Michael P. Lesser

The Morphology-Density Relation in z ~ 1 Clusters

We measure the morphology-density relation (MDR) and morphology-radius relation (MRR) for galaxies in seven z ~ 1 clusters that have been observed with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope. Simulations and independent comparisons of our visually derived morphologies indicate that ACS allows one to distinguish between E, S0, and spiral morphologies down to z850 = 24, corresponding to L/L* = 0.21 and 0.30 at z = 0.83 and 1.24, respectively. We adopt density and radius estimation methods that match those used at lower redshift in order to study the evolution of the MDR and MRR. We detect a change in the MDR between 0.8 < z < 1.2 and that observed at z ~ 0, consistent with recent work; specifically, the growth in the bulge-dominated galaxy fraction, fE+S0, with increasing density proceeds less rapidly at z ~ 1 than it does at z ~ 0. At z ~ 1 and Σ ≥ 500 galaxies Mpc-2, we find fE+S0 = 0.72 ± 0.10. At z ~ 0, an E+S0 population fraction of this magnitude occurs at densities about 5 times smaller. The evolution in the MDR is confined to densities Σ 40 galaxies Mpc-2 and appears to be primarily due to a deficit of S0 galaxies and an excess of Sp+Irr galaxies relative to the local galaxy population. The fE-density relation exhibits no significant evolution between z = 1 and 0. We find mild evidence to suggest that the MDR is dependent on the bolometric X-ray luminosity of the intracluster medium. Implications for the evolution of the disk galaxy population in dense regions are discussed in the context of these observations.

FAINT GALAXIES IN DEEP ADVANCED CAMERA FOR SURVEYS OBSERVATIONS

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 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.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. These observations cover a total area of 26.3 arcmin 2 and have depths close to that of the Hubble Deep Fields in the deepest part of the VV 29 image, with 10 � detection limits for point sources of 27.8, 27.6, and 27.2 AB magnitudes in the g F475W , VF606W ,a ndIF814W bands, respectively. Measuring the faint galaxy number count distribution is a difficult task, with different groups arriving at widely varying results even on the same data set. Here we attempt to thoroughly consider all aspects relevant for faint galaxy counting and photometry, developing methods that 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 Hubble Space Telescope 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, & Madore, that Kron-like magnitudes, such as the ones generated by SExtractor, can miss more than half of the light offaint galaxies, what dramatically affects the slope of the number counts. We show how to correct for this effect, which depends sensitively not only on the characteristics of the observations, but also on the choice of SExtractor parameters. We present catalogs for the VV 29 and NGC 4676 fields with photometry in the F475W, F606W, and F814W 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 � 0:01for 22 25:5 can be well approximated in all our filters by a passive luminosity evolution model based on the COMBO-17 luminosity function (� ¼� 1:5), with a strong merging rate following the prescription of

Advanced camera for the Hubble Space Telescope

The Advanced Camera for the Hubble Space Telescope has three cameras. The first, the Wide Field Camera, will be a high- throughput, wide field, 4096 X 4096 pixel CCD optical and I-band camera that is half-critically sampled at 500 nm. The second, the High Resolution Camera (HRC), is a 1024 X 1024 pixel CCD camera that is critically sampled at 500 nm. The HRC has a 26 inch X 29 inch field of view and 29 percent throughput at 250 nm. The HRC optical path includes a coronagraph that will improve the HST contrast near bright objects by a factor of approximately 10 at 900 nm. The third camera, the solar-blind camera, is a far-UV, pulse-counting array that has a relatively high throughput over a 26 inch X 29 inch field of view. The advanced camera for surveys will increase HSTs capability for surveys and discovery by a factor of approximately 10 at 800 nm.

Hubble Space Telescope ACS Coronagraphic Imaging of the Circumstellar Disk around HD 141569A

Multicolor coronagraphic images of the circumstellar disk around HD 141569A have been obtained with the Hubble Space Telescope Advanced Camera for Surveys. The B, V, and I images show that the disks previously described multiple-ring structure is actually a continuous distribution of dust with a tightly wound spiral structure. Extending from the disk are two, more open spiral arms, one of which appears to reach the nearby binary star HD 141569BC. Diffuse dust is seen up to 1200 AU from HD 141569A. Although planets may exist in the inner region of the disk, tidal interaction with HD 141569BC 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.

STAR FORMATION AT z ~ 6: i-DROPOUTS IN THE ADVANCED CAMERA FOR SURVEYS GUARANTEED TIME OBSERVATION FIELDS

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 arcmin2, or ~0.5 ± 0.1 objects arcmin-2 down to zAB ~ 27.3 (6 σ), or a completeness-corrected ~0.5 ± 0.2 objects arcmin-2 down to zAB ~ 26.5 (including one probable z ~ 6 active galactic nucleus). Combining deep ISAAC data for our RDCS 1252-2927 field (JAB ~ 25.7 and Ks,AB ~ 25.0; 5 σ) and NICMOS data for the Hubble Deep Field-North (J110,AB and H160,AB ~ 27.3, 5 σ), we verify that these dropouts have relatively flat spectral slopes, as one would expect for star-forming objects at z ~ 6. Compared with the average-color (β = -1.3) U-dropout in the Steidel et al. z ~ 3 sample, i-dropouts in our sample range in luminosity from ~1.5L* (zAB ~ 25.6) to ~0.3L* (zAB ~ 27.3) with the exception of one very bright candidate at z850,AB ~ 24.2. The half-light radii vary from 009 to 021, or 0.5 kpc to 1.3 kpc. We derive the z ~ 6 rest-frame UV luminosity density (or star formation rate density) by using three different procedures. All three procedures use simulations based on a slightly lower redshift (z ~ 5) V606-dropout sample from Chandra Deep Field-South ACS images. First, we make a direct comparison of our findings with a no-evolution projection of this V-dropout sample, allowing us to automatically correct for the light lost at faint magnitudes or lower surface brightnesses. We find 23% ± 25% more i-dropouts than we predict, consistent with no strong evolution over this redshift range. Adopting previous results to z ~ 5, this works out to a mere 20% ± 29% drop in the luminosity density from z ~ 3 to z ~ 6. Second, we use the same V-dropout simulations to derive a detailed selection function for our i-dropout sample and compute the UV-luminosity density [(7.2 ± 2.5) × 1025 ergs s-1 Hz-1 Mpc-3 down to zAB ~ 27]. We find a 39% ± 21% drop over the same redshift range (z ~ 3-6), consistent with the first estimate. This is our preferred value and suggests a star formation rate of 0.0090 ± 0.0031 M☉ yr-1 Mpc-3 to zAB ~ 27, or ~0.036 ± 0.012 M☉ yr-1 Mpc-3 by extrapolating the luminosity function to the faint limit, assuming α = -1.6. Third, we follow a very similar procedure, except that we assume no incompleteness, and find a rest-frame continuum luminosity that is ~2-3 times lower than our other two determinations. This final estimate is to be taken as a lower limit and is important if there are modest changes in the colors or surface brightnesses from z ~ 5 to z ~ 6 (the other estimates assume no large changes in the intrinsic selectability of objects). We note that all three estimates are well within the canonical range of luminosity densities necessary for reionization of the universe at this epoch by star-forming galaxies.

HUBBLE SPACE TELESCOPE ADVANCED CAMERA FOR SURVEYS CORONAGRAPHIC IMAGING OF THE AU MICROSCOPII DEBRIS DISK

We present Hubble Space Telescope Advanced Camera for Surveys multicolor coronagraphic images of the recently discovered edge-on debris disk around the nearby (� 10 pc) M dwarf AU Microscopii. The disk is seen between r ¼ 0B75 and 15 00 (7.5–150 AU) from the star. It has a thin midplane with a projected FWHM thickness of 2.5–3.5 AU within r < 50 AU of the star that increases to 6.5–9 AU at r � 75 AU. The disk’s radial brightness profile is generally flat forr < 15 AU, then decreases gradually (I / r � 1:8 )o ut tor � 43 AU, beyond which it falls rapidly (I / r � 4:7 ). Within 50 AU the midplane is straight and aligned with the star, and beyond that it deviates by � 3 � , resulting in a bowed appearance that was also seen in ground-based images. Three-dimensional modeling of the disk shows that the inner region (r < 50 AU) is inclined to the line of sight by less than 1 � and the outer disk by � 3 � . The inclination of the outer disk and moderate forward scattering (g � 0:4) can explain the apparentbow. The intrinsic, deprojected FWHM thickness is 1.5–10 AU, increasing with radius. The models indicate that the disk is clear of dust within � 12 AU of the star, in general agreement with the previous prediction of 17 AU based on the infrared spectral energy distribution. The disk is blue, being 60% brighter at B than I relative to the star. One possible explanation for this is that there is a surplus of very small grains compared with other imaged debris disks that have more neutral or red colors. This may be due to the low radiation pressure exerted by the late-type star. Observations at two epochs show that an extended source seen along the midplane is a background galaxy.

Evolution of the Color-Magnitude Relation in High-Redshift Clusters: Blue Early-Type Galaxies and Red Pairs in RDCS J0910+5422

The color-magnitude relation has been determined for the RDCS J0910+5422 cluster of galaxies at redshift z = 1.106. Cluster members were selected from HST Advanced Camera for Surveys (ACS) images, combined with ground–based near–IR imaging and optical spectroscopy. Postman et al. (2005) morphological classifications were used to identify the early-type galaxies. The observed early–type color–magnitude relation (CMR) in (i775 z850) versus z850 shows an intrinsic scatter in color of 0.060±0.009 mag, within 1 ′ from the cluster X–ray emission center. Both the ellipticals and the S0s show small scatter about the CMR of 0.042 ± 0.010 mag and 0.044± 0.020 mag, respectively. From the scatter about the CMR, a mean luminosity–weighted age t > 3.3 Gyr (zf > 3) is derived for the elliptical galaxies, assuming a simple stellar population modeling (single burst solar metallicity). This is consistent with a previous study of the cluster RDCS1252.9-292 at z=1.24 (Blakeslee et al.). Strikingly, the S0 galaxies in RDCS J0910+5422 are systematically bluer in (i775 z850) by 0.07 ± 0.02 mag, with respect to the ellipticals. The blue S0s are predominantly elongated in shape; the distribution of their ellipticities is inconsistent with a population of axisymmetric disk galaxies viewed at random orientations, suggesting either that they are intrinsically prolate or there is some orientation bias in the S0 classification. The ellipticity distribution as a function of color indicates that the face-on S0s in this particular cluster have likely been classified as elliptical. Thus, if anything, the offset in color between the elliptical and S0 populations may be even more significant. The color offset between S0 and E corresponds to an age difference of � 1 Gyr, for a singleburst solar metallicity model. Alternatively, it could be the result of a different star formation history; a solar metallicity model with an exponential decay in star formation will reproduce the offset for an age of 3.5 Gyr, i.e. the S0s have evolved gradually from star forming progenitors. The color offset could also be reproduced by a factor of �2 difference in metallicity, but the two populations would each need to have very small scatter in metallicity to reproduce the small scatter in color. The early–type population in this cluster appears to be still forming. The blue early-type disk galaxies in RDCS J0910+5422 likely represent the direct progenitors of the more evolved S0s that follow the same red sequence as ellipticals in other clusters. Thirteen red galaxy pairs are observed and the galaxies associated in pairs constitute �40% of the CMR galaxies in this cluster. This finding is consistent with the conclusions of van Dokkum and Tran et al. that most of the early–type galaxies grew from passive red mergers.

ADVANCED CAMERA FOR SURVEYS OBSERVATIONS OF YOUNG STAR CLUSTERS IN THE INTERACTING GALAXY UGC 10214

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 � 3t o 10 Myr. The extreme blue VI (F606WF814W) 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 MV ¼� 14:45, with a half-light radius of 161 pc, and if it is a single star clus- ter, it 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 less than 1:3 � 10 6 M� . Thus, the young stellar knot is unbound and will not evolve into a normal globular cluster. The bright blue clus- ters 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 intracluster media in the distant and younger universe. Subject heading: galaxies: individual (Arp 188, UGC 10214, VV 29) — galaxies: star clusters

Fundamental performance differences between CMOS and CCD imagers: Part II

In depth characterization of CMOS arrays is unveiling many characteristics not observed in CCD imagers. This paper is the first of a series of papers that will discuss unique CMOS characteristics related to fundamental performance differences between CMOS and CCD imagers with emphasis on scientific arrays. The first topic will show that CMOS read noise is ultimately limited by a phenomenon referred to as random telegraph signal (RTS) noise. RTS theory and experimental data discuss its creation, time and frequency domain characteristics, wide variance from pixel to pixel and magnitude on the sensors overall read noise floor. Specific operating parameters that control and lower RTS noise are identified. It is shown how correlated double sampling (CDS) signal processing responds to RTS noise and demonstrate that sub electron CMOS read noise performance is possible. The paper also discusses CMOS sensitivity (V/e-) nonlinearity, an effect not familiar to CCD users. The problem plays havoc on conventional photon transfer analysis that leads to serious measurement errors. New photon transfer relations are developed to deal with the problem. Nonlinearity for custom CMOS pixels is shown to be beneficial for lowering read noise and extending dynamic range. The paper closes with a section on the high performance CMOS array used to generated data products presented.

The Transformation of Cluster Galaxies at Intermediate Redshift

We combine imaging data from the Advanced Camera for Surveys (ACS) with VLT/FORS optical spectroscopy to study the properties of star-forming galaxies in the z = 0.837 cluster Cl 0152-1357. We have morphological information for 24 star-forming cluster galaxies, which range in morphology from late-type and irregular to compact early-type galaxies. We find that while most star-forming galaxies have r625 - i775 colors bluer than 1.0, eight are in the red cluster sequence. Among the star-forming cluster population, we find five compact early-type galaxies that have properties consistent with their identification as progenitors of dwarf elliptical galaxies. The spatial distribution of the star-forming cluster members is nonuniform. We find none within R ~ 500 Mpc of the cluster center, which is highly suggestive of an intracluster medium interaction. We derive star formation rates from [O II] λ3727 line fluxes and use these to compare the global star formation rate of Cl 0152-1357 to other clusters at low and intermediate redshifts. We find a tentative correlation between integrated star formation rates and TX, in the sense that hotter clusters have lower integrated star formation rates. Additional data from clusters with low X-ray temperatures are needed to confirm this trend. We do not find a significant correlation with redshift, suggesting that evolution is either weak or absent between z = 0.2 and 0.8.