[show abstract][hide abstract] ABSTRACT: We report spectropolarimetry of 30 radio-selected broad absorption line (BAL) quasars with the Keck Observatory, 25 from the sample of Becker et al. (2000). Both high and low-ionization BAL quasars are represented, with redshifts ranging from 0.5 to 2.5. The spectropolarimetric properties of radio-selected BAL quasars are very similar to those of radio-quiet BAL quasars: a sizeable fraction (20%) show large continuum polarization (2-10%) usually rising toward short wavelengths, emission lines are typically less polarized than the continuum, and absorption line troughs often show large polarization jumps. There are no significant correlations between polarization properties and radio properties, including those indicative of system orientation, suggesting that BAL quasars are not simply normal quasars seen from an edge-on perspective.
The Astrophysical Journal Supplement Series 06/2010; · 16.24 Impact Factor
[show abstract][hide abstract] ABSTRACT: 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.
The Astrophysical Journal 12/2008; 621(2):651. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We study the dynamical origin of the structures observed in the scattered-light images of the resolved debris disk around HD 141569A. The disk has two conspicuous spiral rings and two large-scale spiral arms. 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. The disk's color, scattering function, and infrared emission suggest that submicron-sized grains dominate the dust population observed in scattered light. CO observations indicate the presence of up to 60 M⊕ of gas. The dust grains are subject to the competing effects of expulsive radiation pressure (β > 1, where β is the ratio of the radiation and gravitational forces) and retentive gas drag. 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 flyby 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 with initial radius 400 AU, a gas mass of 50 M⊕, and β = 4 and suggests that the system is being observed within 4000 yr of the flyby 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 5MJ planet in an eccentric orbit on the planetesimal distribution of HD 141569A.
The Astrophysical Journal 12/2008; 627(2):986. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We investigate the evolution in the distribution of surface brightness, as a function of size, for elliptical and S0 galaxies in the two clusters RDCS J1252.9-2927 (z = 1.237) and RX J0152.7-1357 (z = 0.837). We use multicolor imaging with the Advanced Camera for Surveys on the Hubble Space Telescope to determine these sizes and surface brightnesses. Using three different estimates of the surface brightnesses, we find that we reliably estimate the surface brightness for the galaxies in our sample with a scatter of <0.2 mag and with systematic shifts of 0.05 mag. We construct samples of galaxies with early-type morphologies in both clusters. For each cluster, we use a magnitude limit in a band that closely corresponds to the rest-frame B, to magnitude limit of MB = -18.8 at z = 0, and select only those galaxies within the color-magnitude sequence of the cluster or by using our spectroscopic redshifts. We measure evolution in the rest-frame B surface brightness and find -1.41 ± 0.14 mag from the Coma Cluster of galaxies for RDCS J1252.9-2927 and -0.90 ± 0.12 mag of evolution for RX J0152.7-1357, or an average evolution of (-1.13 ± 0.15)z mag. Our statistical errors are dominated by the observed scatter in the size-surface brightness relation, σ = 0.42 ± 0.05 mag for RX J0152.7-1357 and σ = 0.76 ± 0.10 mag for RDCS J1252.9-2927. We find no statistically significant evolution in this scatter, although an increase in the scatter could be expected. Overall, the pace of luminosity evolution we measure agrees with that of the fundamental plane of early-type galaxies, implying that the majority of massive early-type galaxies observed at z 1 formed at high redshifts.
The Astrophysical Journal 12/2008; 626(2):809. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: 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 arcmin2 and have depths close to that of the Hubble Deep Fields in the deepest part of the VV 29 image, with 10 σ detection limits for point sources of 27.8, 27.6, and 27.2 AB magnitudes in the gF475W, VF606W, and IF814W 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 of faint 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 ± 0.01 for 22 < gF475W < 28, 0.34 ± 0.01 for 22 < VF606W < 27.5, and 0.33 ± 0.01 for 22 < mF814W < 27. The counts do not flatten (except perhaps in the F475W filter), up to the depth of our observations. Our results agree well with those of Bernstein, Freedman, & Madore, who used different data sets and techniques, and show that it is possible to perform consistent measurements of galaxy number counts if the selection effects are properly considered. We find that the faint counts mAB > 25.5 can be well approximated in all our filters by a passive luminosity evolution model based on the COMBO-17 luminosity function (α = -1.5), with a strong merging rate following the prescription of Glazebrook et al., * (1 + Qz), with Q = 4.
The Astrophysical Journal Supplement Series 12/2008; 150(1):1. · 16.24 Impact Factor
[show abstract][hide abstract] ABSTRACT: 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.
The Astrophysical Journal 12/2008; 623(2):721. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We study the internal color properties of a morphologically selected sample of spheroidal galaxies taken from the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) Early Release Observation program of UGC 10214 ("the Tadpole"). By taking advantage of the unprecedented high resolution of the ACS in this very deep data set, we are able to characterize spheroids at subarcsecond scales. Using the V606 and I814 bands, we construct V - I color maps and extract color gradients for a sample of spheroids at I814 < 24 mag. We assess the ability of the ACS to make resolved color studies of galaxies by comparing its results with the multicolor data from the Hubble Deep Fields (HDFs). Here we report that with ACS WFC data using less than ~1/10 the exposure of the WFPC2 HDFs, it is possible to confidently carry out resolved studies of faint galaxies at similar magnitude limits. We also investigate the existence of a population of morphologically classified spheroids that 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 for from metallicity variations. Following the same analysis, we find a similar fraction of early-type systems (~30%-40%) that show nonhomologous internal colors, suggestive of recent star formation activity. We present two statistics for quantifying the internal color variation in galaxies and tracing blue cores, from which we estimate the ratio of nonhomogeneous 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.
The Astrophysical Journal 12/2008; 612(1):202. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: 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 NGC 2915. The clusters, all of which start to resolve into stars, have MV606 = -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 NGC 2915 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 NGC 2915.
The Astrophysical Journal 12/2008; 599(2):L83. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: 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 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 MV = -14.45, with a half-light radius of 161 pc, and if it is a single star cluster, 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 × 106 M☉. 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 intracluster media in the distant and younger universe.
The Astrophysical Journal 12/2008; 585(2):750. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: 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.
The Astrophysical Journal 12/2008; 595(2):589. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: 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 the Hubble Space Telescope Advanced Camera for Surveys (HST ACS) images, combined with ground-based near-IR imaging and optical spectroscopy. 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 elliptical and the S0 galaxies show small scatter about the CMR of 0.042 ± 0.010 and 0.044 ± 0.020 mag, respectively. From the scatter about the CMR, a mean luminosity-weighted age > 3.3 Gyr (zf ≈ 3) is derived for the elliptical galaxies, assuming a simple stellar population modeling (single-burst solar metallicity). Strikingly, the S0 galaxies in RDCS J0910+5422 are systematically bluer in i775 - z850, by 0.07 ± 0.02 mag, than the ellipticals. 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 galaxies corresponds to an age difference of ≈1 Gyr for a single-burst solar-metallicity model. 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 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 elliptical galaxies in other clusters. Thirteen red galaxy pairs are observed, and the galaxies associated in pairs constitute ~40% of the CMR galaxies in this cluster.
The Astrophysical Journal 12/2008; 639(1):81. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We present deep optical imaging of the z = 4.1 radio galaxy TN J1338-1942, obtained using the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope, as well as ground-based near-infrared imaging data from the European Southern Observatory (ESO) Very Large Telescope (VLT). The radio galaxy is known to reside within a large galaxy overdensity (both in physical extent and density contrast). There is good evidence that this "protocluster" region is the progenitor of a present-day rich galaxy cluster. TN J1338 is the dominant galaxy in the protocluster in terms of size and luminosity (in both the optical and near-infrared) and therefore seems destined to evolve into the brightest cluster galaxy. The high spatial resolution ACS images reveal several kiloparsec-scale features within and around the radio galaxy. The continuum light is aligned with the radio axis and is resolved into two clumps in the i775 and z850 bands. These components have luminosities ~109 L☉ and sizes of a few kpc. The estimated nebular continuum, scattered light, synchrotron- and inverse Compton-scattering contributions to the aligned continuum light are only a few percent of the observed total, indicating that the observed flux is likely dominated by forming stars. The estimated star formation rate for the whole radio galaxy is ~200 M☉ yr-1. A simple model in which the jet has triggered star formation in these continuum knots is consistent with the available data. A striking, but small, linear feature is evident in the z850 aligned light and may be indicative of a large-scale shock associated with the advance of the radio jet. The rest of the aligned light also seems morphologically consistent with star formation induced by shocks associated with the radio source, as seen in other high-z radio galaxies (e.g., 4C 41.17). An unusual feature is seen in Lyα emission. A wedge-shaped extension emanates from the radio galaxy perpendicularly to the radio axis. This "wedge" naturally connects to the surrounding asymmetric, large-scale (~100 kpc) Lyα halo. We posit that the wedge is a starburst-driven superwind associated with the first major epoch of formation of the brightest cluster galaxy. The shock and wedge are examples of feedback processes due to both active galactic nucleus and star formation in the earliest stages of massive galaxy formation.
The Astrophysical Journal 12/2008; 630(1):68. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We present new measurements of the galaxy luminosity function (LF) and its dependence on local galaxy density, color, morphology, and clustocentric radius for the massive z = 0.83 cluster MS 1054-0321. Our analyses are based on imaging performed with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST) in the F606W, F775W, and F850LP passbands and extensive spectroscopic data obtained with the Keck Low-Resolution Imaging Spectrograph. Our main results are based on a spectroscopically selected sample of 143 cluster members with morphological classifications derived from the ACS observations. Our three primary findings are (1) the faint-end slope of the LF is steepest in the bluest filter, (2) the LF in the inner part of the cluster (or highest density regions) has a flatter faint-end slope, and (3) the fraction of early-type galaxies is higher at the bright end of the LF, and gradually decreases toward fainter magnitudes. These characteristics are consistent with those in local galaxy clusters, indicating that, at least in massive clusters, the common characteristics of cluster LFs are established at z = 0.83. These results provide additional support for the hypothesis that the formation of galaxies in MS 1054-0321 began at redshifts considerably greater than unity. We also find a 2 σ deficit of intrinsically faint, red galaxies (i775 - z850 ≥ 0.5, Mi > -19) in this cluster. Although the significance is marginal, this trend may suggest that faint, red galaxies (which are common in z < 0.1 rich clusters) have not yet been created in this cluster at z = 0.83. The giant-to-dwarf ratio in MS 1054-0321 starts to increase inwards of the virial radius or when Σ > 30 Mpc-2, coinciding with the environment where the galaxy star formation rate and the morphology-density relation start to appear. A physical process that begins to become effective at around the virial radius or Σ ~ 30 Mpc-2 may thus be responsible for the evolution of color and luminosity of cluster galaxies.
The Astrophysical Journal 12/2008; 621(1):188. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We present F435W (B), F606W (broad V), and F814W (broad I) coronagraphic images of the debris disk around β Pictoris obtained with the Hubble Space Telescope's Advanced Camera for Surveys. These images provide the most photometrically accurate and morphologically detailed views of the disk between 30 and 300 AU from the star ever recorded in scattered light. We confirm that the previously reported warp in the inner disk is a distinct secondary disk inclined by ~5° from the main disk. The projected spine of the secondary disk coincides with the isophotal inflections, or "butterfly asymmetry," previously seen at large distances from the star. We also confirm that the opposing extensions of the main disk have different position angles, but we find that this "wing-tilt asymmetry" is centered on the star rather than offset from it, as previously reported. 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. The disk's northwest region appears 20%-30% redder than its southeast region, which is inconsistent with the notion that forward scattering from the nearer northwest side of the disk should diminish with increasing wavelength. Within ~120 AU, the mF435W - mF606W and mF435W - mF814W colors along the spine of the main disk are ~10% and ~20% redder, respectively, than those of β Pic. These colors increasingly redden beyond ~120 AU, becoming 25% and 40% redder, respectively, than the star at 250 AU. These measurements overrule previous determinations that the disk is composed of neutrally scattering grains. The change in color gradient at ~120 AU nearly coincides with the prominent inflection in the surface brightness profile at ~115 AU and the expected water-ice sublimation boundary. We compare the observed red colors within ~120 AU with the simulated colors of nonicy 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 μm, but the colors are inconsistent with the blue colors expected from grains with porosities 90%. The increasingly red colors beyond the ice sublimation zone may indicate the condensation of icy mantles on the refractory grains, or they may reflect an increasing minimum grain size caused by the cessation of cometary activity.
The Astronomical Journal 12/2007; 131(6):3109. · 4.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: 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 disk's 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.
The Astronomical Journal 12/2007; 126(1):385. · 4.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: The properties of ultracompact dwarf (UCD) galaxy candidates in Abell 1689 (z = 0.183) are investigated, based on deep high-resolution images from the Hubble Space Telescope Advanced Camera for Surveys. A UCD candidate has to be unresolved, have i < 28 mag (MV < -11.5 mag), and satisfy color limits derived from Bayesian photometric redshifts. We find 160 UCD candidates with 22 mag < i < 28 mag. We estimate that about 100 of these are cluster members, based on their spatial distribution and photometric redshifts. For i 26.8 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 (MV -17 mag) are slightly resolved. At the distance of Abell 1689, these two objects would have King profile core radii of 35 pc and reff 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 but also allow for up to four foreground stars among the six brightest UCD candidates. Our findings imply that in Abell 1689 there are ≥10 UCDs with MV < -12.7 mag, probably created by stripping "normal" dwarf or spiral galaxies. Compared with the UCDs in the Fornax Cluster—the location of their original discovery—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. Checking the photometric redshifts of the brightest UCD candidates with spectroscopy would be the next step to definitely confirm the existence of UCDs in Abell 1689.
The Astronomical Journal 12/2007; 128(4):1529. · 4.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: 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.
The Astronomical Journal 12/2007; 125(6):2964. · 4.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: 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 = 075 and 15'' (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 for r < 15 AU, then decreases gradually (I ∝ r-1.8) out to r ≈ 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 apparent bow. 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.
The Astronomical Journal 12/2007; 129(2):1008. · 4.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: We measure the luminosity function of morphologically selected E/S0 galaxies from z = 0.5 to 1.0 using deep high-resolution Advanced Camera for Surveys (ACS) imaging data. Our analysis covers an area of 48 arcmin2 (8 times the area of the Hubble Deep Field North) and extends 2 mag deeper (I ~ 24 mag) than was possible in the Deep Groth Strip Survey (DGSS). Our fields were observed as part of the ACS Guaranteed Time Observations. At 0.5 < z < 0.75, we find M - 5 log h0.7 = -21.1 ± 0.3 and α = -0.53 ± 0.2, and at 0.75 < z < 1.0, we find M - 5 log h0.7 = -21.4 ± 0.2, consistent with 0.3 mag of luminosity evolution (across our two redshift intervals). These luminosity functions are similar in both shape and number density to the luminosity function using morphological selection (e.g., DGSS), but are much steeper than the luminosity functions of samples selected using morphological proxies such as the color or spectral energy distribution (e.g., CFRS, CADIS, or COMBO-17). The difference is due to the "blue," (U - V)0 < 1.7, E/S0 galaxies, which make up to ~30% of the sample at all magnitudes and an increasing proportion of faint galaxies. We thereby demonstrate the need for both morphological and structural information to constrain the evolution of galaxies. We find that the blue E/S0 galaxies have the same average sizes and Sersic parameters as the "red," (U - V)0 > 1.7, E/S0 galaxies at brighter luminosities (MB < -20.1), but are increasingly different at fainter magnitudes, where blue galaxies are both smaller and have lower Sersic parameters. We find differences in both the size-magnitude relation and the photometric plane offset for red and blue E/S0s, although neither red nor blue galaxies give a good fit to the size-magnitude relation. Fits of the colors to stellar population models suggest that most E/S0 galaxies have short star formation timescales (τ < 1 Gyr), and that galaxies have formed at an increasing rate from z ~ 8 until z ~ 2, after which there has been a gradual decline.
The Astronomical Journal 12/2007; 128(5):1990. · 4.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: We present HST ACS observations of the most distant radio galaxy known, TN J0924-2201 at z = 5.2. This radio galaxy has six spectroscopically confirmed Lyα-emitting companion galaxies and appears to lie within an overdense region. The radio galaxy is marginally resolved in i775 and z850, showing continuum emission aligned with the radio axis, similar to what is observed for lower redshift radio galaxies. Both the half-light radius and the UV star formation rate are comparable to the typical values found for Lyman break galaxies at z ~ 4-5. The Lyα emitters are sub-L* galaxies, with deduced star formation rates of 1-10 M☉ yr-1. One of the Lyα emitters is only detected in Lyα. Based on the star formation rate of ~3 M☉ yr-1 calculated from Lyα, the lack of continuum emission could be explained if the galaxy is younger than ~2 Myr and is producing its first stars. Observations in V606i775z850 were used to identify additional Lyman break galaxies associated with this structure. In addition to the radio galaxy, there are 22 V606 break (z ~ 5) galaxies with z850 < 26.5 (5 σ), two of which are also in the spectroscopic sample. We compare the surface density of ~2 arcmin-2 to that of similarly selected V606 dropouts extracted from GOODS and the UDF parallel fields. We find evidence for an overdensity to very high confidence (>99%), based on a counts-in-cells analysis applied to the control field. The excess suggests that the V606 break objects are associated with a forming cluster around the radio galaxy.
The Astrophysical Journal 01/2006; 637(1):58. · 6.73 Impact Factor