Search

We analyze the spectral energy distributions (SEDs) of Lyman break galaxies (LBGs) at z=1-3 selected using the Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) UVIS channel filters. These HST/WFC3 observations cover about 50 sq. arcmin in the GOODS-South field as a part of the WFC3 Early Release Science program. These LBGs at z=1-3 are selected using dropout selection criteria similar to high redshift LBGs. The deep multi-band photometry in this field is used to identify best-fit SED models, from which we infer the following results: (1) the photometric redshift estimate of these dropout selected LBGs is accurate to within few percent; (2) the UV spectral slope (beta) is redder than at high redshift (z>3), where LBGs are less dusty; (3) on average, LBGs at z=1-3 are massive, dustier and more highly star-forming, compared to LBGs at higher redshifts with similar luminosities (0.1L*<~L<~2.5L*), though their median values are similar within 1-sigma uncertainties. This could imply that identical dropout selection technique, at all redshifts, find physically similar galaxies; and (4) stellar masses of these LBGs are directly proportional to their UV luminosities with a logarithmic slope of ~0.46, and star-formation rates are proportional to their stellar masses with a logarithmic slope of ~0.90. These relations hold true --- within luminosities probed in this study --- for LBGs from z~1.5 to 5. The star-forming galaxies selected using other color-based techniques show similar correlations at z~2, but to avoid any selection biases, and for direct comparison with LBGs at z>3, a true Lyman break selection at z~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., Comment: Accepted for publication in ApJ (29 pages, 9 figures)

In the first of a series of forthcoming publications, we present a panchromatic catalog of 102 visually-selected early-type galaxies (ETGs) from observations in the Early Release Science (ERS) program with the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST) of the Great Observatories Origins Deep Survey-South (GOODS-S) field. Our ETGs span a large redshift range, 0.35 < z < 1.5, with each redshift spectroscopically-confirmed by previous published surveys of the ERS field. We combine our measured WFC3 ERS and ACS GOODS-S photometry to gain continuous sensitivity from the rest-frame far-UV to near-IR emission for each ETG. The superior spatial resolution of the HST over this panchromatic baseline allows us to classify the ETGs by their small-scale internal structures, as well as their local environment. By fitting stellar population spectral templates to the broad-band photometry of the ETGs, we determine that the average masses of the ETGs are comparable to the characteristic stellar mass of massive galaxies, 11< log(M [Solar]) < 12. By transforming the observed photometry into the GALEX FUV and NUV, Johnson V, and SDSS g' and r' bandpasses we identify a noteworthy diversity in the rest-frame UV-optical colors and find the mean rest-frame (FUV-V)=3.5 and (NUV-V)=3.3, with 1$\sigma$ standard deviations approximately equal to 1.0. The blue rest-frame UV-optical colors observed for most of the ETGs are evidence for star-formation during the preceding gigayear, but no systems exhibit UV-optical photometry consistent with major recent (<~50 Myr) starbursts. Future publications which address the diversity of stellar populations likely to be present in these ETGs, and the potential mechanisms by which recent star-formation episodes are activated, are discussed., Comment: accepted to the Astrophysical Journal Supplement Series

The Cosmic Origins Spectrograph (COS) was installed in the Hubble Space Telescope in May, 2009 as part of Servicing Mission 4 to provide high sensitivity, medium and low resolution spectroscopy at far- and near-ultraviolet wavelengths (FUV, NUV). COS is the most sensitive FUV/NUV spectrograph flown to date, spanning the wavelength range from 900{\AA} to 3200{\AA} with peak effective area approaching 3000 cm^2. This paper describes instrument design, the results of the Servicing Mission Orbital Verification (SMOV), and the ongoing performance monitoring program., Comment: 9 pages, 7 figures, submitted to Astrophysics and Space Science

The MPF mission will provide a statistical census of exoplanets with masses greater than 0.1 Earth-masses and orbital separations ranging from 0.5AU to infinity. This includes analogs to all the Solar System's planets except for Mercury, as well as most types of planets predicted by planet formation theories. Such a survey will provide results on the frequency of planets around all types of stars except those with short lifetimes. Close-in planets with separations < 0.5 AU are invisible to a space-based microlensing survey, but these can be found by Kepler. Other methods, including ground-based microlensing, cannot approach the comprehensive statistics on the mass and semi-major axis distribution of extrasolar planets that a space-based microlensing survey will provide. The terrestrial planet sensitivity of a ground-based microlensing survey is limited to the vicinity of the Einstein radius at 2-3 AU, and space-based imaging is needed to identify and determine the mass of the planetary host stars for the vast majority of planets discovered by microlensing. Thus, a space-based microlensing survey is likely to be the only way to gain a comprehensive understanding of the architecture of planetary systems, which is needed to understand planet formation and habitability. MPF can accomplish these objectives with proven technology and a cost of $333 million (excluding launch vehicle)., Comment: RFI Response for the Astro2010 Program Prioritization Panel, (The Basis for the Exoplanet Program of the WFIRST Mission)

We present results on the size evolution of passively evolving galaxies at 11.5. We identify 30 galaxies in ~40 square arcmin to H<25 mag. We supplement spectroscopic redshifts from the literature with photometric redshifts determined from the 15-band photometry from 0.22-8 micron. We determine effective radii from Sersic profile fits to the H-band image using an empirical PSF. We find that size evolution is a strong function of stellar mass, with the most massive (M* ~ 10^11 Msol) galaxies undergoing the most rapid evolution from z~2 to the present. Parameterizing the size evolution as (1+z)^{-alpha}, we find a tentative scaling between alpha and stellar mass of alpha ~ -1.8+1.4 log(M*/10^9 Msol). We briefly discuss the implications of this result for our understanding of the dynamical evolution of the red galaxies., Comment: 11 pages, 7 figures, 4 tables. Submitted to ApJ

We present grism spectra of emission-line galaxies (ELGs) from 0.6-1.6 microns from the Wide Field Camera 3 on the Hubble Space Telescope. These new infrared grism data augment previous optical Advanced Camera for Surveys G800L 0.6-0.95 micron grism data in GOODS-South from the PEARS program, extending the wavelength covereage well past the G800L red cutoff. The ERS grism field was observed at a depth of 2 orbits per grism, yielding spectra of hundreds of faint objects, a subset of which are presented here. ELGs are studied via the Ha, [OIII], and [OII] emission lines detected in the redshift ranges 0.22., Comment: Accepted for publication in AJ. Updated to include referee comments. Updated sample using improved reduction contains 23 new galaxies (Table 1; Figures 2 & 3)

We combine new high sensitivity ultraviolet (UV) imaging from the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST) with existing deep HST/Advanced Camera for Surveys (ACS) optical images from the Great Observatories Origins Deep Survey (GOODS) program to identify UV-dropouts, which are Lyman break galaxy (LBG) candidates at z~1-3. These new HST/WFC3 observations were taken over 50 sq.arcmin in the GOODS-South field as a part of the Early Release Science program. The uniqueness of these new UV data is that they are observed in 3 UV/optical (WFC3 UVIS) channel filters (F225W, F275W and F336W), which allows us to identify three different sets of UV-dropout samples. We apply Lyman break dropout selection criteria to identify F225W-, F275W- and F336W-dropouts, which are z~1.7, 2.1 and 2.7 LBG candidates, respectively. Our results are as follows: (1) these WFC3 UVIS filters are very reliable in selecting LBGs with z~2.0, which helps to reduce the gap between the well studied z~>3 and z~0 regimes, (2) the combined number counts agrees very well with the observed change in the surface densities as a function of redshift when compared with the higher redshift LBG samples; and (3) the best-fit Schechter function parameters from the rest-frame UV luminosity functions at three different redshifts fit very well with the evolutionary trend of the characteristic absolute magnitude, and the faint-end slope, as a function of redshift. This is the first study to illustrate the usefulness of the WFC3 UVIS channel observations to select z<3 LBGs. The addition of the new WFC3 on the HST has made it possible to uniformly select LBGs from z~1 to z~9, and significantly enhance our understanding of these galaxies using HST sensitivity and resolution., Comment: Accepted for publication in ApJ (24 pages, 7 figures)

A space-based gravitational microlensing exoplanet survey will provide a statistical census of exoplanets with masses greater than 0.1 Earth-masses and orbital separations ranging from 0.5AU to infinity. This includes analogs to all the Solar System's planets except for Mercury, as well as most types of planets predicted by planet formation theories. Such a survey will provide results on the frequency of planets around all types of stars except those with short lifetimes. Close-in planets with separations < 0.5 AU are invisible to a space-based microlensing survey, but these can be found by Kepler. Other methods, including ground-based microlensing, cannot approach the comprehensive statistics on the mass and semi-major axis distribution of extrasolar planets that a space-based microlensing survey will provide. The terrestrial planet sensitivity of a ground-based microlensing survey is limited to the vicinity of the Einstein radius at 2-3 AU, and space-based imaging is needed to identify and determine the mass of the planetary host stars for the vast majority of planets discovered by microlensing. Thus, a space-based microlensing survey is likely to be the only way to gain a comprehensive understanding of the architecture of planetary systems, which is needed to understand planet formation and habitability. The proposed Microlensing Planet Finder (MPF) mission is an example of a space-based microlensing survey that can accomplish these objectives with proven technology and a cost of under $300 million (excluding launch vehicle)., Comment: 8 pages (including cover), White Paper to the Astro2010 PSF Science Frontier Panel

A space-based gravitational microlensing exoplanet survey will provide a statistical census of exoplanets with masses down to 0.1 Earth-masses and orbital separations ranging from 0.5AU to infinity. This includes analogs to all the Solar System's planets except for Mercury, as well as most types of planets predicted by planet formation theories. Such a survey will provide results on the frequency of planets around all types of stars except those with short lifetimes. Close-in planets with separations < 0.5 AU are invisible to a space-based microlensing survey, but these can be found by Kepler. Other methods, including ground-based microlensing, cannot approach the comprehensive statistics on the mass and semi-major axis distribution of extrasolar planets that a space-based microlensing survey will provide. The terrestrial planet sensitivity of a ground-based microlensing survey is limited to the vicinity of the Einstein radius at 2-3 AU, and space-based imaging is needed to identify and determine the mass of the planetary host stars for the vast majority of planets discovered by microlensing. Thus, a space-based microlensing survey is likely to be the only way to gain a comprehensive understanding of the nature of planetary systems, which is needed to understand planet formation and habitability. The proposed Microlensing Planet Finder (MPF) mission is an example of a space-based microlensing survey that can accomplish these objectives with proven technology and a cost that fits comfortably under the NASA Discovery Program cost cap., Comment: White Paper Submitted to the NASA/NSF ExoPlanet Task Force

(Abridged.) We present F435W (B), F606W (Broad V), and F814W (Broad I) coronagraphic images of the debris disk around Beta Pictoris obtained with HST's Advanced Camera for Surveys. We confirm that the previously reported warp in the inner disk is a distinct secondary disk inclined by ~5 deg from the main disk. The main disk's northeast extension is linear from 80 to 250 AU, but the southwest extension is distinctly bowed with an amplitude of ~1 AU over the same region. Both extensions of the secondary disk appear linear, but not collinear, from 80 to 150 AU. Within ~120 AU of the star, the main disk is ~50% thinner than previously reported. The surface-brightness profiles along the spine of the main disk are fitted with four distinct radial power laws between 40 and 250 AU, while those of the secondary disk between 80 and 150 AU are fitted with single power laws. These discrepancies suggest that the two disks have different grain compositions or size distributions. The F606W/F435W and F814W/F435W flux ratios of the composite disk are nonuniform and asymmetric about both projected axes of the disk. Within ~120 AU, the m_F435W-m_F606W and m_F435W-m_F814W colors along the spine of the main disk are ~10% and ~20% redder, respectively, than those of Beta Pic. These colors increasingly redden beyond ~120 AU, becoming 25% and 40% redder, respectively, than the star at 250 AU. We compare the observed red colors within ~120 AU with the simulated colors of non-icy grains having a radial number density ~r^-3 and different compositions, porosities, and minimum grain sizes. The observed colors are consistent with those of compact or moderately porous grains of astronomical silicate and/or graphite with sizes >0.15-0.20 um, but the colors are inconsistent with the blue colors expected from grains with porosities >90%., Comment: 38 pages (including 21 figures and 4 tables) in EmulateApJ format, accepted for publication in The Astronomical Journal. Full-resolution figures and fully processed FITS images (with error maps) are available at http://acs.pha.jhu.edu/~dag/betapic Version 2: Added 4 references and some clarifying text. Basic facts and conclusions are unchanged

A five square arcminute region around the luminous radio-loud quasar SDSS J0836+0054 (z=5.8) hosts a wealth of associated galaxies, characterized by very red (1.3 < i_775 - z_{850} < 2.0) color. The surface density of these z~5.8 candidates is approximately six times higher than the number expected from deep ACS fields. This is one of the highest galaxy overdensities at high redshifts, which may develop into a group or cluster. We also find evidence for a substructure associated with one of the candidates. It has two very faint companion objects within two arcseconds, which are likely to merge. The finding supports the results of a recent simulation that luminous quasars at high redshifts lie on the most prominent dark-matter filaments and are surrounded by many fainter galaxies. The quasar activity from these regions may signal the buildup of a massive system., Comment: Four figures. The Astrophysical Journal: in press

We study the dynamical origin of the structures observed in the scattered-light images of the resolved debris disk around HD 141569A. We explore the roles of radiation pressure from the central star, gas drag from the gas disk, and the tidal forces from two nearby stars in creating and maintaining these structures. We use a simple one-dimensional axisymmetric model to show that the presence of the gas helps confine the dust and that a broad ring of dust is produced if a central hole exists in the disk. This model also suggests that the disk is in a transient, excited dynamical state, as the observed dust creation rate applied over the age of the star is inconsistent with submillimeter mass measurements. We model in two dimensions the effects of a fly-by encounter between the disk and a binary star in a prograde, parabolic, coplanar orbit. We track the spatial distribution of the disk's gas, planetesimals, and dust. We conclude that the surface density distribution reflects the planetesimal distribution for a wide range of parameters. Our most viable model features a disk of initial radius 400 AU, a gas mass of 50 M_earth, and beta = 4 and suggests that the system is being observed within 4000 yr of the fly-by periastron. The model reproduces some features of HD 141569A's disk, such as a broad single ring and large spiral arms, but it does not reproduce the observed multiple spiral rings or disk asymmetries nor the observed clearing in the inner disk. For the latter, we consider the effect of a 5 M_Jup planet in an eccentric orbit on the planetesimal distribution of HD 141569A., Comment: Accepted to ApJ

The Microlensing Planet Finder (MPF) is a proposed Discovery mission that will complete the first census of extrasolar planets with sensitivity to planets like those in our own solar system. MPF will employ a 1.1m aperture telescope, which images a 1.3 sq. deg. field-of-view in the near-IR, in order to detect extrasolar planets with the gravitational microlensing effect. MPF's sensitivity extends down to planets of 0.1 Earth masses, and MPF can detect Earth-like planets at all separations from 0.7AU to infinity. MPF's extrasolar planet census will provide critical information needed to understand the formation and frequency of extra solar planetary systems similar to our own., Comment: To appear in the Proceedings of the SPIE Astronomical Telescopes and Instrumentation Symposium held in Glascow, Scotland, on 21-25 June, 2004. 12 PDF pages

We measure the luminosity function of morphologically selected E/S0 galaxies from $z=0.5$ to $z=1.0$ using deep high resolution Advanced Camera for Surveys imaging data. Our analysis covers an area of $48\Box\arcmin$ (8$\times$ the area of the HDF-N) and extends 2 magnitudes deeper ($I\sim24$ mag) than was possible in the Deep Groth Strip Survey (DGSS). At $0.51.7$, E/S0 galaxies at brighter luminosities ($M_B<-20.1$), but are increasingly different at fainter magnitudes where `blue' galaxies are both smaller and have lower Sersic parameters. Fits of the colors to stellar population models suggest that most E/S0 galaxies have short star-formation time scales ($\tau<1$ Gyr), and that galaxies have formed at an increasing rate from $z\sim8$ until $z\sim2$ after which there has been a gradual decline., Comment: 39 pages, 21 figures, accepted in AJ

The properties of Ultra Compact Dwarf (UCD) galaxy candidates in Abell 1689 (z=0.183) are investigated, based on deep high resolution ACS images. A UCD candidate has to be unresolved, have i<28 (M_V<-11.5) mag and satisfy color limits derived from Bayesian photometric redshifts. We find 160 UCD candidates with 2226.8 mag, the radial and luminosity distribution of the UCD candidates can be explained well by Abell 1689's globular cluster (GC) system. For i<26.8 mag, there is an overpopulation of 15 +/- 5 UCD candidates with respect to the GC luminosity function. For i<26 mag, the radial distribution of UCD candidates is more consistent with the dwarf galaxy population than with the GC system of Abell 1689. The UCD candidates follow a color-magnitude trend with a slope similar to that of Abell 1689's genuine dwarf galaxy population, but shifted fainter by about 2-3 mag. Two of the three brightest UCD candidates (M_V ~ -17 mag) are slightly resolved. At the distance of Abell 1689, these two objects would have King-profile core radii of ~35 pc and r_eff ~300 pc, implying luminosities and sizes 2-3 times those of M32's bulge. Additional photometric redshifts obtained with late type stellar and elliptical galaxy templates support the assignment of these two resolved sources to Abell 1689. Our findings imply that in Abell 1689 there are at least 10 UCDs with M_V<-12.7 mag. Compared to the UCDs in the Fornax cluster they are brighter, larger and have colors closer to normal dwarf galaxies. This suggests that they may be in an intermediate stage of the stripping process. Spectroscopy is needed to definitely confirm the existence of UCDs in Abell 1689., Comment: 29 pages, 10 figures, accepted for publication in the October 2004 issue of the Astronomical Journal

(Abridged) We study the internal color properties of a morphologically selected sample of spheroidal galaxies taken from HST/ACS ERO program of UGC 10214 (``The Tadpole''). By taking advantage of the unprecedented high resolution of the ACS in this very deep dataset we are able to characterize spheroids at sub-arcseconds scales. Using the V_606W and I_814W bands, we construct V-I color maps and extract color gradients for a sample of spheroids at I_814W < 24 mag. We investigate the existence of a population of morphologically classified spheroids which show extreme variation in their internal color properties similar to the ones reported in the HDFs. These are displayed as blue cores and inverse color gradients with respect to those accounted from metallicity variations. Following the same analysis we find a similar fraction of early-type systems (~30%-40%) that show non-homologous internal colors, suggestive of recent star formation activity. We present two statistics to quantify the internal color variation in galaxies and for tracing blue cores, from which we estimate the fraction of non-homogeneous to homogeneous internal colors as a function of redshift up to z<1.2. We find that it can be described as about constant as a function of redshift, with a small increase with redshift for the fraction of spheroids that present strong color dispersions. The implications of a constant fraction at all redshifts suggests the existence of a relatively permanent population of evolving spheroids up to z~1. We discuss the implications of this in the context of spheroidal formation., Comment: Fixed URL for high resolution version. 13 Pages, 10 Figures. Accepted for Publication in ApJ. Sep 1st issue. Higher resolution version and complete table3B at http://acs.pha.jhu.edu/~felipe/e-prints/Tadpole

We have discovered three globular clusters beyond the Holmberg radius in Hubble Space Telescope Advanced Camera for Surveys images of the gas-rich dark matter dominated blue compact dwarf galaxy NGC2915. The clusters, all of which start to resolve into stars, have M_{V606} = -8.9 to -9.8 mag, significantly brighter than the peak of the luminosity function of Milky Way globular clusters. Their colors suggest a metallicity [Fe/H] ~ -1.9 dex, typical of metal-poor Galactic globular clusters. The specific frequency of clusters is at a minimum normal, compared to spiral galaxies. However, since only a small portion of the system has been surveyed it is more likely that the luminosity and mass normalized cluster content is higher, like that seen in elliptical galaxies and galaxy clusters. This suggests that NGC2915 resembles a key phase in the early hierarchical assembly of galaxies - the epoch when much of the old stellar population has formed, but little of the stellar disk. Depending on the subsequent interaction history, such systems could go on to build-up larger elliptical galaxies, evolve into normal spirals, or in rare circumstances remain suspended in their development to become systems like NGC2915., Comment: ApJ Letters accepted; 6 pages, 2 figures, 3 tables

We present the analysis of the faint galaxy population in the Advanced Camera for Surveys (ACS) Early Release Observation fields VV 29 (UGC 10214) and NGC 4676. Here we attempt to thoroughly consider all aspects relevant for faint galaxy counting and photometry, developing methods which are based on public software and that are easily reproducible by other astronomers. Using simulations we determine the best SExtractor parameters for the detection of faint galaxies in deep HST observations, paying special attention to the issue of deblending, which significantly affects the normalization and shape of the number count distribution. We confirm, as claimed by Bernstein, Freedman and Madore (2002), that Kron-like magnitudes, such as the ones generated by SExtractor, can miss more than half of the light of faint galaxies, what dramatically affects the slope of the number counts. We present catalogs for the VV 29 and NGC 4676 fields with photometry in the g,V and I bands. We also show that combining the bayesian software BPZ with superb ACS data and new spectral templates enables us to estimate reliable photometric redshifts for a significant fraction of galaxies with as few as three filters. After correcting for selection effects, we measure slopes of 0.32+- 0.01 for 22 < g < 28, 0.34+-0.01 for 22< V <27.5 and 0.33+-0.01 for 22 < I < 27. The counts do not flatten (except perhaps in the g-band), up to the depth of our observations. We find that the faint counts m_{AB}> 25.5 can be well approximated in all our filters by a passive luminosity evolution model based on the COMBO-17 luminosity function (\alpha=-1.5), with a strong merging rate following the prescription of Glazebrook et al. (1994), \phi^*\propto (1+Qz), with Q=4., Comment: 21 pages, 20 figures, to appear in ApJS. Several figures heavily degraded to fit astro-ph size limit. Please download full resolution version from http://acs.pha.jhu.edu/~txitxo/FaintGalaxies.ps.gz

Using an i-z dropout criterion, we determine the space density of z~6 galaxies from two deep ACS GTO fields with deep optical-IR imaging. A total of 23 objects are found over 46 arcmin^2, or ~0.5 objects/arcmin^2 down to z~27.3 (6 sigma; all AB mag) (including one probable z~6 AGN). Combining deep ISAAC data for our RDCS1252-2927 field (J~25.7 and Ks~25.0 (5 sigma)) and NICMOS data for the HDF North (JH~27.3 (5 sigma)), we verify that these dropouts have flat spectral slopes. i-dropouts in our sample range in luminosity from ~1.5 L* (z~25.6) to ~0.3 L* (z~27.3) with the exception of one very bright candidate at z~24.2. The half-light radii vary from 0.09" to 0.29", or 0.5 kpc to 1.7 kpc. We derive the z~6 rest-frame UV luminosity density using three different procedures, each utilizing simulations based on a CDF South V dropout sample. First, we compare our findings with a no-evolution projection of this V-dropout sample. We find 23+/-25% more i-dropouts than we predict. Adopting previous results to z~5, this works out to a 20+/-29% drop in the luminosity density from z~3 to z~6. Second, we use these same V-dropout simulations to derive a selection function for our i-dropout sample and compute the UV-luminosity density (7.2+/-2.5 x 10^25 ergs/s/Hz/Mpc^3 down to z~27). We find a 39+/-21% drop over the same redshift range. This is our preferred value and suggests a star formation rate of 0.0090+/-0.0031 M_sol/yr/Mpc^3 to z~27, or ~0.036+/- 0.012 M_sol/yr/Mpc^3 extrapolating the LF to the faint limit. Third, we follow a very similar procedure, but assume no incompleteness, finding a luminosity density which is ~2-3X lower. This final estimate constitutes a lower limit. All three estimates are within the canonical range of luminosity densities necessary for reionization of the universe at this epoch. (abridged), Comment: 36 pages, 13 figures, 2 tables, accepted for publication in ApJ, postscript version with high-resolution figures can be downloaded at http://www.ucolick.org/~bouwens/idropout.ps

Multicolor coronagraphic images of the circumstellar disk around HD141569A have been obtained with the Hubble Space Telescope (HST) Advanced Camera for Surveys. B, V, and I images show that the disk's previously-described multiple-ring structure is actually a continuous distribution of dust with a tightly-wound spiral structure. Two, more open spiral arms extend from the disk, one of which appears to reach the nearby binary star HD141569BC. Diffuse dust is seen up to 1200AU from HD141569A. Although planets may exist in the inner region of the disk, tidal interaction with HD141569BC seems more likely to be the cause of these phenomena. The disk appears redder than the star (B-V = 0.21 and V-I = 0.25), and its color is spatially uniform. A scattering asymmetry factor of g = 0.25-0.35 is derived. The azimuthal density distribution is asymmetric, varying by a factor of ~3 at some radii., Comment: 13 pages, 9 figures, accepted in the Astronomical Journal

We present observations of the first two supernovae discovered with the recently installed Advanced Camera for Surveys (ACS) on the Hubble Space Telescope. The supernovae were found in Wide Field Camera images of the Hubble Deep Field North taken with the F775W, F850LP, and G800L optical elements as part of the ACS guaranteed time observation program. Spectra extracted from the ACS G800L grism exposures confirm that the objects are Type Ia supernovae (SNe Ia) at redshifts z=0.47 and z=0.95. Follow-up HST observations have been conducted with ACS in F775W and F850LP and with NICMOS in the near-infrared F110W bandpass, yielding a total of 9 flux measurements in the 3 bandpasses over a period of 50 days in the observed frame. We discuss many of the important issues in doing accurate photometry with the ACS. We analyze the multi-band light curves using two different fitting methods to calibrate the supernovae luminosities and place them on the SNe Ia Hubble diagram. The resulting distances are consistent with the redshift-distance relation of the accelerating universe model, although evolving intergalactic grey dust remains as a less likely possibility. The relative ease with which these SNe Ia were found, confirmed, and monitored demonstrates the potential ACS holds for revolutionizing the field of high-redshift SNe Ia, and therefore of testing the accelerating universe cosmology and constraining the "epoch of deceleration"., Comment: 11 pages, 8 embedded figures. Accepted for publication in ApJ

We report on Hubble Space Telescope WFPC2 optical and STIS near ultraviolet MAMA observations, and ground-based optical observations of GRB010222, spanning 15 hrs to 71 days. The observations are well-described by a relativistic blast-wave model with a hard electron-energy distribution, p = 1.57, and a jet transition at t_j=0.93 days. These values are slightly larger than previously found as a result of a correction for the contribution from the host galaxy to the late-time ground-based observations and the larger temporal baseline provided by the Hubble Space Telescope observations. The host galaxy is found to contain a very compact core (size <0.25 arcsec) which coincides with the position of the optical transient. The STIS near ultraviolet MAMA observations allow for an investigation of the extinction properties along the line of sight to GRB010222. We find that the far ultraviolet curvature component (c_4) is rather large. In combination with the low optical extinction A_V =0.11 mag, when compared to the Hydrogen column inferred from X-ray observations, we suggest that this is evidence for dust destruction., Comment: ApJ, in press

We present the first Advanced Camera for Surveys (ACS) observations of young star clusters in the colliding/merging galaxy UGC 10214. The observations were made as part of the Early Release Observation (ERO) program for the newly installed ACS during service mission SM3B for the Hubble Space Telescope (HST). Many young star clusters can be identified in the tails of UGC 10214, with ages ranging from ~3 Myr to 10 Myr. The extreme blue V-I (F606W-F814W) colors of the star clusters found in the tail of UGC 10214 can only be explained if strong emission lines are included with a young stellar population. This has been confirmed by our Keck spectroscopy of some of these bright blue stellar knots. The most luminous and largest of these blue knots has an absolute magnitude of M_V = -14.45, with a half-light radius of 161 pc, and if it is a single star cluster, would qualify as a super star cluster (SSC). Alternatively, it could be a superposition of multiple scaled OB associations or clusters. With an estimated age of ~ 4-5 Myr, its derived mass is < 1.3 x 10^6 solar masses. Thus the young stellar knot is unbound and will not evolve into a normal globular cluster. The bright blue clusters and associations are much younger than the dynamical age of the tail, providing strong evidence that star formation occurs in the tail long after it was ejected. UGC 10214 provides a nearby example of processes that contributed to the formation of halos and intra-cluster media in the distant and younger Universe., Comment: 6 pages with embedded figures, ApJ in press

We present the discovery of GRB 020405 made with the Inter-Planetary Network (IPN). With a duration of 60 s, the burst appears to be a typical long duration event. We observed the 75-square acrminute IPN error region with the Mount Stromlo Observatory's 50-inch robotic telescope and discovered a transient source which subsequently decayed and was also associated with a variable radio source. We identify this source as the afterglow of GRB 020405. Subsequent observations by other groups found varying polarized flux and established a redshift of 0.690 to the host galaxy. Motivated by the low redshift we triggered observations with WFPC2 on-board the Hubble Space Telescope (HST). Modeling the early ground-based data with a jet model, we find a clear red excess over the decaying optical lightcurves that is present between day 10 and day 141 (the last HST epoch). This `bump' has the spectral and temporal features expected of an underlying supernova (SN). In particular, the red color of the putative SN is similar to that of the SN associated with GRB 011121, at late time. Restricting the sample of GRBs to those with z<0.7, a total of five bursts, red bumps at late times are found in GRB 970228, GRB 011121, and GRB 020405. It is possible that the simplest idea, namely that all long duration GRBs have underlying SNe with a modest dispersion in their properties (especially peak luminosity), is sufficient to explain the non detections., Comment: (Fixing HST data table; thanks to N. Masetti.) 18 pages, 3 figures (2 in colour). Accepted to appear in the Astrophysical Journal

We present ground-based optical observations of GRB 020124 starting 1.6 hours after the burst, as well as subsequent Very Large Array (VLA) and Hubble Space Telescope (HST) observations. The optical afterglow of GRB 020124 is one of the faintest afterglows detected to date, and it exhibits a relatively rapid decay, $F_\nu\propto t^{-1.60\pm 0.04}$, followed by further steepening. In addition, a weak radio source was found coincident with the optical afterglow. The HST observations reveal that a positionally coincident host galaxy must be the faintest host to date, R>29.5 mag. The afterglow observations can be explained by several models requiring little or no extinction within the host galaxy, A_V~0-0.9 mag. These observations have significant implications for the statistics of the so-called dark bursts (bursts for which no optical afterglow is detected), which are usually attributed to dust extinction within the host galaxy. The faintness and relatively rapid decay of the afterglow of GRB 020124, combined with the low inferred extinction indicate that some dark bursts are intrinsically dim and not dust obscured. Thus, the diversity in the underlying properties of optical afterglows must be observationally determined before substantive inferences can be drawn from the statistics of dark bursts., Comment: Submitted to ApJ; 13 pages; 4 tables; 5 figures

GRB 010921 was the first HETE-2 GRB to be localized via its afterglow emission. The low-redshift of the host galaxy, z=0.451, prompted us to undertake intensive multi-color observations with the Hubble Space Telescope with the goal of searching for an underlying supernova component. We do not detect any coincident supernova to a limit 1.34 mag fainter than SN 1998bw at 99.7% confidence, making this one of the most sensitive searches for an underlying SN. Analysis of the afterglow data allow us to infer that the GRB was situated behind a net extinction (Milky Way and the host galaxy) of A_V ~ 1.8 mag in the observer frame. Thus, had it not been for such heavy extinction our data would have allowed us to probe for an underlying SN with brightness approaching those of more typical Type Ib/c supernovae., Comment: 18 pages, 1 figure. Submitted to Astrophysical Journal Letters

Using observations from an extensive monitoring campaign with the Hubble Space Telescope we present the detection of an intermediate-time flux excess which is redder in color relative to the afterglow of GRB 011121, currently distinguished as the gamma-ray burst with the lowest known redshift. The red ``bump,'' which exhibits a spectral roll-over at ~7200 Angstrom, is well described by a redshifted Type Ic supernova that occurred approximately at the same time as the gamma-ray burst event. The inferred luminosity is about half that of the bright supernova 1998bw. These results serve as compelling evidence for a massive star origin of long-duration gamma-ray bursts. Models that posit a supernova explosion weeks to months preceding the gamma-ray burst event are excluded by these observations. Finally, we discuss the relationship between spherical core-collapse supernovae and gamma-ray bursts., Comment: Published in the Astrophysical Journal (Letters) on 20 May 2002. Seven LaTeX pages, three Postscript figures, one table

We present Hubble Space Telescope (HST) and ground-based observations of a pair of galaxies at redshift 2.38, which are collectively known as 2142-4420 B1 (Francis et al. 1996). The two galaxies are both luminous extremely red objects (EROs), separated by 0.8 arcsec. They are embedded within a 100 kpc scale diffuse Ly-alpha nebula (or blob) of luminosity ~10^44 erg/s. The radial profiles and colors of both red objects are most naturally explained if they are young elliptical galaxies: the most distant yet found. It is not, however, possible to rule out a model in which they are abnormally compact, extremely dusty starbursting disk galaxies. If they are elliptical galaxies, their stellar populations have inferred masses of ~10^11 solar masses and ages of ~7x10^8 years. Both galaxies have color gradients: their centers are significantly bluer than their outer regions. The surface brightness of both galaxies is roughly an order of magnitude greater than would be predicted by the Kormendy relation. A chain of diffuse star formation extending 1 arcsec from the galaxies may be evidence that they are interacting or merging. The Ly-alpha nebula surrounding the galaxies shows apparent velocity substructure of amplitude ~ 700 km/s. We propose that the Ly-alpha emission from this nebula may be produced by fast shocks, powered either by a galactic superwind or by the release of gravitational potential energy., Comment: 33 pages, 9 figures, ApJ in press (to appear in Jun 10 issue)

We present multi-color light-curves of the optical afterglow of GRB 000926. Beginning ~1.5 days after the burst, the light-curves of this GRB steepen measurably. The existence of such achromatic breaks are usually taken to be an important observational signature that the ejecta are not expanding isotropically, but rather have a collimated jet-like geometry. If we interpret the data in this context, we derive an opening angle of 5 degrees, which reduces the energy release compared to an isotropic model by a factor of ~275, to 1.7 x 10^51 erg. To fit the data with a simple jet model requires extinction along the line of sight. The derived A_V is in the range 0.11 - 0.82 mag, depending on the adopted extinction law and whether the electrons giving rise to the optical emission are undergoing synchrotron cooling or not. Since this is in excess of the expected extinction from our Galaxy, we attribute this to the GRB host. We note that this extinction is typical of a galactic disk, and therefore the event likely took place in the disk of its host., Comment: 4 pages, 2 figures, 2 tables. Accepted for publication in ApJ Letters

Visual and ultraviolet spatially resolved (~ 0."1) spectra of SN 1987A obtained on days 3715 and 3743 with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope show that the high-velocity SN debris is colliding with circumstellar gas. Very broad Ly-alpha emission with velocities extending to ~ +/- 20,000 km/s originates inside the inner circumstellar ring and appears to fill most of the surface area within 0."67 +/- 0."03 (0.14 pc at a distance of 50 kpc) of the ring's center. The observed Ly-alpha flux from the shocked ejecta is (1.85 +/- 0.53) 10^{-13} erg/cm2/s and (1.25 +/- 0.51) 10^{-12} erg/cm2/s after correcting for extinction. A spatially unresolved blue-shifted emission feature was discovered in H-alpha (and other lines) on the inner ring at p.a. 31 +/- 8 degree. The H-alpha emission extends to -250 km/s with no corresponding red-shifted emission. This highly localized interaction appears to be the initial contact of the supernova blast wave with an inward protrusion of the inner ring. The broad Ly-alpha emission and the `hot spot' are separate interaction phemonena associated with the reverse and forward shocks, respectively. We also find that the size of the inner ring in forbidden lines of oxygen has a dependence on ionization potential, in agreement with photoionization models of the ring., Comment: 11 pages (LaTeX, aaspp4.sty), 8 figures, accepted for publication in the Astrophysical Journal Letters HST Second Servicing Mission special issue

The star HD72089 is located behind the Vela supernova remnant and shows a complex array of high and low velocity interstellar absorption features arising from shocked clouds. A spectrum of this star was recorded over the wavelength range 1196.4 to 1397.2 Angstroms at a resolving power lambda/Delta lambda = 110,000 and signal-to-noise ratio of 32 by STIS on the Hubble Space Telescope. We have identified 7 narrow components of C I and have measured their relative populations in excited fine-structure levels. Broader features at heliocentric velocities ranging from -70 to +130 km/s are seen in C II, N I, O I, Si II, S II and Ni II. In the high-velocity components, the unusually low abundances of N I and O I, relative to S II and Si II, suggest that these elements may be preferentially ionized to higher stages by radiation from hot gas immediately behind the shock fronts., Comment: 11 pages, 2 figures, Latex. Submitted for the special HST ERO issue of the Astrophysical Journal Letters

We present new FUV/UV observations of the DA white dwarf Wolf 1346 obtained with the Hopkins Ultraviolet Telescope. The atmospheric parameters of this object are estimated from a fit of model atmospheres to several optical spectra to be Teff = 20000 K, log g = 7.90. From the optical spectrum this star is a normal DA without any indications for chemical elements other than hydrogen. The hydrogen line L beta, however, shows a very unusual shape, with a steep red wing and two absorption features on this wing. The shape is reminiscent of the effects of quasi-molecular line broadening, as observed in L alpha in cooler DA white dwarfs. We show that this is indeed the correct explanation, by identifying 4 quasi-molecular satellites caused through perturbations by the H+ ion (H2+ quasi-molecule). The steep red wing is caused by the exponential decline of the line profile beyond the satellite most distant from the line center at 1078 A., Comment: 11 pages Latex with aaspp4 style, 4 postscript figures, as compressed tar file, ApJ Letters, in press