Andrew Dolphin

... Affiliation: AA(Max-Planck-Institut für Astronomie, Heidelberg, Germany), AB(National Optical Astronomy Observatoires, Tucson, USA), AC(UCO/Lick Observatory, UCSC, Santa Cruz, USA). ...And I is the least luminous galaxy in which a globular cluster has been detected so far. ...

Young star clusters provide a unique indicator as to the past star formation of a galaxy. In addition to providing a fossil record of past star formation activity {similarly to the stellar content}, it appears that the amount and nature of cluster formation depends strongly on the environmental conditions. The nature of these objects {indistinguishable from bright or foreground stars from the ground} has made a detailed study difficult in the past; the advent of textitHST and its superior resolution circumvents this problem. Previous textitHST studies have focused on interacting systems and merger remnants {where young clusters are expected to be abundant}, leaving a number of questions unanswered, such as: {1} Is the ratio of massive cluster formation to total star formation constant? {2} Are trends {if any} along the Hubble sequence analogous to those seen in larger OB associations and HII regions? We propose to address these and other questions through a survey of t. he. cluster ...

ABSTRACT We measured the Tip of the Red Giant Branch distances to nine galaxies in the direction to the Virgo cluster using the Advanced Camera for Surveys on the Hubble Space Telescope. These distances put seven galaxies: GR 34, UGC 7512, NGC 4517, IC 3583, NGC 4600, VCC 2037 and KDG 215 in front of the Virgo, and two galaxies: IC 3023, KDG 177 likely inside the cluster. Distances and radial velocities of the galaxies situated between us and the Virgo core clearly exhibit the infall phenomenon toward the cluster. In the case of spherically symmetric radial infall we estimate the radius of the "zero-velocity surface" to be (7.2+-0.7) Mpc that yields the total mass of the Virgo cluster to be (8.0+-2.3) X 10^{14} M_sun in good agreement with its virial mass estimates. We conclude that the Virgo outskirts does not contain significant amounts of dark matter beyond its virial radius.

... Andrew E. Dolphin ... An initial solution to this was to cool the camera from −76◦C to −88◦C to reduce the magnitude of the effect, and to apply a correction that scaled from linearly from zero for starswith Y = 0 to a maximum of 0.04 magnitudes for stars with Y = 800. Holtzman et al. ...

Our understanding of the extragalactic distance scale is limited in large part by uncertainties in the absolute magnitudes of the primary standard candles -- Cepheids and RR Lyraes. As these calibrations are largely independent (Cepheid calibrations are tied to the moving cluster distance to the Pleiades or an assumed LMC distance; RR Lyrae and RGB tip calibrations are tied to

The formation mechanism of galaxy halos has been the source of considerable debate. There is evidence pointing toward both gravitational collapse and accretion as sources of some stars, but the relative importance of these sources are unknown. This question can be addressed with measurements of metallicities and ages of halo stars. Recent ultra-deep HST/ACS observations of a field in the

Observations were made primarily with the WIYN 3.5m telescope at the Kitt Peak National Observatory. In addition, our target fields and standard stars were also observed at the WIYN 0.9m telescope at Kitt Peak. Observations of NGC 2419, Pal 4, and Pal 14, all of which are distant globular clusters in our galaxy and have the additional virtue of having

We describe an archive of stellar photometry measurements that we are constructing from a large fraction of all HST observations of Local Group galaxies. We present the techniques used to derive this database and show examples of the data products, which are available via the WWW. We describe some of the scientific analyses that we are pursuing using this database,

We present the results of a search for short-period variable stars in Leo A. We have found 92 candidate variables: 8 candidate RR Lyrae stars and 84 short-period Cepheids. From the RR Lyraes, we measure a distance modulus of (m-M)0 = 24.51 +/- 0.12, or 0.80 +/- 0.04 Mpc. This discovery of RR Lyraes confirms, for the first time, the presence of an ancient (> ~ 11 Gyr) population in Leo A accounting for at least 0.1% of the galaxy's V luminosity. We have also discovered a halo of old (> ~ 2 Gyr) stars surrounding Leo A, with a scale length roughly 50% larger than that of the dominant young population. The median absolute magnitude of our Cepheid sample is MV = -1.1, fainter than 95.5% of SMC and 98.9% of LMC Cepheids. Their periods are also unusual, with three Cepheids that are deduced to be pulsating in the fundamental mode having periods of under 1 day. Upon examination, the characteristics of the Leo A Cepheids appear to be the natural extension of the classical Cepheid period-luminosity relations to low metallicity, rather than coming from a large population of anomalous Cepheids. We demonstrate that the periods and luminosities are consistent with the expected values of low-metallicity blue helium-burning stars (BHeBs), which populate the instability strip at lower luminosities than do higher-metallicity BHeBs.

Leo A is a nearby (d=690 kpc), very low metallicity (~ 3% solar) isolated dwarf irregular galaxy which is gas rich and forming stars at the present time. Analysis of the evolved stellar populations in HST observations suggest that Leo A is a predominantly young galaxy (~1.5 Gyr old). The presence of a ``young'' galaxy in the Local Group would

The ANGST data set (see associated talk by Dalcanton) from the HST/ACS is providing the deepest color-magnitude diagrams (CMDs) yet measured for several nearby galaxies. These CMDs are already revealing the wide variety of stellar populations present in this volume-limited sample of galaxies. Detailed star formation history measurements on these data are underway, and I will discuss the status of

We present HST/WFPC2 resolved stellar photometry across the disk of the nearby dwarf S0 galaxy NGC 404, which hosts an extended gas disk. The locations of our fields contain a roughly equal mixture of bulge and disk stars. Detailed modeling of the color-magnitude diagrams suggests that 70% of the stellar mass in the NGC 404 disk formed by z 2 (10 Gyr ago) and at least 90% formed prior to z 1 (8 Gyr ago). These results indicate that the stellar populations of the NGC 404 disk are on average significantly older than those of other nearby disk galaxies, suggesting that early and late type disks may have different long-term evolutionary histories, not simply differences in their current and recent star formation rates. Comparisons of the spatial distribution of the young stellar mass and FUV emission in GALEX images show that the brightest FUV regions contain the youngest stellar mass, but some young stars (<160 Myr) lie outside of these regions. Finally, we use our measurements to infer the relationship between the star formation rate and the gas density of the disk at previous epochs. We find that most of the history of the NGC 404 disk is consistent with star formation that has decreased with the gas density according to the Schmidt law. However, 0.5-1 Gyr ago, the star formation rate was unusually low for the inferred gas density, consistent with the possibility that there was a gas accretion event that reignited star formation 0.5 Gyr ago. Such an event could explain why this dwarf S0 galaxy hosts an extended gas disk. Support for this work was provided by NASA through grants GO-10915 and GO-11719 from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555.

We present a measurement of the age distribution of stars residing in nearby spiral disks and dwarf galaxies. With these measurements, we estimate a complete star formation history of the 140 Mpc3 covered by the volume-limited sample of galaxies in the Advanced Camera for Surveys (ACS) Nearby Galaxy Survey Treasury (ANGST). Our sample allows us to separate the contributions by

Some of the most important projects with the most expensive telescope ever built - HST - are hostage to the uncertainty in the absolute calibration of its photometric system. The standard practice in the past has been to calibrate faint objects (i.e., V ~ 26) observed with long exposures against bright stars (V ~ 13) observed with very short exposures. There is now good evidence that non- linearities in the detectors are the main cause of uncertainty in the absolute photometric accuracy of WFPC2 on HST. We propose to use the Mini-Mosaic Imager on the WIYN telescope to obtain observations of faint standards (V ~ 22) for carefully chosen fields that have already been observed with WFPC2. This will allow us to retroactively recover the history of the absolute calibration of WFPC2 throughout its mission , and hence improve the available photometric accuracy for all WFPC2 observations. Future HST observations of the new fields would allow us to follow WFPC2's absolute calibration to the end of its mission. These newly defined standard fields could also serve as the basis for calibrating ACS and WFC3 throughout their missions. This will greatly reduce the error budgets for a wide variety of fundamental projects (e.g., extragalactic Cepheids and RR Lyrae stars; supernovae; globular-cluster ages; metallicity estimates, ...).