Search

Exoplanets orbiting M dwarf stars are a prime target in the search for life in the Universe. M dwarf stars are active, with powerful flares that could adversely impact prospects for life, though there are counter-arguments. Here, we turn flaring to advantage and describe ways in which it can be used to enhance the detectability of planets, in the absence of transits or a coronagraph, significantly expanding the accessible discovery and characterization space. Flares produce brief bursts of intense luminosity, after which the star dims. Due to the light travel time between the star and planet, the planet receives the high intensity pulse, which it re-emits through scattering (a light echo) or intrinsic emission when the star is much fainter, thereby increasing the planet's detectability. The planet's light echo emission can potentially be discriminated from that of the host star by means of a time delay, Doppler shift, spatial shift, and polarization, each of which can improve the contrast of the planet to the star. Scattered light can reveal the albedo spectrum of the planet to within a size scale factor, and is likely to be polarized. Intrinsic emission mechanisms include fluorescent pumping of multiple molecular hydrogen and neutral oxygen lines by intense LyAlpha and LyBeta flare emission, recombination radiation of ionized and photodissociated species, and atmospheric processes such as terrestrial upper atmosphere airglow and near infrared hydroxyl emission. We discuss the feasibility of detecting light echoes and find that under favorable circumstances, echo detection is possible.

We have selected a sample of 11 massive clusters of galaxies observed by the Hubble Space Telescope in order to study the impact of the dynamical state on the intracluster light (ICL) fraction, the ratio of total integrated ICL to the total galaxy member light. With the exception of the Bullet cluster, the sample is drawn from the Cluster Lensing and Supernova Survey and the Frontier Fields program, containing five relaxed and six merging clusters. The ICL fraction is calculated in three optical filters using the CHEFs ICL estimator, a robust and accurate algorithm free of a priori assumptions. We find that the ICL fraction in the three bands is, on average, higher for the merging clusters, ranging between ∼7% and 23%, compared with the ∼2%-11% found for the relaxed systems. We observe a nearly constant value (within the error bars) in the ICL fraction of the regular clusters at the three wavelengths considered, which would indicate that the colors of the ICL and the cluster galaxies are, on average, coincident and, thus, so are their stellar populations. However, we find a higher ICL fraction in the F606W filter for the merging clusters, consistent with an excess of lower-metallicity/younger stars in the ICL, which could have migrated violently from the outskirts of the infalling galaxies during the merger event. © 2018. The American Astronomical Society. All rights reserved., We thank the referee for constructive comments that helped to improve the original manuscript. Y.J.-T. would like to thank Dr. Marc Postman for his support, help, and encouragement during her stay at the STScI, which made this work possible. We gratefully acknowledge the computational support of Dr. Fernando Roig. Y.J.-T. also acknowledges financial support from the Fundacao Carlos Chagas Filho de Amparo a Pesquisa do Estado do Rio de Janeiro (FAPERJ; fellowship Nota 10, PDR-10) through grant E-26/202.835/2016 and the Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior (CAPES; Science without Borders program, Young Talent Fellowship, BJT) through grant A062/2013. R.A.D. acknowledges support from the Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq) through BP grant 312307/2015-2 and the Financiadora de Estudos e Projetos (FINEP) grant REF. 1217/13-01.13.0279.00. Both Y.J.-T. and R.A.D. also acknowledge support from the Spanish National Research Council (CSIC; I-COOP+ 2016 program) through grant COOPB20263 and the Spanish Ministry of Economy, Industry, and Competitiveness (MINECO) through grants AYA2013-48623-C2-1-P and AYA2016-81065-C2-1-P. K.U. acknowledges support from the Ministry of Science and Technology of Taiwan (grant MOST 103-2628-M-001-003-MY3) and from the Academia Sinica Investigator Award. M.M. acknowledges support from the Italian Ministry of Foreign Affairs and International Cooperation, Directorate General for Country Promotion.

At cosmological distances, gravitational lensing can provide a direct measurement of supermassive black hole (SMBH) masses irrespective of their luminosities. Here, we directly estimate the mass of a SMBH in the brightest cluster galaxy (BCG) of MACS J1149+2223.5 at $z=0.54$ through one of the multiply-lensed images of a background spiral galaxy at $z=1.49$ projected close to the BCG. In this particular image, an intrinsically compact region in one of the spiral arms is lensed into an arc that curves towards the BCG center. This arc has a radius of curvature of only $\sim$0."6, betraying the presence of a local compact deflector. Its curvature is most simply reproduced by a point-like object with a mass of $8.4^{+4.3}_{-1.8}\times10^{9}M_\odot$, similar to SMBH masses in local elliptical galaxies having comparable luminosities. The SMBH is noticeably offset by $4.4\pm0.3$ kpc from the BCG light centre, plausibly the result of a kick imparted $\sim2.0\times10^7$ years ago during the merger of two SMBHs, placing it just beyond the stellar core. A similar curvature can be produced by replacing the offset SMBH with a compact galaxy having a mass of $\sim2\times 10^{10}M_\odot$ within a cutoff radius of $50(M/L)_\odot$ to make it undetectable in the deep Hubble Frontiers Fields image, at or close to the cluster redshift; such a lensing galaxy, however, perturbs the adjacent lensed images in an undesirable way., Published version

We present Space Telescope Imaging Spectrograph emission-line spectra of the central regions of the spiral galaxies NGC 1300 and NGC 2748. From the derived kinematics of the nuclear gas we have found evidence for central supermassive black holes in both galaxies. The estimated mass of the black hole in NGC 1300 is 6.6 (+6.3, -3.2) x 10^7 solar masses and in NGC 2748 is 4.4 (+3.5, -3.6) x 10^7 solar masses (both at the 95% confidence level). These two black hole mass estimates contribute to the poorly sampled low-mass end of the nuclear black hole mass spectrum.

We present accurate photometric redshifts for galaxies observed by the Cluster Lensing And Supernova survey with Hubble (CLASH). CLASH observed 25 massive galaxy cluster cores with the Hubble Space Telescope in 16 filters spanning 0.2-1.7 μm. Photometry in such crowded fields is challenging. Compared to our previously released catalogues, we make several improvements to the photometry, including smaller apertures, intracluster light subtraction, point spread function matching and empirically measured uncertainties. We further improve the Bayesian photometric redshift estimates by adding a redder elliptical template and by inflating the photometric uncertainties of the brightest galaxies. The resulting photometric redshift accuracies are dz/(1+z) ~ 0.8, 1.0 and 2.0 per cent for galaxies with I-band F814W AB magnitudes < 18, 20 and 23, respectively. These results are consistent with our expectations. They improve on our previously reported accuracies by a factor of 4 at the bright end and a factor of 2 at the faint end. Our new catalogue includes 1257 spectroscopic redshifts, including 382 confirmed cluster members. We also provide stellar mass estimates. Finally, we include lensing magnification estimates of background galaxies based on our public lens models. Our new catalogue of all 25 CLASH clusters is available via Mikulski Archive for Space Telescopes. The analysis techniques developed here will be useful in other surveys of crowded fields, including the Frontier Fields and surveys carried out with Javalambre-Physics of the Accelerated Universe Astrophysical Survey and James Webb Space Telescope.© 2017 The Authors., We acknowledge the financial support of the Brazilian funding agency FAPESP (Post-doc fellowship - process number 2014/11806-9). Likewise, we also acknowledge the Spanish Ministerio de Educacion y Ciencia through grant AYA2006-14056 BES-2007-16280. We acknowledge the financial support from the Spanish MICINN under the Consolider-Ingenio 2010 Program grant CSD2006-00070: First Science with the GTC. AM acknowledges the CEFCA (Centro de Estudios de Fisica del Cosmos de Aragon) and the IAA-CSIC (Instituto de Astrofisica de Andalucia) institutes for hosting him during the period 2015/2016. PR, MM, AMe and MN acknowledge the financial support from PRIN-INAF 2014 1.05.01.94.02. Likewise, AMacknowledges the STScI (Space Telescope Science Institute) for hosting him during 2010 AugustSeptember, 2011 June-August and 2016 July. We acknowledge the financial support of ESO since most of the spectroscopic redshifts used in the work were based on ESO VLT programme ID 186.A-0798. BA has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement no. 656354.

[abridged] The Hubble and Spitzer Space Telescope surveys of the Frontier Fields (FF) provide extremely deep images around six massive, strong-lensing clusters of galaxies. The ALMA FF survey aims to cover the same fields at 1.1mm, with maps reaching (unlensed) sensitivities of $$5$\sigma$) sources in the first three FF clusters observed by ALMA, based on data from Hubble and Spitzer, the VLT and Herschel. We measure the total photometry in all available bands and determine the photometric redshifts and the physical properties of the counterparts via SED-fitting. In particular, we carefully estimate the FIR photometry using 1.1mm priors to limit the misidentification of blended FIR counterparts, which strongly affect some flux estimates in previous FIR catalogs. We identify robust near-infrared (NIR) counterparts for all 11 sources with K$_s$ detection, the majority of which are quite red, with eight having $F814W-K_s\gtrsim 4$ and five having $F160W-[4.5]\gtrsim3$. From the FIR point of view, all our objects have $z_{phot}$$\sim$1--3, whereas based on the optical SED one object prefers a high-$z$ solution ($z\geq\ $7). Five objects among our sample have spectroscopic redshifts from the GLASS survey for which we can reproduce their SEDs with existing templates. This verification confirms the validity of our photometric redshift methodology. The mean redshift of our sample is $z_{phot}$=1.99$\pm$0.27. All 1.1mm selected objects are massive (10.0$ \approx$1.6) and high dust contents (8.1 $, Comment: 18 pages, 11 figures, accepted for publication in A&A

We present a new strong lensing analysis of the galaxy cluster MACS J1206.2-0847 (MACS 1206), at z=0.44, using deep spectroscopy from CLASH-VLT and VLT/MUSE archival data in combination with imaging from the Cluster Lensing and Supernova survey with Hubble. MUSE observations enable the spectroscopic identification of 23 new multiply imaged sources, extending the previous compilations by a factor of approximately five. In total, we use the positional measurements of 82 spectroscopic multiple images belonging to 27 families at z=1.0-6.1 to reconstruct the projected total mass distribution of MACS 1206. Remarkably, 11 multiple images are found within 50 kpc of the brightest cluster galaxy, making this an unprecedented set of constraints for the innermost projected mass distribution of a galaxy cluster. We thus find that, although dynamically relaxed, the smooth matter component (dark matter plus hot gas) of MACS 1206 shows a significant asymmetry, which closely follows the asymmetric distribution of the stellar component (galaxy members and intracluster light). We determine the value of the innermost logarithmic slope of the projected total mass density profile and find it to be close to the canonical Navarro-Frenk-White value. We demonstrate that this quantity is very robust against different parametrizations of the diffuse mass component; however, this is not the case when only one central image is used in the mass reconstruction. We also show that the mass density profile from our new strong lensing model is in very good agreement with dynamical and X-ray measurements at larger radii, where they overlap., 22 pages, 9 figures, 3 tables, A&A in press, strong lensing model publicly available at http://www.fe.infn.it/u/gbcaminha/M1206_Caminha17

We search for high-redshift dropout galaxies behind the Hubble Frontier Fields (HFF) galaxy cluster MACS J1149.5+2223, a powerful cosmic lens that has revealed a number of unique objects in its field. Using the deep images from the Hubble and Spitzer space telescopes, we find 11 galaxies at z>7 in the MACS J1149.5+2223 cluster field, and 11 in its parallel field. The high-redshift nature of the bright z~9.6 galaxy MACS1149-JD, previously reported by Zheng et al., is further supported by non-detection in the extremely deep optical images from the HFF campaign. With the new photometry, the best photometric redshift solution for MACS1149-JD reduces slightly to z=9.44 +/- 0.12. The young galaxy has an estimated stellar mass of (7 +/- 2)X10E8 Msun, and was formed at z=13.2 +1.9-1.6 when the universe was ~300 Myr old. Data available for the first four HFF clusters have already enabled us to find faint galaxies to an intrinsic magnitude of M(UV) ~ -15.5, approximately a factor of ten deeper than the parallel fields., To appear in the Astrophysical Journal. 20 pages, 6 figures, 3 tables. arXiv admin note: text overlap with arXiv:1402.6743

We present multicolour Hubble Space Telescope images of the powerful z=2.4 radio galaxy MRC 0406-244 and model its complex morphology with several components including a host galaxy, a point source, and extended nebular and continuum emission. We suggest that the main progenitor of this radio galaxy was a normal, albeit massive (M ~10^{11} solar masses), star-forming galaxy. The optical stellar disc of the host galaxy is smooth and well described by a S\'ersic profile, which argues against a recent major merger, however there is also a point-source component which may be the remnant of a minor merger. The half-light radius of the optical disc is constrained to lie in the range 3.5 to 8.2kpc, which is of similar size to coeval star forming galaxies. Biconical shells of nebular emission and UV-bright continuum extend out from the host galaxy along the radio jet axis, which is also the minor axis of the host galaxy. The origin of the continuum emission is uncertain, but it is most likely to be young stars or dust-scattered light from the AGN, and it is possible that stars are forming from this material at a rate of 200^{+1420}_{-110} solar masses per year., Comment: Accepted for publication in MNRAS

Our searches in the CLASH data have found many high-redshift galaxy candidates. One object at z = 6.2 in MACS J0329-02 is quadruply lensed. Another object in MACS J1149+22 is at redshift z = 9.6 and AB = 25.7, the first candidate at z > 9 that is bright enough for spectroscopic observations. The discovery of more than 80 galaxy candidates at z > 7 demonstrates the unique potential of galaxy cluster fields for finding highly magnified, intrinsically faint galaxies at the highest redshifts. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We derive a free-form mass distribution for the unrelaxed cluster A370 (z = 0.375), using the first release of the Hubble Frontier Fields images (76 orbits) and GLASS spectroscopy. Starting from a reliable set of 10 multiply lensed systems, we produce a free-form lens model that identifies ≈80 multiple images. Good consistency is found between models using independent subsamples of these lensed systems, with detailed agreement for the well-resolved arcs. The mass distribution has two very similar concentrations centred on the two prominent brightest cluster galaxies (or BCGs), with mass profiles that are accurately constrained by a uniquely useful system of long radially lensed images centred on both BCGs.We show that the lensing mass profiles of these BCGs are mainly accounted for by their stellar mass profiles, with a modest contribution from dark matter within r < 100 kpc of each BCG. This conclusion may favour a cooled cluster gas origin for BCGs, rather than via mergers of normal galaxies for which darkmatter should dominate over stars. Growth viamerging between BCGs is, however, consistent with this finding, so that stars still dominate over dark matter. We do not observe any significant offset between the positions of the peaks of the dark matter distribution and the light distribution., JL acknowledges a grant from the Seed Funding for Basic Research of the University of Hong Kong (Project No. 201411159166) for this work. JMD acknowledges support of the projects AYA2015-64508-P (MINECO/FEDER, UE), AYA2012-39475-C02-01 and the consolider project CSD2010-00064 funded by the Ministerio de Economia y Competitividad. JL acknowledges, on behalf of his co-I’s, support from the Research Grants Council of Hong Kong through grant 17319316.

We describe an on-going HST program aimed at determining the relationship between the nuclear black hole mass and bulge mass in spiral galaxies. We have selected a volume limited sample of 54 nearby spiral galaxies for which we already have ground based emission line rotation curves, CCD surface photometry and radio maps. We are now obtaining HST/STIS longslit observations of each of the galaxies in the sample in order to determine the nuclear Halpha rotation curve at high (~0.1") spatial resolution. We will use these data to measure the unresolved dark mass concentration at the nucleus of each object. Here we show the first results from observations of objects in the sample., 4 pages, contributed talk at IAU Symposium 205 "Galaxies and their Constituents at the Highest Angular Resolutions", Manchester UK, August 2000. R. Schilizzi, S. Vogel, F. Paresce, M. Elvis eds, ASP Conf. Ser

We derive a free-form mass distribution for the massive cluster AS1063 (z = 0.348) using the completed optical imaging from the Hubble Frontier Fields programmme. Based on a subset of 11 multiply lensed systems with spectroscopic redshift, we produce a lens model that is accurate enough to secure new multiply lensed systems, totalling over a 100 arclets, and to estimate their redshifts geometrically. Consistency is found between this precise model and that obtained using only the subset of lensed sources with spectroscopically measured redshifts. Although a relatively large elongation of the mass distribution is apparent relative to the X-ray map, no significant offset is found between the centroid of our mass distribution and that of the X-ray emission map, suggesting a relatively relaxed state for this cluster. For the well-resolved lensed images, we provide detailed model comparisons to illustrate the precision of our model and hence the reliability of our de-lensed sources. A clear linear structure is associated with one such source extending 23 kpc in length, that could be an example of jet-induced star formation, at redshift z ≈ 3.1., JL acknowledges a grant from the Seed Funding for Basic Research of the University of Hong Kong (Project No. 201411159166) for this work. JMD acknowledges support of the consolider project CSD2010-00064 and AYA2012-39475-C02-01 funded by the Ministerio de Economia y Competitividad. The work of JS was supported at IAP by ERC Project No. 267117 (DARK) hosted by the Pierre and Marie Curie University – Paris 6, and at JHU by NSF grant OIA-1124403.

We present a strong-lensing analysis of the galaxy cluster MS 1358.4+6245 ($z=0.33$), in deep 6-band ACS/HST imaging. In addition to the well-studied system at $z=4.92$, our modelling method uncovers 19 new multiply-lensed images so that a total of 23 images and their redshifts are used to accurately constrain the inner mass distribution. We derive a relatively shallow inner mass profile, $d\log \Sigma/d\log r\simeq -0.33 \pm0.05$ ($r

We present a strong lensing mass model of Abell 1689 which resolves substructures ~25 kpc across (including about ten individual galaxy subhalos) within the central ~400 kpc diameter. We achieve this resolution by perfectly reproducing the observed (strongly lensed) input positions of 168 multiple images of 55 knots residing within 135 images of 42 galaxies. Our model makes no assumptions about light tracing mass, yet we reproduce the brightest visible structures with some slight deviations. A1689 remains one of the strongest known lenses on the sky, with an Einstein radius of RE = 47.0" +/- 1.2" (143 +3/-4 kpc) for a lensed source at zs = 2. We find a single NFW or Sersic prole yields a good fit simultaneously (with only slight tension) to both our strong lensing (SL) mass model and published weak lensing (WL) measurements at larger radius (out to the virial radius). According to this NFW fit, A1689 has a mass of Mvir = 2.0 +0.5/-0.3 x 10^15 Msun / h70 (M200 = 1.8 +0.4/-0.3 x 10^15 Msun / h70) within the virial radius rvir = 3.0 +/- 0.2 Mpc / h70 (r200 = 2.4 +0.1/-0.2 Mpc / h70), and a central concentration cvir = 11.5 +1.5/-1.4 (c200 = 9.2 +/- 1.2). Our SL model prefers slightly higher concentrations than previous SL models, bringing our SL+WL constraints in line with other recent derivations. Our results support those of previous studies which find A1689 has either an anomalously large concentration or significant extra mass along the line of sight (perhaps in part due to triaxiality). If clusters are generally found to have higher concentrations than realized in simulations, this could indicate they formed earlier, perhaps as a result of early dark energy.

The inner mass-profile of the relaxed cluster Abell 1703 is analysed by two very different strong-lensing techniques applied to deep ACS and WFC3 imaging. Our parametric method has the accuracy required to reproduce the many sets of multiple images, based on the assumption that mass approximately traces light. We test this assumption with a fully non-parametric, adaptive grid method, with no knowledge of the galaxy distribution. Differences between the methods are seen on fine scales due to member galaxies which must be included in models designed to search for lensed images, but on the larger scale the general distribution of dark matter is in good agreement, with very similar radial mass profiles. We add undiluted weak-lensing measurements from deep multi-colour Subaru imaging to obtain a fully model-independent mass profile out to the virial radius and beyond. Consistency is found in the region of overlap between the weak and strong lensing, and the full mass profile is well-described by an NFW model of a concentration parameter, $c_{\rm vir}\simeq 7.15\pm0.5$ (and $M_{vir}\simeq 1.22\pm0.15 \times 10^{15}M_{\odot}/h$). Abell 1703 lies above the standard $c$--$M$ relation predicted for the standard $\Lambda$CDM model, similar to other massive relaxed clusters with accurately determined lensing-based profiles.

We report the first results of a study of variable point sources identified using multi-color time-series photometry from Sloan Digital Sky Survey (SDSS) Stripe 82 over a span of nearly 10 years (1998-2007). We construct a light-curve catalog of 221,842 point sources in the R.A. 0-4 h half of Stripe 82, limited to r = 22.0, that have at least 10 detections in the ugriz bands and color errors of < 0.2 mag. These objects are then classified by color and by cross-matching them to existing SDSS catalogs of interesting objects. We use inhomogeneous ensemble differential photometry techniques to greatly improve our sensitivity to variability. Robust variable identification methods are used to extract 6520 variable candidates in this dataset, resulting in an overall variable fraction of ~2.9% at the level of 0.05 mag variability. A search for periodic variables results in the identification of 30 eclipsing/ellipsoidal binary candidates, 55 RR Lyrae, and 16 Delta Scuti variables. We also identify 2704 variable quasars matched to the SDSS Quasar catalog (Schneider et al. 2007), as well as an additional 2403 quasar candidates identified by their non-stellar colors and variability properties. Finally, a sample of 11,328 point sources that appear to be nonvariable at the limits of our sensitivity is also discussed. (Abridged.), Comment: 67 pages, 27 figures. Accepted for publication in ApJS. Catalog available at http://shrike.pha.jhu.edu/stripe82-variables

[Abridged] XMMU J2235.3-2557 is one of the most distant X-ray selected clusters, spectroscopically confirmed at z=1.39. We characterize the galaxy populations of passive members, the thermodynamical properties of the hot gas, its metal abundance and the total mass of the system using imaging data with HST/ACS (i775 and z850 bands) and VLT/ISAAC (J and K_s bands), extensive spectroscopic data obtained with VLT/FORS2, and deep Chandra observations. Out of a total sample of 34 spectroscopically confirmed cluster members, we selected 16 passive galaxies within the central 2' (or 1 Mpc) with ACS coverage, and inferred star formation histories for a sub-sample of galaxies inside and outside the core by modeling their spectro-photometric data with spectral synthesis models, finding a strong mean age radial gradient. Chandra data show a regular elongated morphology, closely resembling the distribution of core galaxies, with a significant cool core. We measure a global X-ray temperature of kT=8.6(-1.2,+1.3) keV (68% c.l.). By detecting the rest-frame 6.7 keV Iron K line, we measure a metallicty Z= 0.26(+0.20,-0.16) Zsun. In the likely hypothesis of hydrostatic equilibrium, we obtain a total mass of Mtot(1, with a baryonic content, both its galaxy population and intra-cluster gas, in a significantly advanced evolutionary stage at 1/3 of the current age of the Universe., Comment: 9 pages, 8 figures, accepted for publication in A&A (v2: typos/language style corrections, updated references)

We present new information on galaxies in the vicinity of luminous radio galaxies and quasars at z=4,5,6. These fields were previously found to contain overdensities of Lyman Break Galaxies (LBGs) or spectroscopic Lyman alpha emitters. We use HST and Spitzer data to infer stellar masses, and contrast our results with large samples of LBGs in more average environments as probed by the Great Observatories Origins Deep Survey (GOODS). The following results were obtained. First, LBGs in both overdense regions and in the field at z=4-5 lie on a very similar sequence in a z'-[3.6] versus [3.6] color-magnitude diagram. This is interpreted as a sequence in stellar mass (log[M*/Msun] = 9-11) in which galaxies become increasingly red due to dust and age as their star formation rate (SFR) increases. Second, the two radio galaxies are among the most massive objects (log[M*/Msun]~11) known to exist at z~4-5, and are extremely rare based on the low number density of such objects as estimated from the ~25x larger area GOODS survey. We suggest that the presence of these massive galaxies and supermassive black holes has been boosted through rapid accretion of gas or merging inside overdense regions. Third, the total stellar mass found in the z=4 ``proto-cluster'' TN1338 accounts for 4, based on a comparison with the massive X-ray cluster Cl1252 at z=1.2. Although future near-infrared observations should determine whether any massive galaxies are currently being missed, one possible explanation for this mass difference is that TN1338 evolves into a smaller cluster than Cl1252. This raises the interesting question of whether the most massive protocluster regions at z>4 remain yet to be discovered., The Astrophysical Journal, In Press (17 pages, 7 figures)

Establishing or ruling out, either through solid mass measurements or upper limits, the presence of intermediate-mass black holes (IMBHs) at the centers of star clusters would profoundly impact our understanding of problems ranging from the formation and long-term dynamical evolution of stellar systems, to the nature of the seeds and the growth mechanisms of supermassive black holes. While there are sound theoretical arguments both for and against their presence in today's clusters, observational studies have so far not yielded truly conclusive IMBH detections nor upper limits. We argue that the most promising approach to solving this issue is provided by the combination of measurements of the proper motions of stars at the centers of Galactic globular clusters and dynamical models able to take full advantage of this type of data set. We present a program based on HST observations and recently developed tools for dynamical analysis designed to do just that., 7 pages; Invited talk at IAU Symposium 266 "Star clusters: basic galactic building blocks" (Rio de Janeiro, 10-14 August 2009), R. de Grijs and R.D. Lepine, eds; v2 reflects a better citation of some references and corrected typos

We conduct a search for z>~7 dropout galaxies behind 11 massive lensing clusters using 21 arcmin**2 of deep HST NICMOS, ACS, and WFPC2 image data. In total, over this entire area, we find only one robust z~7 z-dropout candidate (previously reported around Abell 1689). Four less robust z-dropout and J-dropout candidates are also found. The nature of the four weaker candidates could not be precisely determined due to the limited depth of the available optical data, but detailed simulations suggest that all four could be low-redshift interlopers. We compare these numbers with what we might expect using the z~7 UV luminosity function (LF) determined from field searches. We predict 2.7 z~7 z-dropouts and 0.3 z~9 J-dropouts over our cluster search area, in reasonable agreement with our observational results, given the small numbers. The number of z>~7 candidates we find in the present search are much lower than has been reported in several previous studies of the prevalence of z>~7 galaxies behind lensing clusters. To understand these differences, we examined z>~7 candidates in other studies and conclude that only a small fraction are likely to be z>~7 galaxies. Our findings support models that show that gravitational lensing from clusters is of the most value for detecting galaxies at magnitudes brighter than L* (H, Comment: 9 pages, 3 figures, 5 tables, in press

We use all available deep optical ACS and near-IR data over both the HUDF and the two GOODS fields to search for star-forming galaxies at z>~7 and constrain the UV LF within the first 700 Myrs. Our data set includes ~23 arcmin^2 of deep NICMOS J+H data and ~248 arcmin^2 of ground-based (ISAAC+MOIRCS) data, coincident with ACS optical data of greater or equal depths. In total, we find 8 ~7.3 z-dropouts in our search fields, but no z~9 J-dropout candidates. A careful consideration of a wide variety of different contaminants suggest an overall contamination level of just ~12% for our z-dropout selection. After performing detailed simulations to accurately estimate the selection volumes, we derive constraints on the UV LFs at z~7 and z~9. For a faint-end slope alpha=-1.74, our most likely values for M*(UV) and phi* at z~7 are -19.8+/-0.4 mag and 1.1_{-0.7}^{+1.7} x 10^{-3} Mpc^{-3}, respectively. Our search results for z~9 J-dropouts set a 1 sigma lower limit on M*(UV) of -19.6 mag assuming that phi* and alpha are the same as their values at slightly later times. This lower limit on M*(UV) is 1.4 mag fainter than our best-fit value at z~4, suggesting that the UV LF has undergone substantial evolution over this time period. No evolution is ruled out at 99% confidence from z~7 to z~6 and at 80% confidence from z~9 to z~7. The inferred brightening in M*(UV) with redshift (i.e., M*(UV) = (-21.02+/-0.09) + (0.36+/-0.08)(z - 3.8)) matches the evolution expected in the halo mass function, if the mass-to-light ratio of halos evolves as ~(1+z)**{-1}. Finally, we consider the shape of the UV LF at z>~5 and discuss the implications of the Schechter-like form of the observed LFs, particularly the unexpected abrupt cut-off at the bright end.

We present a new approach to gravitational lens massmap reconstruction. Our massmap solutions perfectly reproduce the positions, fluxes, and shears of all multiple images. And each massmap accurately recovers the underlying mass distribution to a resolution limited by the number of multiple images detected. We demonstrate our technique given a mock galaxy cluster similar to Abell 1689 which gravitationally lenses 19 mock background galaxies to produce 93 multiple images. We also explore cases in which far fewer multiple images are observed, such as four multiple images of a single galaxy. Massmap solutions are never unique, and our method makes it possible to explore an extremely flexible range of physical (and unphysical) solutions, all of which perfectly reproduce the data given. Each reconfiguration of the source galaxies produces a new massmap solution. An optimization routine is provided to find those source positions (and redshifts, within uncertainties) which produce the "most physical" massmap solution, according to a new figure of merit developed here. Our method imposes no assumptions about the slope of the radial profile nor mass following light. But unlike "non-parametric" grid-based methods, the number of free parameters we solve for is only as many as the number of observable constraints (or slightly greater if fluxes are constrained). For each set of source positions and redshifts, massmap solutions are obtained "instantly" via direct matrix inversion by smoothly interpolating the deflection field using a recently developed mathematical technique. Our LensPerfect software is straightforward and easy to use and is made publicly available via our website., 17 pages, 18 figures, accepted by ApJ. Software and full-color version of paper available at http://www.its.caltech.edu/~coe/LensPerfect/

We use the ACS BViz data from the HUDF and all other deep HST ACS fields (including the GOODS fields) to find large samples of star-forming galaxies at z~4 and z~5 and to extend our previous z~6 sample. These samples contain 4671, 1416, and 627 B, V, and i dropouts, respectively, and reach to extremely low luminosities (0.01-0.04 L* or M(UV)~-16 to -17), allowing us to determine the rest-frame UV luminosity function (LF) and faint-end slope alpha at z~4-6 to high accuracy. We find faint-end slopes alpha of -1.73+/-0.05 at z~4, -1.66+/-0.09 at z~5, and -1.74+/-0.16 at z~6 -- suggesting that the faint-end slope is very steep and shows little evolution with cosmic time. We find that M*(UV) brightens considerably in the 0.7 Gyr from z~6 to z~4 (by ~0.7 mag from M*=-20.24+/-0.19 to M*=-20.98+/-0.10). The observed increase in the characteristic luminosity over this range is almost identical to that expected for the halo mass function -- suggesting that the observed evolution is likely due to the hierarchical coalescence and merging of galaxies. The evolution in phi* is not significant. The UV luminosity density at z~6 is modestly lower (0.45+/-0.09 times) than that at z~4 (integrated to -17.5 AB mag) though a larger change is seen in the dust-corrected star-formation rate density. We thoroughly examine published LF results and assess the reasons for their wide dispersion. We argue that the results reported here are the most robust available. The extremely steep faint-end slopes alpha found here suggest that lower luminosity galaxies play a significant role in reionizing the universe. Finally, we consider recent search results for galaxies at z~7-8 and use them to extend our estimates of the evolution in M* from z~7-8 to z~4.

We describe the time- and position-dependent point spread function (PSF) variation of the Wide Field Channel (WFC) of the Advanced Camera for Surveys (ACS) with the principal component analysis (PCA) technique. The time-dependent change is caused by the temporal variation of the $HST$ focus whereas the position-dependent PSF variation in ACS/WFC at a given focus is mainly the result of changes in aberrations and charge diffusion across the detector, which appear as position-dependent changes in elongation of the astigmatic core and blurring of the PSF, respectively. Using >400 archival images of star cluster fields, we construct a ACS PSF library covering diverse environments of the $HST$ observations (e.g., focus values). We find that interpolation of a small number ($\sim20$) of principal components or ``eigen-PSFs'' per exposure can robustly reproduce the observed variation of the ellipticity and size of the PSF. Our primary interest in this investigation is the application of this PSF library to precision weak-lensing analyses, where accurate knowledge of the instrument's PSF is crucial. However, the high-fidelity of the model judged from the nice agreement with observed PSFs suggests that the model is potentially also useful in other applications such as crowded field stellar photometry, galaxy profile fitting, AGN studies, etc., which similarly demand a fair knowledge of the PSFs at objects' locations. Our PSF models, applicable to any WFC image rectified with the Lanczos3 kernel, are publicly available., Accepted to PASP. To appear in December issue. Figures are degraded to meet the size limit. High-resolution version can be downloaded at http://acs.pha.jhu.edu/~mkjee/acs_psf/acspsf.pdf

We examined the morphology-density relations for galaxy samples selected by luminosity and by mass in each of five massive X-ray clusters from z=0.023 to 0.83 for 674 spectroscopically-confirmed members. Rest-frame optical colors and visual morphologies were obtained primarily from Hubble Space Telescope images. Morphology-density relations (MDR) are derived in each cluster from a complete, luminosity-selected sample of 452 galaxies with a magnitude limit M_V < M^{*}_{V} + 1. The change in the early-type fraction with redshift matches previous work for massive clusters of galaxies. We performed a similar analysis, deriving MDRs for complete, mass-selected samples of 441 galaxies with a mass-limit of 10^{10.6} M_{\sun}. Our mass limit includes faint objects, the equivalent of =~1 mag below L^{*} for the red cluster galaxies, and encompasses =~70% of the stellar mass in cluster galaxies. The MDRs in the mass-selected sample at densities of Sigma > 50 galaxies Mpc^{-2} are similar to those in the luminosity-selected sample but show larger early-type fractions. However, the trend with redshift in the fraction of elliptical and S0 galaxies with masses > 10^{10.6} M_{\sun} differs significantly between the mass- and luminosity-selected samples. The clear trend seen in the early-type fraction from z=0 to z=~ 0.8 is not found in mass-selected samples. The early-type galaxy fraction changes much less, and is consistent with being constant at 92% +/- 4% at ��> 500 galaxies Mpc^{-2} and 83 +/- 3% at 50 < ��< 500 galaxies Mpc^{-2}. This suggests that galaxies of mass lower than > 10^{10.6} M_{\sun} play a significant role in the evolution of the early-type fraction in luminosity-selected samples. (Abstract abridged), 18 pages in emulate ApJ format, with 10 color figures, Accepted to ApJ. Version updated to reflect published version, includes new references and a correction to table 3

We present HST ACS coronagraphic observations of HD 100546, a B9.5 star, 103 pc away from the Sun, taken in the F435W, F606W, and F814W bands. Scattered light is detected up to 14" from the star. The observations are consistent with the presence of an extended flattened nebula with the same inclination as the inner disk. The well-known "spiral arms" are clearly observed and trail the rotating disk material. Weaker arms never before reported are also seen. The interarm space becomes brighter, but the structures become more neutral in color at longer wavelengths, which is not consistent with models that assume that they are due to the effects of a warped disk. Along the major disk axis, the colors of the scattered light relative to the star are Δ(F435W - F606W) ≈ 0.0-0.2 mag and Δ(F435W - F814W) ≈ 0.5-1 mag. To explain these colors, we explore the role of asymmetric scattering, reddening, and large minimum sizes on ISM-like grains. We conclude that each of these hypotheses by itself cannot explain the colors. The disk colors are similar to those derived for Kuiper Belt objects, suggesting that the same processes responsible for their colors may be at work here. We argue that we are observing only the geometrically thick, optically thin envelope of the disk, while the optically thick disk responsible for the far-IR emission is undetected. The observed spiral arms are then structures on this envelope. The colors indicate that the extended nebulosity is not a remnant of the infalling envelope but reprocessed disk material.

We report on deep spectroscopy, using LRIS on Keck I and FORS2 on the VLT, of a sample of 22 candidate z similar to 6 Lyman break galaxies (LBGs) selected by the (i)775 - z(850) > 1: 3 dropout criterion. Redshifts could be measured for eight objects. These redshifts are all in the range z = 5: 5-6.1, confirming the efficiency of the i775 - z850 color selection technique. Six of the confirmed galaxies show Ly alpha emission. Assuming that the 14 objects without redshifts are z similar to 6 LBGs that lack detectable Ly alpha emission lines, we infer that the fraction of Ly alpha-emitting LBGs with Ly alpha equivalent widths greater than 20 angstrom among z similar to 6 LBGs is approximate to 30%, similar to that found at z similar to 3. Every Ly alpha-emitting object in our sample is compact, with half-light radii r(hl) 97% confidence. We speculate that the small sizes of the Ly alpha-emitting LBGs are due to these objects being less massive than other LBGs at z similar to 6.

We present deep observations taken with the HST Advanced Camera for Surveys of the central massive galaxy in a forming cluster at z=2.2. The galaxy hosting the powerful radio source MRC 1138-262 is associated with one of the most extensive merger systems known in the early universe. Our HST/ACS image shows many star-forming galaxies merging within a ~200 kpc region that emits both diffuse line emission and continuum in the rest-frame UV. Because this galaxy lives in an overdense environment, it represents a rare view of a brightest cluster galaxy in formation at z>2 which may serve as a testbed for predictions of massive cluster galaxy formation., Contribution to the proceedings of "The Fate of Gas in Galaxies", Dwingeloo, July 2006, with 2 colour figures. To appear in New Astronomy Reviews, Vol. 51 (2007), eds. Morganti, Oosterloo, Villar-Martin & van Gorkom

We present catalogs and images of optical counterparts to the Chandra-selected X-ray sources found in the fields of the five clusters RX J0152-1357, RX J0849+4452, RDCS J0910+5422, MS 1054-0321, and RDCS J1252-2927, which were imaged with the Advanced Camera for Surveys as part of the ACS Guaranteed Time Observer programs. A total of 98 X-ray sources fall within the ACS mosaics, and positive identifications are made for ~96% of them, including confirmed cluster members. We classify the sources as active galactic nuclei (AGNs) or QSOs depending on their X-ray output. The log N(> S)- log S test indicates a significant overdensity of X-ray sources in the RX J0152-1357, RDCS J0910+5422, and MS 1054-0321 fields with respect to the CDF-S, suggesting an association of some X-ray sources with the large-scale structure of the clusters. From the asymmetry and concentration indices, ~52% of the optical counterparts are early-type galaxies, ~35% late-type, and the remainder irregular. A blue core is found in approximately half of the early-type galaxies. From visual examination, approximately 40% of the counterparts possess an unresolved nucleus, a common signature of nuclear activity. A majority of these nuclei are found in near face-on late-type galaxies although selection effects might be important. The X-ray to optical flux ratio of the nuclei correlates with the inclination angle of the late-type galaxies but not of the early types, as expected if dust is significant in the circumnuclear regions. The AGNs possess a ~50% excess of nearby companions compared to the overall galaxy population in the same fields. The surface density of the X-ray sources is highest at projected radii of 1 Mpc from the cluster center and relatively flat at larger radii. We describe the morphology and environment of the cluster members and compare them with the other optical counterparts.

We examine the latest data on the clusterMACSJ0717.5+3745 from the Hubble Frontier Fields campaign. The critically lensed area is the largest known of any lens and very irregular making it a challenge for parametric modelling. Using our free-form method we obtain an accurate solution, identify here many new sets of multiple images, doubling the number of constraints and improving the reconstruction of the dark matter distribution. Our reconstructed mass map shows several distinct central substructures with shallow density profiles, clarifying earlier work and defining well the relation between the dark matter distribution and the luminous and X-ray peaks within the critically lensed region. Using our free-form method, we are able to meaningfully subtract the mass contribution from cluster members to the deflection field to trace the smoothly distributed cluster dark matter distribution. We find four distinct concentrations, three of which are coincident with the luminous matter. The fourth peak has a significant offset from both the closest luminous and X-ray peaks. These findings, together with dynamical data from the motions of galaxies and gas will be important for uncovering the potentially important implications of this extremely massive and intriguing system., JMD acknowledges support of the consolider project CSD2010-00064 and AYA2012-39475-C02-01 funded by the Ministerio de Economia y Competitividad.

We present results from a comprehensive lensing analysis in HST data, of the complete CLASH cluster sample. We identify new multiple-images previously undiscovered allowing improved or first constraints on the cluster inner mass distributions and profiles. We combine these strong-lensing constraints with weak-lensing shape measurements within the HST FOV to jointly constrain the mass distributions. The analysis is performed in two different common parameterizations (one adopts light-traces-mass for both galaxies and dark matter while the other adopts an analytical, elliptical NFW form for the dark matter), to provide a better assessment of the underlying systematics - which is most important for deep, cluster-lensing surveys, especially when studying magnified high-redshift objects. We find that the typical (median), relative systematic differences throughout the central FOV are $\sim40\%$ in the (dimensionless) mass density, $\kappa$, and $\sim20\%$ in the magnification, $\mu$. We show maps of these differences for each cluster, as well as the mass distributions, critical curves, and 2D integrated mass profiles. For the Einstein radii ($z_{s}=2$) we find that all typically agree within $10\%$ between the two models, and Einstein masses agree, typically, within $\sim15\%$. At larger radii, the total projected, 2D integrated mass profiles of the two models, within $r\sim2\arcmin$, differ by $\sim30\%$. Stacking the surface-density profiles of the sample from the two methods together, we obtain an average slope of $d\log (\Sigma)/d\log(r)\sim-0.64\pm0.1$, in the radial range [5,350] kpc. Lastly, we also characterize the behavior of the average magnification, surface density, and shear differences between the two models, as a function of both the radius from the center, and the best-fit values of these quantities., Comment: 35 pages (20 main text pages, plus 15 pages for additional figures and tables); 2 Tables, 17 Figures. V3: accepted version; some minor corrections and additions made. V4: corrected several entries in Table 2. All mass models and magnification maps are made publicly available for the community

The massive cluster MACSJ1149.5+2223(z = 0.544) displays five very large lensed images of a well-resolved spiral galaxy at z = 1.491. It is within one of these images that the first example of a multiply lensed supernova (SN) has been detected recently as part of the Grism Lens-Amplified Survey from Space. The depth of this data also reveals many H II regions within the lensed spiral galaxy which we identify between the five counter-images. Here, we expand the capability of our free-form method to incorporate these H II regions locally, with other reliable lensed galaxies added for a global solution. This improved accuracy allows us to estimate when the Refsdal SN will appear within the other lensed images of the spiral galaxy to an accuracy of ~7 per cent. We predict this SN will reappear in one of the counter-images (RA = 11:49:36.025, Dec. = +22:23:48.11, J2000) and on 2015 November 1 (with an estimated error of ±25 d) it will be at the same phase as it was when it was originally discovered, offering a unique opportunity to study the early phases of this SN and to examine the consistency of the mass model and the cosmological model that have an impact on the time delay prediction., JMD acknowledges support of the consolider project CSD2010-00064 and AYA2012-39475-C02-01 funded by the Ministerio de Economia y Competitividad. AZ was provided by NASA through Hubble Fellowship grant #HST-HF2-51334.001-A awarded by STScI.

We study the photometric and structural properties of spectroscopically confirmed members in the two massive X-ray--selected z=0.83 galaxy clusters MS1054-03 and RXJ0152-1357 using three-band mosaic imaging with the Hubble Space Telescope Advanced Camera for Surveys. The samples include 105 and 140 members of MS1054-03 and RXJ0152-1357, respectively, with ACS F775W magnitude < 24.0. We develop a promising new structural classification method, based on a combination of the best-fit Sersic indices and the normalized root-mean-square residuals from the fits; the resulting classes agree well with the visual ones, but are less affected by galaxy orientation. We examine the color--magnitude relations in detail and find that the color residuals correlate with the local mass density measured from our weak lensing maps; we identify a threshold density of $\Sigma \approx 0.1$, in units of the critical density, above which the star formation appears to cease. For RXJ0152-1357, we also find a trend in the color residuals with velocity, resulting from an offset of about 980 km/s in the mean redshifts of the early- and late-type galaxies. Analysis of the color--color diagrams indicates that a range of star formation time-scales are needed to reproduce the loci of the galaxy colors. We also identify some cluster galaxies whose colors can only be explained by large amounts, $A_V \approx 1$ mag, of internal dust extinction. [Abstract shortened], Comment: 30 pages, emulateapj format; 23 figures, many in color. Accepted by ApJ; scheduled for the 10 June 2006 issue. Some figures degraded; for a higher resolution version, see: http://astro.wsu.edu/blakeslee/z1clusters/

The combined resolution and sensitivity of the Advanced Camera for Surveys deep imaging provides the capability of high-accuracy lens modeling of Abell 1689. Originally based on the technique of Broadhurst and coworkers, our software is designed to provide a precise and efficient method of modeling cluster lenses without assumptions relating the large-scale cluster dark matter to the light. Abell 1689 is robustly modeled using a freely varying cluster halo component consisting of an NFW profile, shapelets (Refregier), and a mass sheet, as well as a galaxy component based on the light. Another improvement over previous modeling techniques is the application ofmagnification-correctedimagemagnitudeconstraints.Themasswithinthe � 50 00 Einsteinradiuswasfoundtobe 2:04 þ0:03

We present a morphological analysis of distant field galaxies using the deep ACS images from the public parallel NICMOS observations of the Hubble Ultra Deep Field obtained in the F435W (B), F606W (V), F775W (i) and F850LP (z) filters. We morphologically segregate galaxies using a combination of visual classification and objective machine based selection. We use the Asymmetry (A) and Central Concentration (C) parameters to characterize galaxies up to z_AB

We present Hubble Space Telescope/Advanced Camera for Surveys (ACS) weak-lensing and Chandra X-ray analyses of MS 1054-0321 at z=0.83, the most distant and X-ray luminous cluster in the Einstein Extended Medium-Sensitivity Survey (EMSS). The high-resolution mass reconstruction through ACS weak-lensing reveals the complicated dark matter substructure in unprecedented detail, characterized by the three dominant mass clumps with the four or more minor satellite groups within the current ACS field. The direct comparison of the mass map with the Chandra X-ray image shows that the eastern weak-lensing substructure is not present in the X-ray image and, more interestingly, the two X-ray peaks are displaced away from the hypothesized merging direction with respect to the corresponding central and western mass clumps, possibly because of ram pressure. In addition, as observed in our previous weak-lensing study of another high-redshift cluster CL 0152-1357 at z=0.84, the two dark matter clumps of MS 1054-0321 seem to be offset from the galaxy counterparts. We examine the significance of these offsets and discuss a possible scenario, wherein the dark matter clumps might be moving ahead of the cluster galaxies. The non-parametric weak-lensing mass modeling gives a projected mass of M(r

Hubble Space Telescope Advanced Camera for Surveys images of the young binary GG Tauri and its circumbinary disk in V and I bandpasses were obtained in 2002 and are the most detailed of this system to date. They confirm features previously seen in the disk including a "gap" apparently caused by shadowing from circumstellar material, an asymmetrical distribution of light about the line of sight on the near edge of the disk, enhanced brightness along the near edge of the disk due to forward scattering, and a compact reflection nebula near the secondary star. New features are seen in the ACS images: two short filaments along the disk, localized but strong variations in the disk intensity ("gaplets"), and a "spur" or filament extending from the reflection nebulosity near the secondary. The back side of the disk is detected in the V band for the first time. The disk appears redder than the combined light from the stars, which may be explained by a varied distribution of grain sizes. The brightness asymmetries along the disk suggest that it is asymmetrically illuminated by the stars due to extinction by nonuniform circumstellar material or that the illuminated surface of the disk is warped by tidal effects (or perhaps both). Localized, time-dependent brightness variations in the disk are also seen.

We present the photometric calibration of the HST Advanced Camera for Surveys (ACS). We give here an overview of the performance and calibration of the 2 CCD cameras, the Wide Field Channel (WFC) and the High Resolution Channel (HRC), and a description of the best techniques for reducing ACS CCD data. On-orbit observations of spectrophotometric standard stars have been used to revise the pre-launch estimate of the instrument response curves to best match predicted and observed count rates. Synthetic photometry has been used to determine zeropoints for all filters in 3 magnitude systems and to derive interstellar extinction values for the ACS photometric systems. Due to the CCD internal scattering of long wavelength photons, the width of the PSF increases significantly in the near-IR and the aperture correction for photometry with near-IR filters depends on the spectral energy distribution of the source. We provide encircled energy curves and a detailed recipe to correct for the latter effect. Transformations between the ACS photometric systems and the UBVRI and WFPC2 systems are presented. In general, two sets of transformations are available: 1 based on the observation of 2 star clusters; the other on synthetic photometry. We discuss the accuracy of these transformations and their sensitivity to details of the spectra being transformed. Initial signs of detector degradation due to the HST radiative environment are already visible. We discuss the impact on the data in terms of dark rate increase, charge transfer inefficiency, and hot pixel population.

High redshift (z >~ 1) clusters are ideal probes to study the formation and evolution of large scale structures and galaxies in the universe. A 10-m class ground based telescope, X-ray observatories (Chandra, XMM-Newton) and HST/ACS are allowing us to perform an unprecedented study of distant massive clusters of galaxies in the redshift range 0.84, Comment: 14 pages, 2 figures. Accepted for publication in Modern Physics Letters A (invited brief review). v2: Order of authors in the .tex version corrected to match that in journal

We investigate the physical nature of blue cores in early-type galaxies through the first multi-wavelength analysis of a serendipitously discovered field blue-nucleated spheroid in the background of the deep ACS/WFC griz multicolor observations of the cluster Abell 1689. The resolved g-r, r-i and i-z color maps reveal a prominent blue core identifying this galaxy as a ``typical'' case study, exhibiting variations of 0.5-1.0 mag in color between the center and the outer regions, opposite to the expectations of reddened metallicity induced gradients in passively evolved ellipticals. From a Magellan-Clay spectrum we secure the galaxy redshift at $z=0.624$. We find a strong X-ray source coincident with the spheroid galaxy. Spectral features and a high X-ray luminosity indicate the presence of an AGN in the galaxy. However, a comparison of the X-ray luminosity to a sample derived from the Chandra Deep Field South displays Lx to be comparable to Type I/QSO galaxies while the optical flux is consistent with a normal star-forming galaxy. We conclude that the galaxy's non-thermal component dominates at high-energy wavelengths while we associate the spheroid blue light with the stellar spectrum of normal star-forming galaxies. We argue about a probable association between the presence of blue cores in spheroids and AGN activity.

Observations of the radio galaxy MRC 0316-257 at z=3.13 and the surrounding field are presented. Using narrow- and broad-band imaging obtained with the VLT, 77 candidate Ly-alpha emitters with a rest-frame equivalent width of > 15 A were selected in a ~7'x7' field around the radio galaxy. Spectroscopy of 40 candidate emitters resulted in the discovery of 33 emission line galaxies of which 31 are Ly-alpha emitters with redshifts similar to that of the radio galaxy, while the remaining two galaxies turned out to be [OII] emitters. The Ly-alpha profiles have widths (FWHM) in the range of 120-800 km/s, with a median of 260 km/s. Where the signal-to-noise was large enough, the Ly-alpha profiles were found to be asymmetric, with apparent absorption troughs blueward of the profile peaks, indicative of absorption along the line of sight of an HI mass of at least 2x10^2 - 5x10^4 M_sun. The properties of the Ly-alpha galaxies (faint, blue and small) are consistent with young star forming galaxies which are still nearly dust free. The volume density of Ly-alpha emitting galaxies in the field around MRC 0316-257 is a factor of 3.3+0.5-0.4 larger compared with the density of field Ly-alpha emitters at that redshift. The velocity distribution of the spectroscopically confirmed emitters has a FWHM of 1510 km/s, which is substantially smaller than the width of the narrow-band filter (FWHM ~ 3500 km/s). The peak of the velocity distribution is located within 200 km/s of the redshift of the radio galaxy. We conclude that the confirmed Ly-alpha emitters are members of a protocluster of galaxies at z~3.13. The size of the protocluster is larger than 3.3x3.3 Mpc^2. The mass of this structure is estimated to be > 3-6x10^14 M_sun and could be the progenitor of a cluster of galaxies similar to e.g. the Virgo cluster. (Abridged), 23 Pages, including 20 PostScript figures. Publiced in Astronomy & Astrophysics. v2: typo fixed and Journal reference added

The M31 satellite galaxy M32 has long been considered an object of unique properties, being the most extreme example of the very rare compact elliptical galaxy class. Here we present the spectroscopic discovery of two M32 twins in the massive galaxy cluster Abell 1689. As these objects are so rare, this is an important step towards a better understanding of the nature of compact galaxies. The two M32 twins had first been detected within our photometric search for ultra compact dwarf galaxy (UCDs) candidates in A1689 (Mieske et al. 2004) with the Advanced Camera for Surveys (ACS). Their luminosities (M_V ~ -17 mag) are very similar to M32 and their surface brightness profiles are consistent with that of M32 projected to A1689's distance. From our ACS imaging we detect several fainter compact galaxy candidates with luminosities intermediate between M32 and the Fornax UCDs. If spectroscopically confirmed as cluster members, this would almost close the gap in the magnitude-surface brightness plane between the region of UCDs and the compact ellipticals, implying a sequence of faint compact galaxies well separated from that of dwarf ellipticals., 5 pages, 5 figures, accepted for publication in A&A letters

We present $V$ and $I$ continuum images and H$\alpha$+[N II] maps of nine early-type galaxies observed with the Wide Field Channel of the Advanced Camera for Surveys on the Hubble Space Telescope. Dust and ionized gas are detected in all galaxies. The optical nebulae are primarily concentrated on the nuclei and extend out to radii of a few hundred parsecs, in compact clumps, filaments, or disks. Two galaxies, NGC 6166 and NGC 6338, also possess diffuse, ionized filaments on kiloparsec scales. The ionized gas is entirely contained within the nuclear disks of ESO 208-G021, NGC 3078, and NGC 7720. In the radio-loud galaxy NGC 6166, emission-line filaments are detected along the radio lobes, possibly as a result of shock ionization. A wide range of ionized gas masses, $M_g\approx7\times10^2-3\times10^6$ $M_\odot$, are calculated from the observed fluxes. Even in this small sample, the orientation of the ionized material correlates well with the major or minor axis of the galaxies, consistent with an external origin for the dust and gas.

The Advanced Camera for Surveys was installed on the Hubble Space Telescope in March 2002. The instrument science team will search for planets using direct imaging with the ACS coronagraph and with precise astrometric and photometric measurement of suitable stars and disks. The camera is operational and performing to expectation.

The Advanced Camera for Surveys (ACS) offers a coronagraphic imaging mode with angular resolution of 0.026″pixel−1. In combination with with the appropriate subtraction of reference star point spread functions (PSF) the coronagraph is capable of achieving contrast ratios of ∼1000. We present some of the first ACS observations of the optically thin debris disk HD141569A and discuss new results from these 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.

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)

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.

The nearby and luminous QSO 3C 273 was imaged in 2002 July with the High Resolution Channel (HRC) of the Advanced Camera for Surveys (ACS) in coronagraphic mode in F475W (g), F606W (V), and F814W (I) as part of the Early Release Observations (ERO) program. After subtraction of the remaining PSF of the QSO, these images offer the most detailed view yet of the morphology and colors of the host galaxy of this QSO. We find that the central light distribution is elongated along the jet axis and its outer edge is delineated by an arc, centered on the jet at a radius of ~26 from the QSO and bluer than the surrounding galaxy. This system is embedded in an extended galactic halo. Compared with early-type galaxies of similar redshifts and luminosities, the light distribution of 3C 273 is flatter in the core, likely from suppression by dust, but similar in the outer halo. The QSO is displaced from the isophotal center of the galaxy by ~14. Previously known emission-line extensions are confirmed and new morphological features are identified, including a dramatic spiral-shaped plume, two faint filaments, a dust lane, and a knot along the jet axis. Part of the inner jet is unambiguously detected in all three bandpasses, and its morphology matches that of a MERLIN radio map. Different mechanisms that could explain the morphology of 3C 273 are considered, such as scattered QSO radiation, a face-on disk, and a merger event.