and James Webb Space Telescopes: Testing the Bullet-dwarf Origin of the Trail

   https://doi.org/10.3847/2041-8213/ae77ef  

   Tang, Yimeng; Anand, Gagandeep S; Romanowsky, Aaron J; van_Dokkum, Pieter G; Bundy, Kevin A (June 2026, The Astrophysical Journal Letters)

   Abstract NGC 1052-DF2 and DF4 are two ultradiffuse galaxies deficient in dark matter (DM) and reported as part of a remarkable linear trail of dwarf galaxies in the NGC 1052 field. Recently, NGC 1052-DF9 has been identified as the third galaxy missing DM along the trail. This structure may have been formed in a high-velocity head-on collision between two gas-rich dwarfs, known as the “bullet-dwarf” scenario. However, the trail overlaps in projection with a foreground system, the NGC 1035 group at ∼13 Mpc, raising suspicions that the trail is an artifact of this superposition. DF2 and DF4 have been found to be at distances of 21.7 ± 1.2 and 20.0 ± 1.6 Mpc, respectively, using the tip of the red giant branch (TRGB) method with deep Hubble Space Telescope (HST) imaging, but the distances to other trail dwarfs remain unknown. In this Letter, we use HST imaging to obtain surface brightness fluctuation (SBF) distance estimates for eight candidate trail dwarfs, as well as for the giant galaxies NGC 1052 and NGC 1035. We find that the dwarfs are all at ∼20 Mpc and are not associated with the foreground NGC 1035 group. However, for DF2, we derive an SBF distance of 17.7 ± 1.4 Mpc, inconsistent with the published HST TGRB distance (21.7 ± 1.2 Mpc). Meanwhile, James Webb Space Telescope (JWST) observations of DF2 offer a second, and potentially more accurate, TRGB distance of 17.6 ± 0.6 Mpc. While this value matches our SBF result, it is clear that uniform JWST imaging of the remaining trail dwarfs is critically needed. 

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4. The Hubble Constant from Infrared Surface Brightness Fluctuation Distances*

   https://doi.org/10.3847/1538-4357/abe86a  

   Blakeslee, John P.; Jensen, Joseph B.; Ma, Chung-Pei; Milne, Peter A.; Greene, Jenny E. (April 2021, The Astrophysical Journal)

   Abstract We present a measurement of the Hubble constant H 0 from surface brightness fluctuation (SBF) distances for 63 bright, mainly early-type galaxies out to 100 Mpc observed with the WFC3/IR on the Hubble Space Telescope (HST). The sample is drawn from several independent HST imaging programs using the F110W bandpass, with the majority of the galaxies being selected from the MASSIVE survey. The distances reach the Hubble flow with a median statistical uncertainty per measurement of 4%. We construct the Hubble diagram with these IR SBF distances and constrain H 0 using four different treatments of the galaxy velocities. For the SBF zero-point calibration, we use both the existing tie to Cepheid variables, updated for consistency with the latest determination of the distance to the Large Magellanic Cloud from detached eclipsing binaries, and a new tie to the tip of the red giant branch (TRGB) calibrated from the maser distance to NGC 4258. These two SBF calibrations are consistent with each other and with theoretical predictions from stellar population models. From a weighted average of the Cepheid and TRGB calibrations, we derive H 0 = 73.3 ± 0.7 ± 2.4 km s −1 Mpc −1 , where the error bars reflect the statistical and systematic uncertainties. This result accords well with recent measurements of H 0 from Type Ia supernovae, time delays in multiply lensed quasars, and water masers. The systematic uncertainty could be reduced to below 2% by calibrating the SBF method with precision TRGB distances for a statistical sample of massive early-type galaxies out to the Virgo cluster measured with the James Webb Space Telescope. 

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5. PHANGS–ALMA: Arcsecond CO(2–1) Imaging of Nearby Star-forming Galaxies

   https://doi.org/10.3847/1538-4365/ac17f3  

   Leroy, Adam K.; Schinnerer, Eva; Hughes, Annie; Rosolowsky, Erik; Pety, Jérôme; Schruba, Andreas; Usero, Antonio; Blanc, Guillermo A.; Chevance, Mélanie; Emsellem, Eric; et al (November 2021, The Astrophysical Journal Supplement Series)

   Abstract We present PHANGS–ALMA, the first survey to map CO J = 2 → 1 line emission at ∼1″ ∼100 pc spatial resolution from a representative sample of 90 nearby ( d ≲ 20 Mpc) galaxies that lie on or near the z = 0 “main sequence” of star-forming galaxies. CO line emission traces the bulk distribution of molecular gas, which is the cold, star-forming phase of the interstellar medium. At the resolution achieved by PHANGS–ALMA, each beam reaches the size of a typical individual giant molecular cloud, so that these data can be used to measure the demographics, life cycle, and physical state of molecular clouds across the population of galaxies where the majority of stars form at z = 0. This paper describes the scientific motivation and background for the survey, sample selection, global properties of the targets, Atacama Large Millimeter/submillimeter Array (ALMA) observations, and characteristics of the delivered data and derived data products. As the ALMA sample serves as the parent sample for parallel surveys with MUSE on the Very Large Telescope, the Hubble Space Telescope, AstroSat, the Very Large Array, and other facilities, we include a detailed discussion of the sample selection. We detail the estimation of galaxy mass, size, star formation rate, CO luminosity, and other properties, compare estimates using different systems and provide best-estimate integrated measurements for each target. We also report the design and execution of the ALMA observations, which combine a Cycle 5 Large Program, a series of smaller programs, and archival observations. Finally, we present the first 1″ resolution atlas of CO emission from nearby galaxies and describe the properties and contents of the first PHANGS–ALMA public data release. 

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