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

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.

The Gas-Star Formation Cycle in Nearby Star-forming Galaxies II. Resolved Distributions of CO and H$α$ Emission for 49 PHANGS Galaxies

“…(c) Optical radius from the Lyon-Meudon Extragalactic Database (LEDA). ( d) & (e): SFR and M * (Leroy et al 2021b). (f) Aperture-corrected total molecular gas mass based on the PHANGS-ALMA observations (Leroy et al 2021a).…”

“…The galaxies are lying on or near the star-forming main sequence. More details on the survey design and scientific motivation are presented in Leroy et al (2021b). In this work, we focus on the molecular gas and ionized (Hα) gas observed by the PHANGS-ALMA and PHANGS-Hα (narrowband) surveys, respectively.…”

“…(g) Atomic gas mass from LEDA. (h) Offset from the star-forming main sequence ∆MS (Catinella et al 2018;Leroy et al 2021b). (i) Hubble type from LEDA.…”

“…The left panel of Figure 1 shows the SFR-M * relation for our sample overlaid on a sample of local galaxies from the xCOLD GASS survey (grey circles; Saintonge et al 2017). The integrated SFR and M * are derived based on GALEX and WISE (Leroy et al 2019(Leroy et al , 2021b. The line in the figure represents the local star-forming main sequence derived by Leroy et al (2019).…”

Dependence of $X_{\rm CO}$ on metallicity, intensity, and spatial scale in a self-regulated interstellar medium

“…Meanwhile, the CO(2-1) line has become more widely used as an alternative to CO(1-0) in recent years. For example, the recent high-resolution survey of nearby star-forming galaxies, PHANGS-ALMA (Leroy et al 2021), chose CO(2-1) over CO(1-0) for better sensitivity. In this case, inferring H 2 requires a two-step process: the observed CO(2-1) line intensity (W 21 ) is first converted into W 10 with an adopted line ratio…”