Hubble Space Telescope WFPC2 Images Research Articles

A Candidate Dark Matter Deficient Dwarf Galaxy in the Fornax Cluster Identified through Overluminous Star Clusters

Two low surface brightness (LSB) dwarf galaxies were identified recently as having little or no dark matter (DM), provoking widespread interest in their formation histories. These galaxies also host populous systems of star clusters that are on average larger and more luminous than typical globular clusters (GCs). We report an initial attempt to identify new candidate DM-deficient dwarfs via their unusual GC systems. Using a large catalog of LSB galaxies from the Dark Energy Survey, we inspect their Dark Energy Camera Legacy Survey (DECaLS) imaging and identify FCC 224 as a candidate found on the outskirts of the Fornax cluster. We analyze the GC system using DECaLS and archival Hubble Space Telescope WFPC2 imaging, and find an apparent population of overluminous GCs. More detailed follow-up of FCC 224 is in progress.

The Quadratic Programming Method for Extracting Emission Line Maps from Line-blended Narrowband Images

Abstract We present a method to separate blended line emission from a set of narrowband (NB) images whose distinct throughput profiles overlap with each other. This method, based on the principle of quadratic programming (QP), treats line blending as a linearly constrained quadratic optimization problem and finds the solution by imposing a condition that the desired individual line intensities are always positive while allowing measurement uncertainties. We demonstrate the method for the quintessential example of line blending in the Hα complex by the Hα and [N ii] lines using the archived Hubble Space Telescope WFPC2 and WFC3 NB images of the planetary nebula NGC 6720, in comparison with its genuine line maps extracted from a spectral cube obtained by our own long-slit scan mapping observations with the Gemini Multi-Object Spectrograph at Gemini-North. Such line-blended NB images typically permit only qualitative line diagnostics, which can lead to a factor of a few difference in the line flux ratios. This novel QP method, however, can extract individual line emission maps as long as the number of line-blended but well-calibrated NB images of differing throughputs is equal to or grater than the number of emission line maps to be extracted. The proposed QP method provides a viable alternative to the existing line emission calibration methods: a simple but innovative way to enable quantitative fully 2D plasma diagnostics at the pixel scale of input NB images available from various instruments, telescopes, and archives.

CRBLASTER: A Parallel-Processing Computational Framework for Embarrassingly Parallel Image-Analysis Algorithms

ABSTRACTThe development of parallel-processing image-analysis codes is generally a challenging task that requires complicated choreography of interprocessor communications. If, however, the image-analysis algorithm is embarrassingly parallel, then the development of a parallel-processing implementation of that algorithm can be a much easier task to accomplish because, by definition, there is little need for communication between the compute processes. I describe the design, implementation, and performance of a parallel-processing image-analysis application, called crblaster, which does cosmic-ray rejection of CCD images using the embarrassingly parallel l.a.cosmic algorithm. crblaster is written in C using the high-performance computing industry standard Message Passing Interface (MPI) library. crblaster uses a two-dimensional image partitioning algorithm that partitions an input image into N rectangular subimages of nearly equal area; the subimages include sufficient additional pixels along common image partition edges such that the need for communication between computer processes is eliminated. The code has been designed to be used by research scientists who are familiar with C as a parallel-processing computational framework that enables the easy development of parallel-processing image-analysis programs based on embarrassingly parallel algorithms. The crblaster source code is freely available at the official application Web site at the National Optical Astronomy Observatory. Removing cosmic rays from a single 800 × 800 pixel Hubble Space Telescope WFPC2 image takes 44 s with the IRAF script lacos_im.cl running on a single core of an Apple Mac Pro computer with two 2.8 GHz quad-core Intel Xeon processors. crblaster is 7.4 times faster when processing the same image on a single core on the same machine. Processing the same image with crblaster simultaneously on all eight cores of the same machine takes 0.875 s—which is a speedup factor of 50.3 times faster than the IRAF script. A detailed analysis is presented of the performance of crblaster, using between 1 and 57 processors on a low-power Tilera 700 MHz 64 core TILE64 processor.

Z∼ 7-10 GALAXIES BEHIND LENSING CLUSTERS: CONTRAST WITH FIELD SEARCH RESULTS

We conduct a search for z 7 dropout galaxies behind 11 massive lensing clusters using 21 arcmin2 of deep Hubble Space Telescope 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 are likely to be low-redshift interlopers. By contrast, we estimate that our robust candidate A1689-zD1 has <0.2% probability of being a low-redshift interloper. 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 is much lower than that which 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 27) where the LF is expected to be very steep. Use of these clusters to constrain the faint-end slope or determine the full LF is likely of less value due to the shallower effective slope measured for the LF at fainter magnitudes, as well as significant uncertainties introduced from modeling both the gravitational lensing and incompleteness.

The connection between globular cluster systems and their host galaxy and environment: a case study of the isolated elliptical NGC 821★

In an effort to probe the globular cluster (GC) system of an isolated elliptical galaxy, a comprehensive analysis of the NGC 821 GC system was performed. New imaging from the WIYN Mini-Mosaic imager, supplemented with Hubble Space Telescope (HST) WFPC2 images reveals a GC system similar to those found in counterpart ellipticals located in high-density environments. To put these results into the context of galaxy formation, a robustly determined census of GC systems is presented and analysed for galaxies spanning a wide range of masses (> M*), morphologies and environments. Results from this meta-study: (1) confirm previous findings that the number of GCs normalized by host galaxy stellar mass increases with host stellar mass. Spiral galaxies in the sample show smaller relative GC numbers than those of massive ellipticals, suggesting the GC systems of massive ellipticals were not formed from major spiral–spiral mergers; (2) indicate that GC system numbers per unit galaxy baryon mass increases with host baryon mass and that GC formation efficiency may not be universal as previously thought; (3) suggest previously reported trends with environment may be incorrect due to sample bias or the use of galaxy stellar masses to normalize GC numbers. Thus claims for environmentally dependent GC formation efficiencies should be revisited; (4) in combination with weak-lensing halo mass estimates, suggest that GCs formed in direct proportion to the halo mass; (5) are consistent with theoretical predictions whereby the local epoch of reionization did not vary significantly with environment or host galaxy type.

Supernova Remnants in the Magellanic Clouds. IX. Multiwavelength Analysis of the Physical Structure of N49

We present a multiwavelength analysis of the supernova remnant N49 in the Large Magellanic Cloud. Using high-resolution Hubble Space Telescope WFPC2 images of Hα, [S II], and [O III] emission, we study the morphology of the remnant and calculate the rms electron densities in different regions. We detect an offset of [O III] and Hα emission peaks of about 0.5'' and discuss possible scenarios that could give rise to such high values. The kinematics of the remnant is analyzed by matching individual kinematic features in the echelle spectra obtained at the Cerro Tololo Inter-American Observatory with the morphological features revealed in the WFPC2 images. We detect a narrow Hα emission component and identify it as diffuse preshock recombination radiation, and discrete broad emission features that correspond to the shocked gas in filaments. The overall expansion of the remnant is about 250 km s-1. The dense clouds are shocked up to line-of-sight velocities of 250 km s-1, and the less dense gas up to 300 km s-1. A few cloudlets have even higher radial velocities, reaching up to 350 km s-1. We confirm the presence of the cavity in the remnant and identify the center of explosion. Using archival Chandra and XMM-Newton data, we observe the same trends in surface brightness distribution for the optical and X-ray images. We carry out a spectral analysis of three regions that represent the most significant optical features.

Gaps in the Cloud Cover? Comparing Extinction Measures in Spiral Disks

Dust in galaxies can be mapped either by the FIR/submillimeter emission, the optical or infrared reddening of starlight, or the extinction of a known background source. We compare two dust extinction measurements for a set of 15 sections in 13 nearby galaxies to determine the scale of the dusty interstellar medium (ISM) responsible for disk opacity: one using stellar reddening and the other a known background source. In our earlier papers, we presented extinction measurements of 29 galaxies, based on calibrated counts of distant background objects identified though foreground disks in Hubble Space Telescope WFPC2 images. For the 13 galaxies that overlap with the Spitzer Infrared Nearby Galaxies Survey, we now compare these results with those obtained from an I - L color map. Our goal is to determine whether or not a detected distant galaxy indicates a gap in the dusty ISM, and hence to better understand the nature and geometry of the disk extinction. We find that distant galaxies are predominantly in low-extinction sections marked by the color maps, indicating that their number depends both on the cloud cover of Spitzer-resolved dust structures, mostly the spiral arms, and a diffuse, unresolved underlying disk. We note that our infrared color map [E(I - L)] underestimates the overall dust presence in these disks severely because it implicitly assumes the presence of a dust screen in front of the stellar distribution.

A Correlation between Galaxy Morphology and MgiiHalo Absorption Strength

(Abridged) We compared the quantified morphological properties of 37 intermediate redshift MgII absorption selected galaxies to the properties of the absorbing halo gas, observed in the spectra of background quasars. The galaxy morphologies were measured using GIM2D modeling of Hubble Space Telescope WFPC-2 images and the absorbing gas properties were obtained from HIRES/Keck and UVES/VLT quasar spectra. We found a 3.1 sigma correlation between galaxy morphological asymmetries normalized by the quasar-galaxy projected separations, A/D, and the MgII rest-frame equivalent widths. Saturation effects cause increased scatter in the relationship with increasing W_r(2796). We defined a subsample for which the fraction of saturated pixels in the absorption profiles is f_sat<0.5. The correlation strengthened to 3.3 sigma. We also find a paucity of small morphological asymmetries for galaxies selected by MgII absorption as compared to those of the general population of field galaxies, as measured in the Medium Deep Survey. The K-S probability that the two samples are drawn from the same galaxy population is ruled out at a 99.8% confidence level. The A/D-W_r(2796) correlation suggests a connection between the processes that perturb galaxies and the quantity of gas in their halos, normalized by the impact parameter. Since the perturbations are minor, it is clear that dramatic processes or events are not required for a galaxy to have an extended halo; the galaxies appear "normal". We suggest that common, more mild processes that populate halos with gas, such as satellite galaxy merging, accretion of the local cosmic web, and longer-range galaxy-galaxy interactions, consequently also induce the observed minor perturbations in the galaxies.

HST observations of azimuthal asymmetry in Saturn's rings

From 378 Hubble Space Telescope WFPC2 images obtained between 1996–2004, we have measured the detailed nature of azimuthal brightness variations in Saturn's rings. The extensive geometric coverage, high spatial resolution ( ∼ 300 km px −1 ), and photometric precision of the UBVRI images have enabled us to determine the dependence of the asymmetry amplitude and longitude of minimum brightness on orbital radius, ring elevation, wavelength, solar phase angle, and solar longitude. We explore a suite of dynamical models of self-gravity wakes for two particle size distributions: a single size and a power law distribution spanning a decade in particle radius. From these N-body simulations, we calculate the resultant wake-driven brightness asymmetry for any given illumination and viewing geometry. The models reproduce many of the observed properties of the asymmetry, including the shape and location of the brightness minimum and the trends with ring elevation and solar longitude. They also account for the “tilt effect” in the A and B rings: the change in mean ring brightness with effective ring opening angle, | B eff | . The predicted asymmetry depends sensitively on dynamical ring particle properties such as the coefficient of restitution and internal mass density, and relatively weakly on photometric parameters such as albedo and scattering phase function. The asymmetry is strongest in the A ring, reaching a maximum amplitude A ∼ 25 % near a = 128 , 000 km . Here, the observations are well-matched by an internal particle density near 450 kg m −3 and a narrow particle size distribution. The B ring shows significant asymmetry (∼5%) in regions of relatively low optical depth ( τ ∼ 0.7 ). In the middle and outer B ring, where τ ≫ 1 , the asymmetry is much weaker ( ∼ 1 % ), and in the C ring, A < 0.5 % . The asymmetry diminishes near opposition and at shorter wavelengths, where the albedo of the ring particles is lower and multiple-scattering effects are diminished. The asymmetry amplitude varies strongly with ring elevation angle, reaching a peak near | B eff | = 10 ° in the A ring and at | B eff | = 15 – 20 ° in the B ring. These trends provide an estimate of the thickness of the self-gravity wakes responsible for the asymmetry. Local radial variations in the amplitude of the asymmetry within both the A and B rings are probably caused by regional differences in the particle size distribution.

AHubble Space TelescopeArchival Survey of Feathers in Spiral Galaxies

We present a survey of spiral arm extinction substructures referred to as feathers in 223 spiral galaxies using Hubble Space Telescope WFPC2 images. The sample includes all galaxies in the RC3 catalog with cz < 5000 km s-1, BT < 15, i < 60°, and types Sa-Sd with well-exposed broadband WFPC2 images. The detection frequency of delineated, periodic feathers in this sample is 20% (45 of 223). This work is consistent with Lynds, who concluded that feathers are common in prototypical Sc galaxies; we find that feathers are equally common in Sb galaxies. Sb-Sc galaxies without clear evidence for feathers either had poorer quality images, or flocculent or complex structure. We did not find clearly defined feathers in any Scd-Sd galaxy. The probability of detecting feathers was highest (83%) for spiral galaxies with well-defined primary dust lanes (the lanes that line the inner edge of an arm); well-defined primary dust lanes were only noted in Sab-Sc galaxies. Consistent with earlier work, we find that neighboring feathers tend to have similar shapes and pitch angles. OB associations are often found lining feathers, and many feathers transition to the stellar substructures known as spurs (Elmegreen). We find that feathers are coincident with interarm filaments strikingly revealed in Spitzer 8 μm images. Comparison with CO (1-0) maps of NGC 0628 and NGC 5194 from BIMA SONG shows that feathers originate at the primary dust lane coincident with gas surface density peaks. Contrary to the appearance at 8 μm, the CO maps show that gas surface density in feathers decreases rapidly with distance from the primary dust lane. We also find that the spacing between feathers decreases with increasing gas surface density, consistent with formation via a gravitational instability.

Integral Field Spectroscopy of the Luminous Infrared Galaxy Arp 299 (IC 694 + NGC 3690)

The luminous infrared galaxy Arp 299 (IC 694 + NGC 3690) is studied using optical integral field spectroscopy obtained with the INTEGRAL system, together with archival Hubble Space Telescope WFPC2 and NICMOS images. The stellar and ionized gas morphology shows λ-dependent variations due to the combined effects of the dust internal extinction and the nature and spatial distribution of the different ionizing sources. The two-dimensional ionization maps have revealed an off-nuclear conical structure of about 4 kpc in length, characterized by high-excitation conditions and a radial gradient in the gas electron density. The apex of this structure coincides with B 1 region of NGC 3690, which in turn presents Seyfert-like ionization, high extinction, and a high velocity dispersion. These results strongly support the hypothesis that B1 is the true nucleus of NGC 3690, where an AGN is located. In the circumnuclear regions H II-like ionization dominates, while LINER-like ionization is found elsewhere. The Hα-emitting sources with ages from 3.3 to 7.2 × 106 yr, have masses of between 6 and 680 × 106 M⊙; and contribute about 45% to the bolometric luminosity (extinction corrected). The ionized (Hα) and neutral (Na D) gas velocity fields show similar structure on scales of several hundred to about 1 kpc, indicating that these gas components are kinematically coupled. The kinematic structure is complex and on scales of about 0.2 kpc does not appear to be dominated by the presence of ordered, rotational motions. The large velocity dispersion measured in NGC 3690 indicates that this galaxy is the most massive of the system. The low velocity amplitude and dispersion of the interface suggest that the ionized gas is slowly rotating or in a close to quiescent phase. © 2006. The American Astronomical Society. All rights reserved.

The globular cluster system of NGC 5846 revisited: colours, sizes and X-ray counterparts

Context. NGC 5846 is a giant elliptical galaxy with a previously well studied globular cluster system (GCS), known to have a bimodal colour distribution with a remarkably high red fraction. Aims. Here we revisit the central galaxy regions searching for new globular cluster (GC) candidates, and measure magnitudes, colours and sizes for them. We also search for their X-ray counterparts. Methods. We use archival Hubble Space Telescope WFPC2 images, from which we modelled and subtracted the host light distribution to increase the available sample of GCs. We performed photometry on the central objects, and measured sizes and coordinates for the entire GC system known in this galaxy. Results. We detect two dozen previously unknown GC candidates in the central regions. Reliable sizes are obtained for about 60 GCs; their typical effective radii are in the range 3-5 pc. The largest clusters are located in the central regions. We find 7 X-ray counterparts to GCs, most of them in the central region. They are among the most luminous X-ray sources in NGC 5846. They are also optically luminous, compact and belong to the red subpopulation. Conclusions. The GCS of NGC 5846 is composed of relatively luminous X-ray sources.

The Recent Star Formation in NGC 6822 fromHubble Space TelescopeImaging

We present Hubble Space Telescope WFPC2 and STIS imaging of the low-metallicity galaxy NGC 6822, performed as part of a study of the young stellar populations in the galaxies of the Local Group. Eleven WFPC2 pointings, with some overlap, cover two regions extending over 19 and 13 arcmin2, respectively, off the galaxy center. The filters used are F170W, F255W, F336W, F439W, and F555W. One 25'' × 25'' field observed with the STIS FUV- and NUV-MAMA includes Hodge's OB 8 association and the H II region Hubble V, contained in field 1 of Bianchi et al.; this previous study provides additional WFPC2 four-band photometry. We derive the physical parameters of the stars in the fields and the extinction by comparing the photometry to grids of model magnitudes. The environments studied in this work include one of the most luminous (in Hα) H II regions in the Local Group (Hubble V) with a compact star cluster, a typical OB association (OB 15), the sparse field population, and the outskirts of NGC 6822. In the WFPC2 fields, most of the hot massive stars are found in the Hodge OB 15 association, at about 5' (0.7 kpc) east of the galaxy center, extending ≈90'' (≈200 pc) in our imaging. The color-magnitude diagram indicates a young age, ≤10 Myr, for this association, where we measure 70 stars hotter than ~16,000 K (earlier than mid-B spectral type) according to their photometric colors. In the compact H II region Hubble V, we measure 80 stars brighter than mNUV < 22.5 mag and find most of them to have high temperatures. The density (per unit area) of hot massive stars in the core of the OB 8 association is higher than in OB 15 by a factor of 12, while the total stellar mass formed is similar (≈4 or 7 × 103 M⊙, when extrapolated to a mass range of 1-100 or 0.1-100 M⊙, respectively). In both OB 15 and OB 8 massive star candidates are found. In the general field outside of the OB 15 association, we find few hot massive stars (0.7 arcmin-2) and several A-type supergiants. No massive star candidates are found in the WFPC2 fields outside the main galaxy body (≈105-145 [1.5-2 kpc] from the galaxy center), where the population is dominated by foreground stars, at least down to V ~ 22. At fainter magnitudes, we measure in these outer fields a significantly larger number of stars than the model for Milky Way foreground objects would predict. The average extinction is found to vary among the three environments studied: E(B - V) = 0.22 in the outer regions, E(B - V) = 0.27 in the fields east of the galaxy main bar, and E(B - V) = 0.40 in the H II region Hubble V. A quantitative discussion of the applicability of the reddening-free-index method for photometric determination of stellar parameters is provided for the filters used in this work, based on our grids of stellar models.

AHubble Space TelescopeSnapshot Survey of Dynamically Close Galaxy Pairs in the CNOC2 Redshift Survey

We compare the structural properties of two classes of galaxies at intermediate redshift: those in dynamically close galaxy pairs, and those that are isolated. Both samples are selected from the Canadian Network for Observational Cosmology field galaxy redshift survey (CNOC2) and have redshifts in the range 0.1 < z < 0.6. Hubble Space Telescope WFPC2 images were acquired as part of a snapshot survey and were used to measure bulge fraction and asymmetry for these galaxies. We find that paired and isolated galaxies have identical distributions of bulge fractions. Conversely, we find that paired galaxies are much more likely to be asymmetric (RT + RA ≥ 0.13) than isolated galaxies. Assuming that half of these pairs are unlikely to be close enough to merge, we estimate that 40% ± 11% of merging galaxies are asymmetric, compared with 9% ± 3% of isolated galaxies. The difference is even more striking for strongly asymmetric (RT + RA ≥ 0.16) galaxies: 25% ± 8% for merging galaxies versus 1% ± 1% for isolated galaxies. We find that strongly asymmetric paired galaxies are very blue, with rest-frame B-R colors close to 0.80, compared with a mean (B-R)0 of 1.24 for all paired galaxies. In addition, asymmetric galaxies in pairs have strong [O II] λ3727 emission lines. We conclude that close to half of the galaxy pairs in our sample are in the process of merging and that most of these mergers are accompanied by triggered star formation.

Hubble Space Telescope imaging of globular cluster candidates in low surface brightness dwarf galaxies

Fifty-seven nearby low surface brightness dwarf galaxies (−10 > MV > −16) were searched for globular cluster candidates (GCCs) using Hubble Space Telescope WFPC2 imaging in V and I. The sample consists of 18 dwarf spheroidal (dSph), 36 irregular (dIrr), and 3 “transition” type (dIrr/dSph) galaxies with angular sizes less than 3.7 kpc situated at distances 2−6 Mpc in the field and in the nearby groups: M 81, Centaurus A, Sculptor, Canes Venatici I cloud. We find that ∼50% of dSph, dIrr/dSph, and dIrr galaxies contain GCCs. The fraction of GCCs located near the center of dwarf spheroidal galaxies is >2 times higher than for dIrrs. The mean integral color of GCCs in dSphs, (V − I)0 = 1.04 ± 0.16 mag, coincides with the corresponding value for Galactic globular clusters and is similar to the blue globular cluster sub-populations in massive early-type galaxies. The color distribution for GCCs in dIrrs shows a clear bimodality with peaks near (V − I)0 = 0.5 and 1.0 mag. Blue GCCs are presumably young with ages t −6.5 mag in both dSph and dIrr galaxies, reminiscent of excess populations of faint globular clusters in nearby Local Group spiral galaxies. The measurement of structural parameters using King-profile fitting reveals that most GCCs have structural parameters similar to extended outer halo globular clusters in the Milky Way and M 31, as well as the recently discovered population of “faint fuzzy” clusters in nearby lenticular galaxies.

UltravioletHubble Space TelescopeObservations of the Jet in M87

We present new ultraviolet photometry of the jet in M87 obtained from Hubble Space Telescope (HST) WFPC2 imaging. We combine these ultraviolet data with previously published photometry for the knots of the jet in radio, optical, and X-ray and fit three theoretical synchrotron models to the full data set. The synchrotron models consistently overpredict the flux in the ultraviolet when fitted over the entire data set. We show that if the fit is restricted to the radio through ultraviolet data, the synchrotron models can provide a good match to the data. The break frequencies of these fits are much lower than previous estimates. The implied synchrotron lifetimes for the bulk of the emitting population are longer than in earlier work but still much shorter than the estimated kinematic lifetimes of the knots. The observed X-ray flux cannot be successfully explained by the simple synchrotron models that fit the ultraviolet and optical fluxes. We discuss the possible implications of these results for the physical properties of the M87 jet. We also observe increased flux for the HST-1 knot that is consistent with previous results for flaring. This observation fills in a significant gap in the time coverage early in the history of the flare and therefore sets constraints on the initial brightening of the flare.

Discovery of Optical Emission in the Hot Spots of Three 3CR Quasars: High‐Energy Particle Acceleration in Powerful Radio Hot Spots

Archival Hubble Space Telescope WFPC2 images were used to search for optical emission associated with the radio jets of a number of luminous quasars. From this search, we report new optical hot spot detections in the well-known blazar 3C 454.3 and the lobe-dominated quasars 3C 275.1 and 3C 336. We also find optical emission in the vicinity of the hot spot in 3C 208 but believe this is a chance alignment. Optical emission from the arcsecond-scale jet in 3C 454.3 is also detected. Multifrequency archival radio data from the VLA and MERLIN are analyzed, and the synchrotron spectra of these high-power hot spots are presented. We estimate that their break frequencies are in the range of 1010-1011 Hz, with large uncertainties due to the wide gap in frequency coverage between the radio and optical bands. We also calculate their equipartition magnetic fields and find that the anticorrelation between break frequency and magnetic field found by Brunetti et al. for lower power hot spots extends to these high-power hot spots. This supports their model of hot spots based on shock acceleration and synchrotron losses.

Infall, the Butcher‐Oemler Effect, and the Descendants of Blue Cluster Galaxies atz ∼ 0.6

Using wide-field Hubble Space Telescope WFPC2 imaging and extensive Keck LRIS spectroscopy, we present a detailed study of the galaxy populations in MS 2053-04, a massive, X-ray-luminous cluster at z = 0.5866 ± 0.0011. Analysis of 149 confirmed cluster members shows that MS 2053 is composed of two structures that are gravitationally bound to each other; their respective velocity dispersions are 865 ± 71 km s-1 (113 members) and 282 ± 51 km s-1 (36 members). MS 2053's total dynamical mass is 1.2 × 1015 M☉. MS 2053 is a classic Butcher-Oemler cluster with a high fraction of blue members (24% ± 5%) and an even higher fraction of star-forming members (44% ± 7%), as determined from their [O II] λ3727 emission. The number fraction of blue/star-forming galaxies is much higher in the infalling structure than in the main cluster. This result is the most direct evidence to date that the Butcher-Oemler effect is linked to galaxy infall. In terms of their colors, luminosities, estimated internal velocity dispersions, and [O II] λ3727 equivalent widths, the infalling galaxies are indistinguishable from the field population. MS 2053's deficit of S0 galaxies combined with its overabundance of blue spirals implies that many of these late-type galaxies will evolve into S0 members. The properties of the blue cluster members in both the main cluster and infalling structure indicate that they will evolve into low-mass, L 3.

Spatially Resolved Spectroscopy and Coronagraphic Imaging of the TW Hydrae Circumstellar Disk

We present the first spatially resolved spectrum of scattered light from the TW Hydrae protoplanetary disk. This nearly face-on disk is optically thick, surrounding a classical T Tauri star in the nearby 10 Myr old TW Hya association. The spectrum was taken with the Hubble Space Telescope (HST) STIS CCD, providing resolution R ~ 360 over the wavelength range 5250-10300 Å. Spatially resolved spectroscopy of circumstellar disks is difficult because of the high contrast ratio between the bright star and faint disk. Our novel observations provide optical spectra of scattered light from the disk between 40 and 155 AU from the star. The scattered light has the same color as the star (gray scattering) at all radii except the innermost region. This likely indicates that the scattering dust grains are larger than about 1 μm all the way out to large radii. From the spectroscopic data, we also obtained radial profiles of the integrated disk brightness at two position angles, over almost the same region as previously observed in HST WFPC2 and NICMOS coronagraphic images (35 to 173 AU from the star). The profiles have the same shape as the earlier ones, but show a small azimuthal asymmetry in the disk not previously noted. Our STIS broadband coronagraphic images of TW Hya confirm the reality of this asymmetry, and show that the disk surface brightness inside 140 AU has a sinusoidal dependence on azimuthal angle. The maximum brightness occurs at a position angle of 2336 ± 57 east of north. This might be caused by the combination of forward scattering and an increase in inclination in the inner region of the disk, suggesting that the TW Hya disk has a warp like that seen in the β Pictoris debris disk.

Location of the Optical Reverse Shock in the Cassiopeia A Supernova Remnant

We use two epochs of Hubble Space Telescope WFPC2 images separated by 2 yr to determine the location and propagation of the reverse shock (RS) in the young supernova remnant Cassiopeia A (Cas A). The images trace optical line emission from fast-moving knots and filaments of highly processed ejecta as they cross the RS, become heated and compressed, and radiatively cool. At numerous positions around the optical shell, new emission features are seen in the 2002 images that were not yet visible in the 2000 exposures. In a few instances emission features seen in the first epoch have completely disappeared in the second epoch. We concentrate on two regions along the rim of the main emission shell in Cas A for close inspection: one in the northwestern part of the shell and another along the southwestern part of it. In these regions the RS is viewed almost edge-on, and its precise position has been measured. The RS is coherent in these regions over arcminute (~1 pc) scales but is highly distorted perpendicular to the direction of expansion. We find the RS to be generally expanding at 50%-60% of the ~5500 km s-1 bulk velocity of the optical ejecta. We present shock models for the ejecta that are consistent with the high densities and short cooling times observed in the optical knots of the Cas A remnant.