Transcript Cycle 21 Treasury program LEGUS (HST/GO

D. Calzetti1, J.C. Lee2, A. Adamo3, A. Aloisi2, J. Andrews1, T. M. Brown2, R. Chandar4, C. Christian2, M. Cignoni2, G. Clayton5,
R. da Silva6, S.E. de Mink7, C. Dobbs8, B. Elmegreen9, D. Elmegreen10, A. Evans11, M. Fumagalli7,23, J.S. Gallagher12,
D. Gouliermis13, E.K. Grebel13, A. Herrero14, B. Hilbert2, D.A. Hunter15, K.E. Johnson11, R.C. Kennicutt16, H. Kim17, D. Lennon18,
K. Levay2, C. Martin19, M. Krumholz6, P. Nair20, A. Nota21, A. Pellerin22, J. Prieto23, M. Regan2, J. Ryon12, E. Sabbi2, D. Schaerer24,
D. Schiminovich25, L.J. Smith21, D. Thilker26, M. Tosi27, S. Van Dyk19, R. Walterbos28, B.C. Whitmore2, A. Wofford29, S. Bright2, L. Ubeda2
(1U. of Massachusetts; 2STScI; 3MPIA; 4U. of Toledo; 5Louisiana State U.; 6UC Santa Cruz; 7Carnegie Obs.; 8U. of Exeter; 9IBM; 10Vassar College; 11U. of Virginia; 12U. of Wisconsin; 13U. of Heidelberg; 14IA de Canarias; 15Lowell Obs.;
16IoA
Cambridge; 17Arizona State U.; 18ESA-ESAC; 19CalTech; 20U. of Alabama; 21STScI/ESA; 22SUNY-Geneseo; 23Princeton U.; 24Geneva Obs.; 25Columbia U.; 26Johns Hopkins U.; 27INAF-Bologna Obs.; 28NMSU; 29UPMC-CNRS/IAP)
The Survey
Abstract
legus.stsci.edu
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.
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).
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.
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.
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), and
morphological type (Figure 4). Cover 104 in stellar mass, and ~100 in
oxygen abundance.
Range in internal structure (presence/absence of bars, rings) and
interaction state.
Pre-existing GALEX, Spitzer, WISE, and ground-based Ha images.
Observations
NGC6503, 5 Mpc, NUV,B, I
N
 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.
Science Goals
 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.
 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 Ha imaging.
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) .
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).
15”
A Taste of Science
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.
Data Products
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
N
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 Ha (0.6563 mm) and Pb (1.282 mm) emission lines enables
accurate foreground dust extinction estimates.
 Photometric measurements for the 10 brightest clusters plus the heavily dustenshrouded Cluster 11 (Figure 7) yield SEDs from 0.15 mm (UV) to 1.1 mm (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.
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.
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.