We present Hubble Space Telescope photometry for three fields in the outer disk of the Large Magellanic Cloud (LMC) extending approximately 4 mag below the faintest main-sequence turnoff. We cannot ...detect any strongly significant differences in the stellar populations of the three fields based on the morphologies of the color-magnitude diagrams, the luminosity functions, and the relative numbers of stars in different evolutionary stages. Our observations therefore suggest similar star formation histories in these regions, although some variations are certainly allowed. The fields are located in two regions of the LMC: one is in the northeast and two are located in the northwest. Under the assumption of a common star formation history, we combine the three fields with ground-based data at the same location as one of the fields to improve statistics for the brightest stars. We compare this stellar population with those predicted from several simple star formation histories suggested in the literature, using a combination of the R-method of Bertelli et al. (1992) and comparisons with the observed luminosity function. The only model we consider that is not rejected by the observations is one in which the star formation rate is roughly constant for most of the LMC's history and then increases by a factor of 3 about 2 Gyr ago. Such a model has roughly equal numbers of stars older and younger than 4 Gyr, and thus is not dominated by young stars. This star formation history, combined with a closed-box chemical evolution model, is consistent with observations that the metallicity of the LMC has doubled in the past 2 Gyr. (Author)
We present an analysis of compact star clusters in deep Hubble Space Telescope Wide Field Planetary Camera 2 images of NGC 1275. B- and R-band photometry of roughly 3000 clusters shows a bimodality ...in the B - R colors, suggesting that distinct old and young cluster populations are present. The small spread in the colors of the blue clusters is consistent with the hypothesis that they are a single-age population, with an inferred age of 0.1 to 1 Gyr. The luminosity function shows increasing numbers of blue clusters to the limit of our photometry, which reaches several magnitudes past the turnover predicted if the cluster population is identical to current Galactic globular clusters seen at a younger age. The blue clusters have a spatial distribution that is more centrally peaked than that of the red clusters. The individual clusters are slightly resolved, with core radii less than about 0.75 pc if they have modified Hubble profiles. We estimate the specific frequencies of the old and young populations and discuss the uncertainties in these estimates. We find that the specific frequency of the young population in NGC 1275 is currently larger than that of the old population and will remain so as the young population evolves, even if the majority of the low-mass clusters are eventually destroyed. If the young population formed during a previous merger, this suggests that mergers can increase the specific frequency of globular clusters in a galaxy. (Author)
We present an F606W-F814W color-magnitude diagram for the Draco dwarf spheroidal galaxy based on HST Wide Field Planetary Camera 2 (WFPC2) images. The luminosity function is well sampled to 3 mag ...below the turnoff. We see no evidence for multiple turnoffs and conclude that, at least over the field of view of the WFPC2, star formation was primarily single-epoch. If the observed number of blue stragglers is due to extended star formation, then roughly 6 percent (upper limit) of the stars could be half as old as the bulk of the galaxy. The color difference between the red giant branch and the turnoff is consistent with an old population and is very similar to that observed in the old, metal-poor Galactic globular clusters M68 and M92. Despite its red horizontal branch, Draco appears to be older than M68 and M92 by 1.6 +/- 2.5 Gyr, lending support to the argument that the 'second parameter' that governs horizontal-branch morphology must be something other than age. Draco's observed luminosity function is very similar to that of M68, and the derived initial mass function is consistent with that of the solar neighborhood. (Author)
We analyze the spectral energy distributions (SEDs) of Lyman break galaxies (LBGs) at z ~ or = 1-3 selected using the Hubble Space Telescope (HST)Wide Field Camera 3 (WFC3) UVIS channel filters. ...These HST/WFC3 observations cover about 50 arcmin2 in the GOODS-South field as a part of the WFC3 Early Release Science program. These LBGs at z ~ or = 1-3 are selected using dropout selection criteria similar to high-redshift LBGs. The star-forming galaxies selected using other color-based techniques show similar correlations at z ~ or = 2, but to avoid any selection biases, and for direct comparison with LBGs at z > 3, a true Lyman break selection at z ~ or = 2 is essential. The future HST UV surveys, both wider and deeper, covering a large luminosity range are important to better understand LBG properties and their evolution.
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