Carol A. Christian

LEGACY EXTRAGALACTIC UV SURVEY (LEGUS) WITH THE HUBBLE SPACE TELESCOPE. I. SURVEY DESCRIPTION

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 near-ultraviolet 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(λ8024 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 (lesssim50 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.

Hubble Space Telescope Advanced Camera for Surveys Mosaic of the Prototypical Starburst Galaxy M82

ABSTRACT In 2006 March, the Hubble Heritage Team obtained a large, four‐filter (B, V, I, and Hα), 6 point mosaic data set of the prototypical starburst galaxy NGC 3034 (M82) with the Advanced Camera for Surveys on board the Hubble Space Telescope (HST). The resulting color composite Heritage image was released in 2006 April to celebrate HST’s 16th anniversary. Cycle 15 HST proposers were encouraged to submit General Observer and Archival Research proposals to complement and/or analyze this unique data set. Since our M82 mosaics represent a significant investment of expert processing beyond the standard archival products, we also released our drizzle‐combined FITS data as a High‐Level Science Product via the Multimission Archive at STScI in 2006 December. This paper documents the key aspects of the observing program and image processing: calibration, image registration and combination (drizzling), and the rejection of cosmic rays and detector artifacts.

Hubble Space Telescope Photometry of the Metal-rich Globular Clusters NGHC 6624 and NGC 6637

We have observed the metal-rich globular clusters NGC 6624 and NGC 6637 (M69) using the planetary camera of the WFPC2 on the Hubble Space Telescope (HST). Observations of the Ca II triplet lines in giant stars in these clusters show that NGC 6624 and NGC 6637 have metallicities on the Zinn and West scale of [Fe/H] = -0.63 ? 0.09 and -0.65 ? 0.09, only slightly more metal rich than 47 Tuc [Fe/H] = -0.71 ? 0.07. For clusters of identical (or nearly so) metallicity, one can make a direct comparison of the color-magnitude diagrams to derive the relative ages of the clusters. From the color-magnitude diagrams derived from the HST photometry, we find that NGC 6624 and NGC 6637 differ in age by less than 0.5 Gyr. Their color-magnitude diagrams are also compared with those of 47 Tuc and NGC 6352, and while these latter diagrams are of somewhat lower quality, they are consistent with all of these clusters having the same ages. Adopting an apparent distance modulus of 13.40 and reddening E(B-V) = 0.04 for 47 Tuc, the new Yale isochrones yield an age for the clusters of 14 Gyr. The positions of NGC 6624 and NGC 6637 in the Galaxy suggest that they belong to the bulge population of globular clusters. The only other bulge clusters that have been dated so far are the more metal rich clusters NGC 6528 and NGC 6553, which also appear to be very old. Consequently, the age-metallicity relation of the bulge may be very steep. The close similarity of the ages and metallicities of NGC 6624 and NGC 6637 to the thick-disk globular clusters 47 Tuc and NGC 6352 indicates that the age-metallicity relations of these populations intersect. We briefly discuss the possibility that these populations had a common origin.

Hubble Space Telescope Observations of the Interacting Galaxies NGC 2207 and IC 2163

Original article can be found at: http://www.journals.uchicago.edu/AJ/--Copyright American Astronomical Society

HST observations of the interacting galaxies NGC 2207 and IC 2163

Original article can be found at: http://www.journals.uchicago.edu/AJ/--Copyright American Astronomical Society

The M31 globular cluster G219 - Resolved V and I CCD photometry

The VI CCD photometry of individual stars in the M31 globular cluster G219 and the surrounding halo field is presented. The color-magnitude diagram for G219 shows that the cluster is at least as metal poor as M15, consistent with the metallicity estimates from spectroscopy and IR photometry of its integrated light. The halo field appears to be more metal rich than the cluster itself and shows an indication of a clumping of stars that is interpreted as the red end of the horizontal branch. The mean metallicity of the halo field appears similar to that of 47 Tuc, although a considerable range in metallicity is present. Combined with information from related studies, this latter result is consistent with the idea that the M31 halo field stars are relatively metal rich and exhibit only a slight metallicity gradient out to a significant galactocentric distance. 23 refs.

Legacy ExtraGalactic UV Survey with The Hubble Space Telescope: Stellar Cluster Catalogs and First Insights Into Cluster Formation and Evolution in NGC 628

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 628 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 mass-independent 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 mass-dependent component is necessary to fully describe the YSC disruption process in NGC 628.

A High-resolution Multiband Survey of Westerlund 2 with the Hubble Space Telescope. I. Is the Massive Star Cluster Double?

We present first results from a high resolution multi-band survey of the Westerlund 2 region with the Hubble Space Telescope. Specifically, we imaged Westerlund 2 with the Advanced Camera for Surveys through the

Photometry of giant-branch stars in the M31 globular cluster G1

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