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We present the largest catalog to-date of star clusters and compact associations in nearby galaxies. We have performed a V-band-selected census of clusters across the 38 spiral galaxies of the PHANGS-HST Treasury Survey, and measured integrated, aperture-corrected NUV-U-B-V-I photometry. This work has resulted in uniform catalogs that contain $\sim$20,000 clusters and compact associations which have passed human inspection and morphological classification, and a larger sample of $\sim$100,000 classified by neural network models. Here, we report on the observed properties of these samples, and demonstrate that tremendous insight can be gained from just the observed properties of clusters, even in the absence of their transformation into physical quantities. In particular, we show the utility of the UBVI color-color diagram, and the three principal features revealed by the PHANGS-HST cluster sample: the young cluster locus, the middle-age plume, and the old globular cluster clump. We present an atlas of maps of the 2D spatial distribution of clusters and compact associations in the context of the molecular clouds from PHANGS-ALMA. We explore new ways of understanding this large dataset in a multi-scale context by bringing together once-separate techniques for the characterization of clusters (color-color diagrams and spatial distributions) and their parent galaxies (galaxy morphology and location relative to the galaxy main sequence). A companion paper presents the physical properties: ages, masses, and dust reddenings derived using improved spectral energy distribution (SED) fitting techniques., Comment: 48 pages, 26 figures, article in review at APJ

A primary new capability of JWST is the ability to penetrate the dust in star forming galaxies to identify and study the properties of young star clusters that remain embedded in dust and gas. In this paper we combine new infrared images taken with JWST with our optical HST images of the star-bursting barred (Seyfert2) spiral galaxy NGC 1365. We find that this galaxy has the richest population of massive young clusters of any known galaxy within 30 Mpc, with $\sim$ 30 star clusters that are more massive than 10$^6$ Msolar and younger than 10 Myr. Sixteen of these clusters are newly discovered from our JWST observations. An examination of the optical images reveals that 4 of 30 ($\sim$13$\%$) are so deeply embedded that they cannot be seen in the I band (AV $\gt$ 10 mag), and that 11 of 30 ($\sim$37$\%$) are missing in the HST B band, so age and mass estimates from optical measurements alone are challenging. These numbers suggest that massive clusters in NGC 1365 remain obscured in the visible for $\sim$ 1.3 $\pm$ 0.7 Myr, and are either completely or partially obscured for $\sim$ 3.7 $\pm$ 1.1 Myr. We also use the JWST observations to gain new insights into the triggering of star cluster formation by the collision of gas and dust streamers with gas and dust in the bar. The JWST images reveal previously unknown structures (e.g., bridges and overshoot regions from stars that form in the bar) that help us better understand the orbital dynamics of barred galaxies and associated star-forming rings. Finally, we note that the excellent spatial resolution of the NIRCAM F200W filter provides a better way to separate barely resolved compact clusters from individual stars based on their sizes., Comment: 17 pages, 9 figures, accepted for publications as part of PHANGS-JWST ApJL Focus Issue

The PHANGS collaboration has been building a reference dataset for the multi-scale, multi-phase study of star formation and the interstellar medium in nearby galaxies. With the successful launch and commissioning of JWST, we can now obtain high-resolution infrared imaging to probe the youngest stellar populations and dust emission on the scales of star clusters and molecular clouds ($\sim$5-50 pc). In Cycle 1, PHANGS is conducting an 8-band imaging survey from 2-21$\mu$m of 19 nearby spiral galaxies. CO(2-1) mapping, optical integral field spectroscopy, and UV-optical imaging for all 19 galaxies have been obtained through large programs with ALMA, VLT/MUSE, and Hubble. PHANGS-JWST enables a full inventory of star formation, accurate measurement of the mass and age of star clusters, identification of the youngest embedded stellar populations, and characterization of the physical state of small dust grains. When combined with Hubble catalogs of $\sim$10,000 star clusters, MUSE spectroscopic mapping of $\sim$20,000 HII regions, and $\sim$12,000 ALMA-identified molecular clouds, it becomes possible to measure the timescales and efficiencies of the earliest phases of star formation and feedback, build an empirical model of the dependence of small dust grain properties on local ISM conditions, and test our understanding of how dust-reprocessed starlight traces star formation activity, all across a diversity of galactic environments. Here we describe the PHANGS-JWST Treasury survey, present the remarkable imaging obtained in the first few months of science operations, and provide context for the initial results presented in the first series of PHANGS-JWST publications., Comment: Re-submitted after addressing minor comments from referee. To be published as part of PHANGS-JWST ApJL Focus Issue

Emission lines with a double-peak (DP) shape, detected in the centre of galaxies, have been extensively used in the past to identify peculiar kinematics such as dual active galactic nuclei, outflows or mergers. From a large DP galaxy sample, a connection to minor merger galaxies with ongoing star formation was suggested. To gain a better understanding of different mechanisms creating a DP signature, we here explore synthetic SDSS spectroscopic observations computed from disc models and simulations. We show how a DP signature is connected to the central part of the rotation curve of galaxies, which is mostly shaped by the stellar bulge. We, furthermore, find that bars can create strong DP emission-line signatures when viewed along their major axis. Major mergers can form a central rotating disc in late post-coalescence merger stages (1\,Gyr after the final coalescence), which creates a DP signature. Minor mergers tend to show a DP feature with no correlation to the galaxy inclination within 350\,Myr after the final coalescence. Comparisons of these scenarii with observations disfavour major mergers, since these show predominantly elliptical and only a few S0 morphologies. Furthermore, at such a late merger stage the enhanced star formation is most likely faded. Bars and minor mergers, on the other hand, can be compared quite well with the observations. Both observations coincide with increased star formation found in observations, and minor mergers in particular do not show any dependency with the observation direction. However, observations resolving the galaxy kinematics spatially are needed to distinguish between the discussed possibilities. More insight into the origin of DP will be gained by a broader comparison with cosmological simulations. The understanding of the DP origin can provide important tools to study the mass growth of galaxies in future high redshift surveys., Comment: 22 pages, 19 figures, article accepted to A&A

The earliest stages of star formation occur enshrouded in dust and are not observable in the optical. Here we leverage the extraordinary new high-resolution infrared imaging from JWST to begin the study of dust-embedded star clusters in nearby galaxies throughout the local volume. We present a technique for identifying dust-embedded clusters in NGC 7496 (18.7 Mpc), the first galaxy to be observed by the PHANGS-JWST Cycle 1 Treasury Survey. We select sources that have strong 3.3$\mu$m PAH emission based on a $\rm F300M-F335M$ color excess, and identify 67 candidate embedded clusters. Only eight of these are found in the PHANGS-HST optically-selected cluster catalog and all are young (six have SED-fit ages of $\sim1$ Myr). We find that this sample of embedded cluster candidates may significantly increase the census of young clusters in NGC 7496 from the PHANGS-HST catalog -- the number of clusters younger than $\sim$2 Myr could be increased by a factor of two. Candidates are preferentially located in dust lanes, and are coincident with peaks in PHANGS-ALMA CO (2-1) maps. We take a first look at concentration indices, luminosity functions, SEDs spanning from 2700A to 21$\mu$m, and stellar masses (estimated to be between $\sim10^4-10^5 M_{\odot}$). The methods tested here provide a basis for future work to derive accurate constraints on the physical properties of embedded clusters, characterize the completeness of cluster samples, and expand analysis to all 19 galaxies in the PHANGS-JWST sample, which will enable basic unsolved problems in star formation and cluster evolution to be addressed., Comment: 12 pages, 6 figures; accepted for publication in ApJL as part of PHANGS-JWST First Results Special Issue

In a recent work, a large sample of double-peak (DP) emission line galaxies have been identified from the SDSS. While the two peaks could represent two kinematic components, they may be linked to the large bulges which their host galaxies tend to have. Star-forming DP galaxies display a central star formation enhancement and have been discussed to be compatible with a sequence of recent minor mergers. In order to probe merger induced star formation mechanisms, we conducted observations of the molecular gas content of 35 star-forming DP galaxies in the upper part of the main sequence (MS) of star formation with the IRAM 30m telescope. Including similar galaxies 0.3 dex above MS and with existing molecular gas observations from the literature, we finally obtain a sample of 52 such galaxies. We succeed in fitting the same kinematic parameters to the optical ionised and molecular gas emission lines for 10 (19%) galaxies. We find a central star formation enhancement resulting most likely from a galaxy merger or galaxy interaction, which is indicated by an excess of gas extinction found in the centre. This star formation is traced by radio continuum emissions at 150MHz, 1.4 and 3GHz, which are all three linearly correlated in log with the CO luminosity with the same slope. The 52 DP galaxies are found to have a significantly larger amount of molecular gas and larger depletion times, hence a lower star formation efficiency, than the expected values at their distance of the MS. This is consistent with a scenario of minor mergers increasing the mass of bulges and driving gas to the centre. We also exclude the inwards directed gas migration and central star formation enhancement to be the origin of a bar morphology. Hence, these 52 DP galaxies could be the results of recent minor mergers that funnelled molecular gas towards their centre, triggering star formation, but with a moderate efficiency., Comment: 33 pages, 19 figures, accepted by A&A

Given their prominent role in galaxy evolution, it is of paramount importance to unveil galaxy interactions and merger events and to investigate the underlying mechanisms. The use of high-resolution data makes it easier to identify merging systems, but it can still be challenging when the morphology does not show any clear galaxy-pair or gas bridge. Characterising the origin of puzzling kinematic features can help to reveal complicated systems. Here, we present a merging galaxy, MaNGA 1-114955, in which we highlighted the superimposition of two distinct rotating discs along the line of sight. These counter-rotating objects both lie on the star-forming main sequence but display perturbed stellar velocity dispersions. The main galaxy presents off-centred star formation as well as off-centred high-metallicity regions supporting the scenario of recent starbursts, while the secondary galaxy hosts a central starburst which coincides with an extended radio emission, in excess with respect to star formation expectations. Stellar mass as well as dynamical mass estimates agree towards a mass ratio within the visible radius of 9:1 for these interacting galaxies. We suggest we are observing a pre-coalescence stage of a merger. The primary galaxy has accreted gas through a past first pericentre passage about 1 Gyr ago, and more recently from the secondary gas-rich galaxy, which exhibits an underlying active galactic nucleus (AGN). Our results demonstrate how a galaxy can hide another one and the relevance of a multi-component approach to study ambiguous systems. We anticipate our method to be efficient at unveiling the mechanisms taking place in a sub-sample of galaxies observed by the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey, all exhibiting kinematic features of puzzling origin in their gas emission lines., Comment: 13 pages, 14 figures. Accepted for publication in Astronomy and Astrophysics

Mergers can be detected as double-peak narrow emission line galaxies but they are difficult to disentangle from disc rotations and gas outflows. We aim to properly detect such galaxies and distinguish the underlying mechanisms. Relying on RCSED, we developed an automated selection procedure and found 5663 double-peak emission line galaxies at z<0.34 corresponding to 0.8% of the parent database. To characterise these galaxies, we built a single-peak no-bias control sample (NBCS) with the same redshift and stellar mass distributions as the double-peak sample (DPS). These two samples are indeed very similar in terms of absolute magnitude, [OIII] luminosity, colour-colour diagrams, age and specific star formation rate, metallicity, and environment. We find an important excess of S0 galaxies in the DPS, not observed in the NBCS, and which cannot be accounted for by the environment, as most of these galaxies are isolated or in poor groups. Similarly, we find a relative deficit of pure discs in the DPS late-type galaxies, that are preferentially of Sa type. In parallel, we observe a systematic central excess of star formation and extinction for DP galaxies. Finally, there are noticeable differences in the kinematics: the gas velocity dispersion is correlated with the galaxy inclination in the NBCS, whereas this relation does not hold for the DPS. Furthermore, the DP galaxies show larger stellar velocity dispersions and they deviate from the Tully-Fisher relation for both late-type and S0 galaxies. These discrepancies can be reconciled if one considers the two peaks as two different components. Considering the morphological biases in favour, bulge-dominated galaxies and star-formation central enhancement, we suggest a scenario of multiple sequential minor mergers driving the increase of the bulge size, leading to larger fractions of S0 galaxies and a deficit of pure disc galaxies., Comment: 30 pages, 27 figures, article accepted to A&A

We present a study of 58 double-peaked emission line galaxies for which one of the components is suppressed in [OIII]5008 or significantly weaker than the other one. Accordingly, the two components are classified differently in the BPT diagram. We show that the strong [OIII] component coincides with the stellar velocity and the suppressed component is off-centred in 66% of the galaxies, while in 12% of them it is the opposite. The analysis of their morphology reveals that about half of the sample is composed of S0, the rest is composed in equal part of mergers and late-type galaxies. We discuss that these characteristics exclude rotating discs and suggest different stages of merging. It is possible that the number of mergers is underestimated if the double nuclei are not resolved. Tidal features are detected in the outskirts of some S0 galaxies. This high fraction of S0 is surprising, as in addition most of the galaxies are isolated and the others in small groups. All these galaxies, hosting an AGN component, are massive, lie on the star forming sequence, and exhibit an enhanced star formation in their centre. While we cannot exclude outflows, these galaxies exhibit a spectra, which do not correspond to usual outflow observations characterised by high gas velocities, and the standard deviations of the two peaks are comparable. In parallel, these characteristics are compatible with ultimate stages of galaxy merging, where the two nuclei are too close to be detected or dynamical disturbances might be present in post-mergers like massive S0., Comment: 8 Pages, 12 figures, Letter accepted to A&A