What are the faintest distant galaxies we can see with the Hubble Space Telescope (HST) now, before the launch of the James Webb Space Telescope? This is the challenge taken up by the Frontier Fields, a Director's discretionary time campaign with HST and the Spitzer Space Telescope to see deeper into the universe than ever before. The Frontier Fields combines the power of HST and Spitzer with the natural gravitational telescopes of massive highmagnification clusters of galaxies to produce the deepest observations of clusters and their lensed galaxies ever obtained. Six clusters-Abell 2744, MACSJ0416.1-2403, MACSJ0717.5+3745, MACSJ1149.5+2223, Abell S1063, and Abell 370-have been targeted by the HST ACS/WFC and WFC3/IR cameras with coordinated parallel fields for over 840 HST orbits. The parallel fields are the second-deepest observations thus far by HST with 5σ point-source depths of ∼29th ABmag. Galaxies behind the clusters experience typical magnification factors of a few, with small regions magnified by factors of 10-100. Therefore, the Frontier Field cluster HST images achieve intrinsic depths of ∼30-33 mag over very small volumes. Spitzer has obtained over 1000 hr of Director's discretionary imaging of the Frontier Field cluster and parallels in IRAC 3.6 and 4.5 μm bands to 5σ point-source depths of ∼26.5, 26.0 ABmag. We demonstrate the exceptional sensitivity of the HST Frontier Field images to faint high-redshift galaxies, and review the initial results related to the primary science goals.

We present the results of a detailed analysis of the properties of dwarf O-type stars in a metal-poor environment. High-resolution, high-quality, ultraviolet and optical spectra of six O-type stars in the H II region NGC 346 have been obtained from a spectroscopic survey of O stars in the SMC. Stellar parameters and chemical abundances have been determined using NLTE line-blanketed photospheric models calculated with Tlusty. Additionally, we have modeled the spectra with the NLTE line-blanketed wind code, CMFGEN, to derive wind parameters. Stellar parameters and chemical abundances, and in particular iron abundances, obtained with the two NLTE codes compare quite favorably. This

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We report new observations of Zn II and Cr II absorption lines in 10 damped Lyman α systems (DLAs), mostly at redshift z abs > ∼ 2.5 . By combining these results with those from our earlier survey and other recent data, we construct a sample of 34 measurements (or upper limits) of the Zn abundance relative to hydrogen [Zn/H]; the sample includes more than one third of the total number of DLAs known. The plot of the abundance of Zn as a function of redshift reinforces the two main findings of our previous study. (1) Damped Lyman α systems are mostly metal-poor, at all redshifts sampled; the column density weighted mean for the whole data set is [Zn/H] = −1.13 ± 0.38 (on a logarithmic scale), or approximately 1/13 of solar.(2) There is a large spread, by up to two orders of magnitude, in the metallicities we measure at essentially the same redshifts. We propose that damped Lyman α systems are drawn from a varied population of galaxies of different morphological types and at different stages of chemical evolution, supporting the idea of a protracted epoch of galaxy formation. At redshifts z > ∼ 2 the typical metallicity of the damped Lyman α systems is in agreement with expectations based on the consumption of H I gas implied by the recent measurements of Ω DLA by Storrie-Lombardi et al. (1996a), and with the metal ejection rates in the universe at these epochs deduced by Madau (1996) from the ultraviolet luminosities of high redshift galaxies revealed by deep imaging surveys. There are indications in our data for an increase in the mean metallicity of the damped Lyman α systems from z > 3 to ≈ 2, consistent with the rise in the comoving star formation rate indicated by the relative numbers of U and B drop-outs in the Hubble Deep Field.Although such comparisons are still tentative, it appears that these different avenues

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