The PHANGS-HST Survey: Physics at High Angular resolution in Nearby
  GalaxieS with the Hubble Space Telescope
  The PHANGS program is building the first dataset to enable the multi-phase,
multi-scale study of star formation across the nearby spiral galaxy population.
This effort is enabled by large survey programs with ALMA, VLT/MUSE, and 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 five band 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
multi-scale 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, HII 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, highlighting 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.
These high-level science products will seed a broad range of investigations, in
particular, the study of embedded stellar populations and dust with JWST, for
which a PHANGS Cycle 1 Treasury program to obtain eight band 2-21 $\mu$m
imaging has been approved.
