The Cycle 21 Treasury program LEGUS (HST/GO-13364) is the first HSTUV Atlas of nearby galaxies, aimed at the thorough investigation of starformation and its relation with galaxy environment, from the scales ofindividual stars to those of ~kpc clustered structures.
The 154-orbits program is obtaining NUV,U,B,V,I images of 50 star-forminggalaxies in the distance range 4-12 Mpc, covering the full range ofmorphology, star formation rate (SFR), mass, metallicity, internal structure, andinteraction state found in the local Universe. The imaging survey will yield:
accurate recent (<50 Myr) star formation histories (SFHs) from resolvedmassive stars,
the extinction-corrected ages and masses of star clusters and associations.
These extensive inventories of massive stars, clustered systems, and SFHs willbe used for a number of science goals (listed to the right), and will be deliveredto the community (see Data Products below).
LEGUS observations will inform theories of star formation and galaxyevolution, and improve the understanding of the physical underpinning of thegas-star formation relation and the nature of the clumpy star formation at highredshift.
LEGUS will provide the reference survey and the foundation for futureobservations with JWST and with ALMA.
Abstract
Multi-band aligned and resampled images ormosaics.
Multi-band, merged photometric catalog of allsources (stars, clusters, and blended objects) foreach galaxy., including locations, photometricuncertainties, and sizes.
For each galaxy, a catalog of cluster ages, masses,extinction, and uncertainties, as derived frommulti-band SED fitting (both deterministic andstochastic cluster models).
One-stop-shop for all ancillary GALEX, Spitzer,WISE, and ground-based Himaging.
Most LEGUS galaxies are well-known and well-studied,representative of star-forming galaxies in the distance range 4-12 Mpc.
Inclination < 70 degrees and Galactic Latitude > 20 degrees.
Span full local range in: SFR (factor ~103), sSFR (~103), andmorphological type (Figure 4). Cover 104 in stellar mass, and ~100 inoxygen abundance.
Range in internal structure (presence/absence of bars, rings) andinteraction state.
Pre-existing GALEX, Spitzer, WISE, and ground-based H images.
Figure 2:Footprint of the WFC3/F275W andF336W mosaic for NGC5457 (blue),overlaid on the archival ACS/B,V,I footprints(red outlines); the location of the parallelACS 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 NASAcontract 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 throughnew WFC3 observations (Figure 1) or archival data.
10 galaxies (NGC0628, 1291, 1313, 1512/10, 4258,4594, 5194/95, 5457, 6744, 7793) observed with 2-4pointings, for a total of 63 pointings (Figure 2).
Primary Exposures:
WFC3/F275W and F336W;
WFC3/F438W (40 galaxies) or archivalACS/WFC/F435W;
WFC3/F555W and F814 W (28 galaxies) orarchival ACS/WFC/F555W and F814W.
Typically: 1 orbit in NUV, and ½ orbit each inU,B,V,I.
Parallel Exposures: ACS/F435W and F814W.
STATUS: as of November 2013, about ½ of theLEGUS sample has been observed and is availablein 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 beenobtained 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 thebottom (T=10).
Figure 5: Color-magnitude diagrams for the galaxy NGC6503 (Figure 1), shown inmultiple band combinations for the horizontal axis. Point-sources are detected downto S/N=3
Figure 6: (U-B) vs. (V-I) two-color diagram for cluster candidates in NGC 6503 from ourSExtractor catalog. The diagram shows objects with mF555W>26, and FWHMF555W>2.1 pixels. TheSED 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 starformation clumps.
Star Cluster Formation and Evolution: univariatefunctions of mass and time, and bi-variate functions;dependency on galactic environment (Figure 6).
Resolved Massive Stars: recent-past (<50 Myr) starformation histories from CMDs (Figure 5); calibration ofUV SFRs; massive stars IMF and its environmentalvariations.
Legacy: Supernova Precursors. Multi-band imaging foridentification of progenitors of future supernova events.
Figure 7: Three-color composite (NUV, B, I) ofthe central region of NGC5253, located at 3.7Mpc distance, using a combination of WFC3 andACS images. The 10 brightest clusters and themost dust-attenuated cluster (# 11) are marked byyellow circles and numbered. The size of thecircles matches the size of the apertures used forphotometric measurements. The largest circlesare 0.5” in radius, corresponding to ~9 pc inlinear size.
NGC5253 is a very nearby dwarf Irregular, hosting a central burst of starformation
LEGUS observations at F275W and F336W are joined by ACS/WFC and HRCand WFC3/IR images from different programs
Availability of both H (0.6563 m) and P(1.282 m) emission lines enablesaccurate 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 providepreliminary estimates on masses and ages.
Preliminary results (confirming earlier findings):
All clusters are young (ages<20 Myr), with masses between ~5,000 and 20,000Mo;
Clusters 5 and 11 are consistent with ages < 2 Myr, and significant dustattenuation.
legus.stsci.edu
Figure 4: Below:Distribution of the Legus sample physical properties, in terms of SFR andsSFR (SFR/mass) as a function of morphological type T and stellar mass, respectively. LEGUSis 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 moderatelyextincted Cluster 2, with an estimated age of 5 Myr and mass of ~15,000 Mo; and the dust-ensroudedCluster 11, with a very preliminary age estimate of 1 Myr and mass estimate of about 4,000 Mo.