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 similar to 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 mu m imaging has been approved.

The CO-to-H-2 conversion factor (alpha(CO)) is critical to studying molecular gas and star formation in galaxies. The value of a co has been found to vary within and between galaxies, but the specific environmental conditions that cause these variations are not fully understood. Previous observations on similar to kiloparsec scales revealed low values of alpha(CO) in the centers of some barred spiral galaxies, including NGC 3351. We present new Atacama Large Millimeter/ submillimeter Array Band 3, 6, and 7 observations of (CO)-C-12, (CO)-C-13, and (CO)-O-18 lines on 100 pc scales in the inner similar to 2 kpc of NGC 3351. Using multiline radiative transfer modeling and a Bayesian likelihood analysis, we infer the H-2 density, kinetic temperature, CO column density per line width, and CO isotopologue abundances on a pixel-by-pixel basis. Our modeling implies the existence of a dominant gas component with a density of 2-3 x 10(3) cm(-3) in the central similar to 1 kpc and a high temperature of 30-60 K near the nucleus and near the contact points that connect to the bar-driven inflows. Assuming a CO/H-2 abundance of 3 x 10(-4), our analysis yields alpha(CO) 0.5-2.0 M-circle dot (K km s(-1) pc(2))(-1) with a decreasing trend with galactocentric radius in the central similar to 1 kpc. The inflows show a substantially lower alpha(CO) less than or similar to 0.1 M-circle dot (K km s(-1) pc(2))(-1) likely due to lower optical depths caused by turbulence or shear in the inflows. Over the whole region, this gives an intensity-weighted alpha(CO) of similar to 1.5 M-circle dot (K km s(-1) pc(2))(-1) which is similar to previous dustmodeling-based results at kiloparsec scales. This suggests that low alpha(CO) on kiloparsec scales in the centers of some barred galaxies may be due to the contribution of low-optical-depth CO emission in bar-driven inflows.

The relative distribution of molecular gas and star formation in galaxies gives insight into the physical processes and timescales of the cycle between gas and stars. In this work, we track the relative spatial configuration of CO and H alpha emission at high resolution in each of our galaxy targets and use these measurements to quantify the distributions of regions in different evolutionary stages of star formation: from molecular gas without star formation traced by H alpha to star-forming gas, and to H ii regions. The large sample, drawn from the Physics at High Angular resolution in Nearby GalaxieS ALMA and narrowband H alpha (PHANGS-ALMA and PHANGS-H alpha) surveys, spans a wide range of stellar masses and morphological types, allowing us to investigate the dependencies of the gas-star formation cycle on global galaxy properties. At a resolution of 150 pc, the incidence of regions in different stages shows a dependence on stellar mass and Hubble type of galaxies over the radial range probed. Massive and/or earlier-type galaxies in our sample exhibit a significant reservoir of molecular gas without star formation traced by H alpha, while lower-mass galaxies harbor substantial H ii regions that may have dispersed their birth clouds or formed from low-mass, more isolated clouds. Galactic structures add a further layer of complexity to the relative distribution of CO and H alpha emission. Trends between galaxy properties and distributions of gas traced by CO and H alpha are visible only when the observed spatial scale is MUCH LESS-THAN500 pc, reflecting the critical resolution requirement to distinguish stages of the star formation process.

Feedback from massive stars plays a key role in molecular cloud evolution. After the onset of star formation, the young stellar population is exposed by photoionization, winds, supernovae, and radiation pressure from massive stars. Recent observations of nearby galaxies have provided the evolutionary timeline between molecular clouds and exposed young stars, but the duration of the embedded phase of massive star formation is still ill-constrained. We measure how long massive stellar populations remain embedded within their natal cloud, by applying a statistical method to six nearby galaxies at resolution, using CO, Spitzer 24, and H alpha emission as tracers of molecular clouds, embedded star formation, and exposed star formation, respectively. We find that the embedded phase (with CO and 24 emission) lasts for 2-7 Myr and constitutes of the cloud lifetime. During approximately the first half of this phase, the region is invisible in H alpha, making it heavily obscured. For the second half of this phase, the region also emits in H alpha and is partially exposed. Once the cloud has been dispersed by feedback, 24 emission no longer traces ongoing star formation, but remains detectable for another 2-9 Myr through the emission from ambient CO-dark gas, tracing star formation that recently ended. The short duration of massive star formation suggests that pre-supernova feedback (photoionization and winds) is important in disrupting molecular clouds. The measured time-scales do not show significant correlations with environmental properties (e.g. metallicity). Future JWST observations will enable these measurements routinely across the nearby galaxy population.

The feedback from young stars (i.e. pre-supernova) is thought to play a crucial role in molecular cloud destruction. In this paper, we assess the feedback mechanisms acting within a sample of 5810 Hii regions identified from the PHANGS-MUSE survey of 19 nearby (<20Mpc) star-forming, main-sequence spiral galaxies [log(M-star/M-circle dot) = 9.4-11]. These optical spectroscopic maps are essential to constrain the physical properties of the Hii regions, which we use to investigate their internal pressure terms. We estimate the photoionized gas (P-therm), direct radiation (P-rad), and mechanical wind pressure (P-wind), which we compare to the confining pressure of their host environment (P-de). The Hii regions remain unresolved within our similar to 50-100pc resolution observations, so we place upper (P-max) and lower (P-min) limits on each of the pressures by using a minimum (i.e. clumpy structure) and maximum (i.e. smooth structure) size, respectively. We find that the P-max measurements are broadly similar, and for P-min the P-therm is mildly dominant. We find that the majority of Hii regions are overpressured, P-tot/P-de = (P-therm + P-wind + P-rad)/P-de > 1, and expanding, yet there is a small sample of compact Hii regions with P-tot,P-max/P-de < 1 (similar to 1 per cent of the sample). These mostly reside in galaxy centres (R-gal < 1kpc), or, specifically, environments of high gas surface density; log(Sigma(gas)/M(circle dot)pc(-2)) similar to 2.5 (measured on kpc-scales). Lastly, we compare to a sample of literature measurements for P-therm and P(rad)to investigate how dominant pressure term transitions over around 5dex in spatial dynamic range and 10dex in pressure.

Extensive archival Hubble Space Telescope, Spitzer Space Telescope, and Large Binocular Telescope imaging of the recent intermediate-luminosity transient, AT 2019krl in M74, reveal a bright optical and mid-infrared progenitor star. While the optical peak of the event was missed, a peak was detected in the infrared with an absolute magnitude of M (4.5 mu m) = -18.4 mag, leading us to infer a visual-wavelength peak absolute magnitude of -13.5 to -14.5. The pre-discovery light curve indicated no outbursts over the previous 16 yr. The colors, magnitudes, and inferred temperatures of the progenitor best match a 13-14 M (circle dot) yellow or blue supergiant (BSG) if only foreground extinction is taken into account, or a hotter and more massive star if any additional local extinction is included. A pre-eruption spectrum of the star reveals strong H alpha and [N ii] emission with wings extending to +/- 2000 km s(-1). The post-eruption spectrum is fairly flat and featureless with only H alpha, Na i D, [Ca ii], and the Ca ii triplet in emission. As in many previous intermediate-luminosity transients, AT 2019krl shows remarkable observational similarities to luminous blue variable (LBV) giant eruptions, SN 2008S-like events, and massive-star mergers. However, the information about the pre-eruption star favors either a relatively unobscured BSG or a more extinguished LBV with M > 20 M-circle dot likely viewed pole-on.

We use an unprecedented sample of about 23 000 H II regions detected at an average physical resolution of 67 pc in the PHANGS-MUSE sample to study the extragalactic H II region H alpha luminosity function (LF). Our observations probe the star-forming disk of 19 nearby spiral galaxies with low inclination and located close to the star formation main sequence at z = 0. The mean LF slope, alpha, in our sample is = 1.73 with a sigma of 0.15. We find that alpha decreases with the galaxy's star formation rate surface density, Sigma(SFR), and argue that this is driven by an enhanced clustering of young stars at high gas surface densities. Looking at the H II regions within single galaxies, we find that no significant variations occur between the LF of the inner and outer part of the star-forming disk, whereas the LF in the spiral arm areas is shallower than in the inter-arm areas for six out of the 13 galaxies with clearly visible spiral arms. We attribute these variations to the spiral arms increasing the molecular clouds' arm-inter-arm mass contrast and find suggestive evidence that they are more evident for galaxies with stronger spiral arms. Furthermore, we find systematic variations in a between samples of H II regions with a high and low ionization parameter, q, and argue that they are driven by the aging of H II regions.

We use integral field spectroscopy from the PHANGS-MUSE survey, which resolves the ionised interstellar medium structure at similar to 50 pc resolution in 19 nearby spiral galaxies, to study the origin of the diffuse ionised gas (DIG). We examine the physical conditions of the diffuse gas by first removing morphologically defined H II regions and then binning the low-surface-brightness areas to achieve significant detections of the key nebular lines in the DIG. A simple model for the leakage and propagation of ionising radiation from H II regions is able to reproduce the observed distribution of H alpha in the DIG. This model infers a typical mean free path for the ionising radiation of 1.9 kpc for photons propagating within the disc plane. Leaking radiation from H II regions also explains the observed decrease in line ratios of low-ionisation species ([S II]/H alpha, [N II]/H alpha, and [O I]/H alpha) with increasing H alpha surface brightness (sigma(H alpha)). Emission from hot low-mass evolved stars, however, is required to explain: (1) the enhanced low-ionisation line ratios observed in the central regions of some of the galaxies in our sample; (2) the observed trends of a flat or decreasing [O III]/H beta with sigma(H alpha); and (3) the offset of some DIG regions from the typical locus of H II regions in the Baldwin-Phillips-Terlevich (BPT) diagram, extending into the area of low-ionisation (nuclear) emission-line regions (LI[N]ERs). Hot low-mass evolved stars make a small contribution to the energy budget of the DIG (2% of the galaxy-integrated H alpha emission), but their harder spectra make them fundamental contributors to [O III] emission. The DIG might result from a superposition of two components, an energetically dominant contribution from young stars and a more diffuse background of harder ionising photons from old stars. This unified framework bridges observations of the Milky Way DIG with LI(N)ER-like emission observed in nearby galaxy bulges.

We provide new planetary nebula luminosity function (PNLF) distances to 19 nearby spiral galaxies that were observed with VLT/MUSE by the PHANGS collaboration. Emission line ratios are used to separate planetary nebulae (PNe) from other bright [O III] emitting sources like compact supernovae remnants (SNRs) or H II regions. While many studies have used narrowband imaging for this purpose, the detailed spectral line information provided by integral field unit (IFU) spectroscopy grants a more robust way of categorizing different [O III] emitters. We investigate the effects of SNR contamination on the PNI.F and find that we would fail to classify all objects correctly, when limited to the same data narrowband imaging provides. However, the few misclassified objects usually do not fall on the bright end of the luminosity function, and only in three cases does the distance change by more than 1 sigma. We find generally good agreement with literature values from other methods. Using metallicity constraints that have also been derived from the same IFU data, we revisit the PNLF zero-point calibration. Over a range of 8.34 < 12 + log (O/H) < 8.59, our sample is consistent with a constant zero-point and yields a value of M* = -4.542(-0.059)(+0.103) mag, within to of other literature values. MUSE pushes the limits of PNLF studies and makes galaxies beyond 20 Mpc accessible for this kind of analysis. This approach to the PNLF shows great promise for leveraging existing archival IFU data on nearby galaxies.

Understanding the spatial distribution of metals within galaxies allows us to study the processes of chemical enrichment and mixing in the interstellar medium. In this work, we map the 2D distribution of metals using a Gaussian Process Regression (GPR) for 19 star-forming galaxies observed with the Very Large Telescope/Multi Unit Spectroscopic Explorer (VLF-MUSE) as a part of the PRANGS-MUSE survey. We find that 12 of our 19 galaxies show significant 2D metallicity variation. Those without significant variations typically have fewer metallicity measurements, indicating this is due to the dearth of H II regions in these galaxies, rather than a lack of higher-order variation. After subtracting a linear radial gradient, we see no enrichment in the spiral arms versus the disc. We measure the 50 per cent correlation scale from the two-point correlation function of these radially subtracted maps, finding it to typically be an order of magnitude smaller than the fitted GPR kernel scale length. We study the dependence of the two-point correlation scale length with a number of global galaxy properties. We find no relationship between the 50 per cent correlation scale and the overall gas turbulence, in tension with existing theoretical models. We also find more actively star-forming galaxies, and earlier type galaxies have a larger 50 per cent correlation scale. The size and stellar mass surface density do not appear to correlate with the 50 per cent correlation scale, indicating that perhaps the evolutionary state of the galaxy and its current star formation activity is the strongest indicator of the homogeneity of the metal distribution.