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 cloud-scale density, velocity dispersion, and gravitational boundedness of the interstellar medium (ISM) vary within and among galaxies. In turbulent models, these properties play key roles in the ability of gas to form stars. New high fidelity, high resolution surveys offer the prospect to measure these quantities across galaxies. We present a simple approach to make such measurements and to test hypotheses that link small-scale gas structure to star formation and galactic environment. Our calculations capture the key physics of the Larson scaling relations, and we show good correspondence between our approach and a traditional "cloud properties" treatment. However, we argue that our method is preferable in many cases because of its simple, reproducible characterization of all emission. Using, low-J 12 CO data from recent surveys, we characterize the molecular ISM at 60 pc resolution in the Antennae, the Large Magellanic Cloud, M31, M33, M51, and M74. We report the distributions of surface density, velocity dispersion, and gravitational boundedness at 60 pc scales and show galaxy-to-galaxy and intra-galaxy variations in each. The distribution of flux as a function of surface density appears roughly lognormal with a 1σ width of ∼0.3 dex, though the center of this distribution varies from galaxy to galaxy. The 60 pc resolution line width and molecular gas surface density correlate well, which is a fundamental behavior expected for virialized or free-falling gas. Varying the measurement scale for the LMC and M31, we show that the molecular ISM has higher surface densities, lower line widths, and more self-gravity at smaller scales.

Carbon monoxide (CO) provides crucial information about the molecular gas properties of galaxies. While 12 CO has been targeted extensively, isotopologues such as 13 CO have the advantage of being less optically thick and observations have recently become accessible across full galaxy discs. We present a comprehensive new dataset of 13 CO(1-0) observations with the IRAM 30-m telescope of the full discs of 9 nearby spiral galaxies from the EMPIRE survey at a spatial resolution of ∼1.5 kpc. 13 CO(1-0) is mapped out to 0.7 − 1 r 25 and detected at high signal-to-noise throughout our maps. We analyse the 12 CO(1-0)-to-13 CO(1-0) ratio (ℜ) as a function of galactocentric radius and other parameters such as the 12 CO(2-1)-to-12 CO(1-0) intensity ratio, the 70-to-160 µm flux density ratio, the star-formation rate surface density, the star-formation efficiency, and the CO-to-H 2 conversion factor. We find that ℜ varies by a factor of 2 at most within and amongst galaxies, with a median value of 11 and larger variations in the galaxy centres than in the discs. We argue that optical depth effects, most likely due to changes in the mixture of diffuse/dense gas, are favored explanations for the observed ℜ variations, while abundance changes may also be at play. We calculate a spatially-resolved 13 CO(1-0)-to-H 2 conversion factor and find an average value of 1.0 × 10 21 cm −2 (K km s −1 ) −1 over our sample with a standard deviation of a factor of 2. We find that 13 CO(1-0) does not appear to be a good predictor of the bulk molecular gas mass in normal galaxy discs due to the presence of a large diffuse phase, but it may be a better tracer of the mass than 12 CO(1-0) in the galaxy centres where the fraction of dense gas is larger.

We describe the processing of the PHANGS-ALMA survey and present the PHANGS-ALMA pipeline, a public software package that processes calibrated interferometric and total power data into science-ready data products. PHANGS-ALMA is a large, high-resolution survey of CO(2-1) emission from nearby galaxies. The observations combine ALMA's main 12 m array, the 7 m array, and total power observations, and use mosaics of dozens to hundreds of individual pointings. We describe the processing of the u − v data, imaging and deconvolution, linear

We present ALMA 12 CO (J=1-0, 3-2 and 6-5), 13 CO (J=1-0) and C 18 O (J=1-0) observations of the local Ultra Luminous Infrared Galaxy, IRAS 13120-5453. The morphologies of the three isotopic species differ, where 13 CO shows a hole in emission towards the center. We measure integrated brightness temperature line ratios of 12 CO/ 13 CO ≥ 60 (exceeding 200) and 13 CO/C 18 O ≤ 1 in the central region. Assuming optical thin emission, C 18 O is more abundant than 13 CO in several regions. The abundances within the central 500 pc are consistent with enrichment of the ISM via a young starburst (<7Myr), a top-heavy initial mass function or a combination of both.