Legacy extragalactic UV survey (LEGUS) with the hubble space telescope. I. Survey description

This is the final version of the article. Available from the publisher via the DOI in this record.The American Astronomical Society. All rights reserved. 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 ∼kiloparsec-size clustered structures. Five-band imaging from the nearultraviolet to the I band with the Wide-Field Camera 3 (WFC3), plus parallel optical imaging with the Advanced Camera for Surveys (ACS), is being collected for selected pointings of 50 galaxies within the local 12 Mpc. The filters used for the observations with the WFC3 are F275W (λ2704 A˚), F336W(λ3355 A˚), F438W(λ4325 A˚), F555W(λ5308 A˚), and F814W(ë8024 A˚); the parallel observations with the ACS use the filters F435W (λ4328 A˚), F606W(λ5921 A˚), and F814W(λ8057 A˚). The multiband 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 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 scientific results. Because LEGUS will provide a reference survey and a foundation for future observations with the James Webb Space Telescope and with ALMA, a large number of data products are planned for delivery to the community.Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, under NASA Contract NAS 5–26555. These observations are associated with Program 13364. Support for Program 13364 was provided by NASA through a grant from the Space Telescope Science Institute. This research has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. S.d.M. acknowledges support for this work by NASA through Einstein Fellowship grant PF3-140105. C.L.D. acknowledges funding from the European Research Council for the FP7 ERC starting grant project, LOCALSTAR. D.A.G. kindly acknowledges financial support by the German Research Foundation through grant GO 1659/3-1. A.H. acknowledges support by the Spanish MINECO under Project Grant AYA2012-39364-C02-1. J.E.R. gratefully acknowledges the support of the National Space Grant College and Figure 6. Hierarchical structures traced by the UV-bright stars in NGC 6503 are shown on a three-color image (UV, B, I) of the galaxy. The UVbright population has been selected from the CMD of the galaxy, using the region delimited by −2 ⩽ NUV−U ⩽ 2 mag and brighter than absolute magnitude MNUV −2.5 mag. The four-color contours (blue, cyan, green, and yellow, in order of increasing physical scale, separated by a factor of 2 in smoothing kernel FWHM) delimit regions having significant difference in the smoothed surface density of the UV-bright stars between one scale and the next. The largest regions have sizes ∼700 pc. Our method links together any spatially associated overdensities detected at arbitrary scale into composite hierarchical structures. The field of view of the image is ∼3.3 kpc × 1.6 kpc. Figure 7. Two-point angular correlation function of the young stars (<100 Myr; blue line) and old stars (>500 Myr; red line) for the whole extent of NGC 6503. The horizontal gray dotted line is the expected two-point angular correlation function of a randomly distributed population. The monotonically decreasing functions imply that the stellar distributions follow a hierarchical pattern, with the young stars more strongly clustered than the old stars across galactic scales up to projected sizes of at least 100″ (equivalent to ∼2.75 kpc). 21 The Astronomical Journal, 149:51 (25pp), 2015 February Calzetti et al. Fellowship Program and the Wisconsin Space Grant Consortium. A.W. acknowledges funding from the European Research Council under the European Communityʼs Seventh Framework Programme (FP7/2007-2013 Grant Agreement 321323)