Abstract
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 μm imaging has been approved.
Highlights
How do stars form from the complex multi-phase interstellar medium (ISM) in galaxies? This question lies at the heart of astrophysics, as star formation is a key process governing the evolution of baryons in the universe (Peroux & Howk 2020)
PHANGS-HST provides a critical augmentation to the HST archive for nearby spiral galaxies for which both star clusters and molecular clouds can be efficiently detected by HST and Atacama Large Millimeter/submillimeter Array (ALMA) over galactic scales
With the transformative capabilities of ALMA and HST working in concert, PHANGS will help bridge the fields of star formation and galaxy evolution by investigating how small-scale physics, which create the basic quanta of star formation, may depend on the physical conditions of the greater galactic environment and conspire to produce galaxy scaling relationships
Summary
How do stars form from the complex multi-phase interstellar medium (ISM) in galaxies? This question lies at the heart of astrophysics, as star formation is a key process governing the evolution of baryons in the universe (Peroux & Howk 2020). PHANGS is charting the connections between giant molecular clouds, Hii regions, and young stars throughout a diversity of galactic environments in the local universe by combining observations from new Treasury programs with ALMA, VLT/MUSE, and HST, with supporting data including ground-based Hα, VLA Hi, Astrosat FUV/NUV wide-field imaging, and the wealth of panchromatic ground- and space-based survey observations collected for the nearby galaxies in the sample over the past three decades. This high-level description is intended to provide the framework for a series of papers, as summarized, which document each of the major components in detail, in particular new methods for selecting star cluster candidates, morphological classification of candidates, and identification of stellar associations over a range of physical scales. Magnitudes in this and other PHANGS-HST pipeline papers are given in the Vega system, to facilitate comparison with prior HST studies of resolved stellar populations, unless otherwise noted
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