We report the large effort that is producing comprehensive high-level young star cluster (YSC) catalogs for a significant fraction of galaxies observed with the Legacy ExtraGalactic UV Survey (LEGUS) Hubble treasury program. We present the methodology developed to extract cluster positions, verify their genuine nature, produce multiband photometry (from NUV to NIR), and derive their physical properties via spectral energy distribution fitting analyses. We use the nearby spiral galaxy NGC 628 as a test case for demonstrating the impact that LEGUS will have on our understanding of the formation and evolution of YSCs and compact stellar associations within their host galaxy. Our analysis of the cluster luminosity function from the UV to the NIR finds a steepening at the bright end and at all wavelengths suggesting a dearth of luminous clusters. The cluster mass function of NGC 628The 1 is consistent with a power-law distribution of slopes~-2 and a truncation of a few times 10 5  M . After their formation, YSCs and compact associations follow different evolutionary paths. YSCs survive for a longer time frame, confirming their being potentially bound systems. Associations disappear on timescales comparable to hierarchically organized star-forming regions, suggesting that they are expanding systems. We find massindependent cluster disruption in the inner region of NGC 628, while in the outer part of the galaxy there is little or no disruption. We observe faster disruption rates for low mass (10 4  M ) clusters, suggesting that a massdependent component is necessary to fully describe the YSC disruption process in NGC 628.Astrophysical Journal, 841:131 (26pp), 2017 June 1 https:

The Legacy ExtraGalactic UV Survey (LEGUS) is a Cycle 21 Treasury program on the Hubble Space Telescope, aimed at the investigation of star formation and its relation with galactic environment in nearby galaxies, from the scales of individual stars to those of ∼kpc-size clustered structures. Five-band imaging, from the near-ultraviolet to the I-band, with the Wide Field Camera 3, plus parallel optical imaging with the Advanced Camera for Surveys, is being collected for selected pointings of 50 galaxies within the local 12 Mpc. The filters used for the observations with the Wide Field Camera 3 are: F275W(λ2,704Å), F336W(λ3,355Å), F438W(λ4,325Å), F555W(λ5,308Å), and F814W(λ8,024Å); the parallel observations with the Advanced Camera for Surveys use the filters: F435W(λ4,328Å), F606W(λ5,921Å), and F814W(λ8,057Å). The multi-band images are yielding accurate recent ( 50 Myr) star formation histories from resolved massive stars and the extinction-corrected ages and masses of star clusters and associations. The extensive inventories of massive stars and clustered systems will be used to investigate the spatial and temporal evolution of star formation * Einstein Fellow within galaxies. This will, in turn, inform theories of galaxy evolution and improve the understanding of the physical underpinning of the gas-star formation relation and the nature of star formation at high redshift. This paper describes the survey, its goals and observational strategy, and the initial science results. Because LEGUS will provide a reference survey and a foundation for future observations with JWST and with ALMA, a large number of data products are planned for delivery to the community.

We present a study of the M 83 cluster population, covering the disc of the galaxy between radii of 0.45 and 4.5 kpc. We aim to probe the properties of the cluster population as a function of distance from the galactic centre. We observe a net decline in cluster formation efficiency (Γ, i.e. the amount of star formation happening in bound clusters) from about 26% in the inner region to 8% in the outer part of the galaxy. The recovered Γ values within different regions of M 83 follow the same Γ versus star formation rate density relation observed for entire galaxies. We also probe the initial cluster mass function (ICMF) as a function of galactocentric distance. We observe a significant steepening of the ICMF in the outer regions (from −1.90 ± 0.11 to −2.70 ± 0.14) and for the whole galactic cluster population (slope of −2.18 ± 0.07) of M 83. We show that this change of slope reflects a more fundamental change of the 'truncation mass' at the high-mass end of the distribution. This can be modelled as a Schechter function of slope −2 with an exponential cut-off mass (M c ) that decreases significantly from the inner to the outer regions (from 4.00 to 0.25 × 10 5 M ⊙ ) while the galactic M c is ≈ 1.60 × 10 5 M ⊙ . The trends in Γ and ICMF are consistent with the observed radial decrease of the Σ(H 2 ), hence in gas pressure. As gas pressure declines cluster formation becomes less efficient. We conclude that the host galaxy environment appears to regulate 1) the fraction of stars locked in clusters; 2) the upper mass limit of the ICMF, consistently described by a near-universal slope −2 truncated at the high-mass end.

The study of young massive clusters can provide key information for the formation of globular clusters, as they are often considered analogues. A currently unanswered question in this field is how long these massive clusters remain embedded in their natal gas, with important implications for the formation of multiple populations that have been used to explain phenomena observed in globular clusters. We present an analysis of ages and masses of the young massive cluster population of M83. Through visual inspection of the clusters, and comparison of their spectral energy distributions (SEDs) and position in colour-colour space, the clusters are all exposed (no longer embedded) by <4 Myr, most likely less, indicating that current proposed age spreads within older clusters are unlikely. We also present several methods of constraining the ages of very young massive clusters. This can often be difficult using SED fitting due to a lack of information to disentangle age-extinction degeneracies and possible inaccurate assumptions in the models used for the fitting. The individual morphology of the Hα around each cluster has a significant effect on the measured fluxes, which contributes to inaccuracies in the age estimates for clusters younger than 10 Myr using SED fitting. This is due to model uncertainties and aperture effects. Our methods to help constrain ages of young clusters include using the near-infrared and spectral features, such as Wolf-Rayet stars.