The Cycle 21 Treasury program LEGUS (HST/GO-13364) is the first HST UV Atlas of nearby galaxies, aimed at the thorough investigation of star formation and its relation with galaxy environment, from the scales of individual stars to those of ~kpc clustered structures.

The 154-orbits program is obtaining NUV,U,B,V,I images of 50 star-forming galaxies in the distance range 4-12 Mpc, covering the full range of morphology, star formation rate (SFR), mass, metallicity, internal structure, and interaction state found in the local Universe. The imaging survey will yield: accurate recent (<50 Myr) star formation histories (SFHs) from resolved massive stars, the extinction-corrected ages and masses of star clusters and associations.

These extensive inventories of massive stars, clustered systems, and SFHs will be used for a number of science goals (listed to the right), and will be delivered to the community (see Data Products below).

LEGUS observations will inform theories of star formation and galaxy evolution, and improve the understanding of the physical underpinning of the gas-star formation relation and the nature of the clumpy star formation at high redshift. LEGUS will provide the reference survey and the foundation for future observations with JWST and with ALMA.

Abstract

Multi-band aligned and resampled images or mosaics.

Multi-band, merged photometric catalog of all sources (stars, clusters, and blended objects) for each galaxy., including locations, photometric uncertainties, and sizes.

For each galaxy, a catalog of cluster ages, masses, extinction, and uncertainties, as derived from multi-band SED fitting (both deterministic and stochastic cluster models).

One-stop-shop for all ancillary GALEX, Spitzer, WISE, and ground-based Himaging.

Figure 2:Footprint of the WFC3/F275W and F336W mosaic for NGC5457 (blue), overlaid on the archival ACS/B,V,I footprints (red outlines); the location of the parallel ACS is also shown (magenta) .

Acknowledgments: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, Inc., under NASA contract NAS 5-26555. These observations are associated with program GO -13364.This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by JPL, CalTech, under contract with the National Aeronautics and Space Administration.

Observations

Data Products

NGC6503, 5 Mpc, NUV,B, I NUV,U,B,V,I images for each galaxy, either through

new WFC3 observations (Figure 1) or archival data.

10 galaxies (NGC0628, 1291, 1313, 1512/10, 4258, 4594, 5194/95, 5457, 6744, 7793) observed with 2-4 pointings, for a total of 63 pointings (Figure 2).

Primary Exposures: WFC3/F275W and F336W; WFC3/F438W (40 galaxies) or archival

ACS/WFC/F435W; WFC3/F555W and F814 W (28 galaxies) or

archival ACS/WFC/F555W and F814W. Typically: 1 orbit in NUV, and ½ orbit each in

U,B,V,I. Parallel Exposures: ACS/F435W and F814W.

STATUS: as of November 2013, about ½ of the LEGUS sample has been observed and is available in the MAST archive.

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Figure 1: Top Panel: three color-composite (blue=NUV, green=B, and red=I) of the LEGUS galaxy NGC6503; all images have been obtained with WFC3. Bottom Panel: the WFC3 footprint (blue) and the parallel ACS footprint (magenta) for this galaxy.

The Survey

Science Goals

A Taste of Science

Figure 3: Left: two color-composite GALEX FUV+NUV images of the LEGUS galaxies, ordered by morphological type, from early spirals at the top (Sa, T>0) to Irregulars at the bottom (T=10).

Figure 5: Color-magnitude diagrams for the galaxy NGC6503 (Figure 1), shown in multiple band combinations for the horizontal axis. Point-sources are detected down to S/N=3

Figure 6: (U-B) vs. (V-I) two-color diagram for cluster candidates in NGC 6503 from our SExtractor catalog. The diagram shows objects with mF555W>26, and FWHMF555W>2.1 pixels. The SED models shown are from CB09 (Bruzual & Charlot 2011, Astr. Source Code Library 1104.005) with solar metallicity (Z=0.017, orange line) and ~2x solar metallicity (Z=0.04, red line).

Hierarchy of Star Formation: evolution of structures, timescales and dependency on local galactic environment; characteristic scales; relation to high-z kpc-scale star formation clumps.

Star Cluster Formation and Evolution: univariate functions of mass and time, and bi-variate functions; dependency on galactic environment (Figure 6).

Resolved Massive Stars: recent-past (<50 Myr) star formation histories from CMDs (Figure 5); calibration of UV SFRs; massive stars IMF and its environmental variations.

Legacy: Supernova Precursors. Multi-band imaging for identification of progenitors of future supernova events.

Figure 7: Three-color composite (NUV, B, I) of the central region of NGC5253, located at 3.7 Mpc distance, using a combination of WFC3 and ACS images. The 10 brightest clusters and the most dust-attenuated cluster (# 11) are marked by yellow circles and numbered. The size of the circles matches the size of the apertures used for photometric measurements. The largest circles are 0.5” in radius, corresponding to ~9 pc in linear size.

NGC5253 is a very nearby dwarf Irregular, hosting a central burst of star formation

LEGUS observations at F275W and F336W are joined by ACS/WFC and HRC and WFC3/IR images from different programs

Availability of both H (0.6563 m) and P(1.282 m) emission lines enables accurate foreground dust extinction estimates.

Photometric measurements for the 10 brightest clusters plus the heavily dust-enshrouded Cluster 11 (Figure 7) yield SEDs from 0.15 m (UV) to 1.1 m (H).

Modeling of SEDs using synthetic populations + dust attenuation provide preliminary estimates on masses and ages.

Preliminary results (confirming earlier findings): All clusters are young (ages<20 Myr), with masses between ~5,000 and

20,000 Mo; Clusters 5 and 11 are consistent with ages < 2 Myr, and significant dust

attenuation.

legus.stsci.edu

Figure 4: Below:Distribution of the Legus sample physical properties, in terms of SFR and sSFR (SFR/mass) as a function of morphological type T and stellar mass, respectively. LEGUS is complementary to archival HST imaging holdings, for galaxies within 12-15 Mpc.

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Figure 8: Photometry and SED modeling for two of the clusters shown in Figure 7. The moderately extincted Cluster 2, with an estimated age of 5 Myr and mass of ~15,000 Mo; and the dust-ensrouded Cluster 11, with a very preliminary age estimate of 1 Myr and mass estimate of about 4,000 Mo.