Wide Field Planetary Camera 2 Images Research Articles

QSO number counts to B < 24.5 from hubble space telescope WFPC2 images

We have conducted a survey for faint quasars to B < 24.5, z < 2.1 based on archival high-latitude Hubble Space Telescope Wide Field Planetary Camera 2 (WFPC2) images covering 0.04 deg 2 of the sky in three filters spanning 2300–9600 Å. Taking advantage of the superb 0.1″ resolution of WFPC2, we have developed an approach to select quasar candidates based on their morphology and position in multicolor space. We derive cumulative surface densities and compare them with our previous survey in the Groth–Westphal Strip and other faint quasar surveys that utilize very different selection approaches. In most cases the agreement with these surveys is good, however, we find approximately twice as many quasar candidates to B = 24.5 as the COMBO-17 survey. Although we are confident that our approach eliminates most contaminating stars and galaxies, spectroscopic follow-up will be required to determine the level of residual contamination and confirm or refute this excess.

A Hubble Space Telescope Survey of the Disk Cluster Population of M31. I. WFPC2 Pointings11Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5‐26555.

ABSTRACTAs a follow‐up to the automated cluster search carried out by Williams & Hodge, we have examined 39 Hubble Space Telescope Wide Field Planetary Camera 2 (WFPC2) pointings to locate and study a comprehensive collection of disk clusters. The Williams technique was effective in finding young clusters, but not intermediate‐age or old clusters. Our searches have shown that M31 has large numbers of these intermediate and older open clusters, most of them undetected by both the Williams survey and other ground‐based searches. We present a catalog of 343 clusters detected on the WFPC2 images. Extrapolation from our data indicates that the entire disk of M31 contains approximately 80,000 star clusters. We have carried out integrated multicolor photometry of these clusters to ascertain their properties and to compare their properties with cluster systems of other galaxies. We show the cluster luminosity function, the color‐magnitude diagram, the formation function, and the size distribution. Cluster densities and colors show trends with disk position. An age distribution is derived and, although the ages are very uncertain for the fainter clusters, there is evidence for cluster dynamical destruction at about the same rate as in our Galaxy.

VIMOS-IFU survey ofz~ 0.2 massive galaxy clusters

We present extensive multi-color imaging and low-resolution VIMOS integral field unit (IFU) spectroscopic observations of the X-ray luminous cluster Abell 2667 (). An extremely bright giant gravitational arc () is easily identified as part of a triple image system, and other fainter multiple images are also revealed by the Hubble Space Telescope Wide Field Planetary Camera-2 images. The VIMOS-IFU observations cover a field of view of and enable us to determine the redshift of all galaxies down to . Furthermore, redshifts could be identified for some sources down to . In particular we identify 21 members in the cluster core, from which we derive a velocity dispersion of km s-1, corresponding to a total mass of within a kpc (30 arcsec) radius. Using the multiple images constraints and priors on the mass distribution of cluster galaxy halos we construct a detailed lensing-mass model leading to a total mass of within the Einstein radius (16 arcsec). The lensing mass and dynamical mass are in good agreement, although the dynamical one is much less accurate. Within a 110 kpc radius, we find a rest-frame K-band M/L ratio of . Comparing these measurements with published X-ray analysis is, however, less conclusive. Although the X-ray temperature matches the dynamical and lensing estimates, the published NFW mass model derived from the X-ray measurement with its low concentration of cannot account for the large Einstein radius observed in this cluster. A higher concentration of ∼6 would, however, match the strong lensing measurements. These results very likely reflect the complex structure of the cluster mass distribution, underlying the importance of panchromatic studies from small to large scale in order to better understand cluster physics.

Cluster Populations in Abell 2125 and 2218

We combine new narrowband photometry with archival Wide Field Planetary Camera 2 (WFPC2) data for A2218 (z = 0.18) and A2125 (z = 0.25), two clusters with intermediate redshifts but very different cluster properties, in order to examine the evolution of galaxy populations. A2218 is a dense, elliptical-rich cluster (Bautz-Morgan type II) similar to the Coma Cluster in its evolutionary appearance, whereas A2125 is a less dense, more open cluster (Bautz-Morgan type II–III), although similar in richness to A2218. The color-magnitude relation indicates that A2125 has a more developed blue population than A2218 (the Butcher-Oemler effect), although both clusters have significant numbers of blue galaxies (ranging in star formation rate from normal, star-forming disks to starburst systems) as compared to a present-day cluster. The colors of the red populations are identical in A2125 and A2218 and are well fitted by passive evolution models. We are able, for the first time, to combine archived WFPC2 images with our narrowband photometry for a color, morphological, and structural analysis of the blue Butcher-Oemler population. We find the blue population to be composed of two subpopulations, a bright, spiral population and a fainter, dwarf starburst population. A2125 is richer in bright starburst systems, a phenomenon apparently induced by the cluster's younger dynamical state. In addition, a majority of the S0 population in A2125 and A2218 is composed of bulge + disk systems, whereas nearby clusters such as Coma are composed primarily of lenticulars (pure disk S0s). The structural parameters of the S0 bulges in A2125 and A2218 are identical to the structure parameters of cluster ellipticals, suggesting that the disks of some S0s in intermediate-redshift clusters are stripped away, with the leftover bulges evolving into present-day ellipticals.

Cosmic Gravitational Shear from the Hubble Space Telescope Medium Deep Survey

We present a measurement of cosmic shear on scales ranging from 10'' to 2' in 347 Wide Field Planetary Camera 2 (WFPC2) images of random fields. Our result is based on shapes measured via image fitting and on a simple statistical technique; careful calibration of each step allows us to quantify our systematic uncertainties and to measure the cosmic shear down to very small angular scales. The WFPC2 images provide a robust measurement of the cosmic shear signal decreasing from 5.2% at 10'' to 2.2% at 130''.

From Globular Clusters to Tidal Dwarfs: Structure Formation in the Tidal Tails of Merging Galaxies

Using V and I images obtained with the Wide Field Planetary Camera 2 (WFPC2) of the Hubble Space Telescope, we investigate compact stellar structures within tidal tails. Six regions of tidal debris in the four classic Toomre sequence mergers: NGC 4038/39 (Antennae), NGC 3256, NGC 3921, and NGC 7252 (Atoms for Peace) have been studied in order to explore how the star formation depends on the local and global physical conditions. These mergers sample a range of stages in the evolutionary sequence and tails with and without embedded tidal dwarf galaxies. The six tails are found to contain a variety of stellar structures, with sizes ranging from those of globular clusters up to those of dwarf galaxies. From V and I WFPC2 images, we measure the luminosities and colors of the star clusters. NGC 3256 is found to have a large population of blue clusters (0.2 V-I 0.9), particularly in its western tail, similar to those found in the inner region of the merger. In contrast, NGC 4038/39 has no clusters in the observed region of the tail, only less luminous point sources likely to be individual stars. NGC 3921 and NGC 7252 have small populations of clusters along their tails. A significant cluster population is clearly associated with the prominent tidal dwarf candidates in the eastern and western tails of NGC 7252. The cluster-rich western tail of NGC 3256 is not distinguished from the others by its dynamical age or by its total H I mass. However, the mergers that have few clusters in the tail all have tidal dwarf galaxies, while NGC 3256 does not have prominent tidal dwarfs. We speculate that star formation in tidal tails may manifest itself either in small structures like clusters along the tail or in large structures such as dwarf galaxies, but not in both. Also, NGC 3256 has the highest star formation rate of the four mergers studied, which may contribute to the high number of star clusters in its tidal tails.

Detection of Optical Synchrotron Emission from the Radio Jet of 3C 279

We report the detection of optical and ultraviolet emission from the kiloparsec scale jet of the well-known quasar 3C~279. A bright knot, discovered in archival V and U band {\it Hubble Space Telescope} Faint Object Camera images, is coincident with a peak in the radio jet $\sim0.6\arcsec$ from the nucleus. The detection was also confirmed in Wide Field Planetary Camera-2 images. Archival Very Large Array and MERLIN radio data are also analyzed which help to show that the high-energy optical/UV continuum, and spectrum, are consistent with a synchrotron origin from the same population of relativistic electrons responsible for the radio emission.

Investigation of Gravitational Lens Mass Models

We have previously reported the discovery of strong gravitational lensing by faint elliptical galaxies using the Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope, and here we investigate their potential usefulness in placing constraints on lens mass models. We compare various ellipsoidal surface mass distributions, including those with and without a core radius, as well as models in which the mass distributions are assumed to have the same axis ratio and orientation as the galaxy light. We also study models that use a spherical mass distribution having various profiles, both empirical and following those predicted by cold dark matter simulations. These models also include a gravitational shear term. The model parameters and associated errors have been derived by two-dimensional analysis of the observed WFPC2 images. The maximum likelihood procedure iteratively converges simultaneously on the model for the lensing elliptical galaxy and the lensed image components. The motivation for this study was to distinguish between these mass models with this technique. However, we find that, despite using the full image data rather than just locations and integrated magnitudes, the lenses are fitted equally well with several of the mass models. Each of the mass models generates a similar configuration but with a different magnification and cross-sectional area within the caustic, and both of these latter quantities govern the discovery probability of lensing in the survey. These differences contribute to considerable cosmic scatter in any estimate of the cosmological constant, Λ, using gravitational lenses.

Kinematics of 1200 Kilometer per Second Jets in He 3‐1475

Spectroscopic observations of a proto-planetary nebula He 3-1475 with the Space Telescope Imaging Spectrograph reveal the kinematics of its high-velocity (1200 km s-1) jets. The jets are formed at a large (0.15 pc) distance from its central star by collimation of an asymmetric stellar wind in a pair of conical shocks seen in Wide Field Planetary Camera 2 (WFPC2) images. The jets consist of several pairs of knots symmetrically distributed with respect to the central star, with most knots exhibiting a head-tail morphology. Large (up to 650 km s-1) radial velocity gradients are seen within the knots on subarcsecond scales, with velocities decreasing from the knot heads toward their trailing tails. These large velocity gradients are a sign of efficient deceleration of jets by a much slower bipolar outflow. The inclination angle of the bipolar outflow is equal to 40°, based on Doppler shifts of the scattered stellar Hα line. Its velocity is equal to 140 km s-1 at a distance of 0.23 pc from the star and increases monotonically with the radial distance from the star. A comparison of new WFPC2 [N II] λ6584 images with older WFPC2 images reveals expansion of the jets. The measured jet proper motions in combination with their radial velocities imply that He 3-1475 is a Galactic bulge star at a distance of 8 kpc, located 800 pc above the Galactic plane. Its very high luminosity (25,000 L☉) implies that He 3-1475 must be significantly more massive than a typical asymptotic giant branch star within the Galactic bulge, perhaps because of a past mass transfer and/or a merger event in an interacting binary system.

Hubble Space TelescopeObservations of the Host Galaxies of BL Lacertae Objects

Six BL Lac objects from the complete 1 Jy radio-selected sample of 34 objects were observed in cycle 5 with the Hubble Space Telescope (HST) Wide-Field Planetary Camera 2 (WFPC2) to an equivalent limiting flux of μ~26 mag arcsec-2. Here we report results for the second half of this sample, as well as new results for the first three objects, discussed previously by Falomo and coworkers. In addition, we have analyzed in the same way HST images of three X-ray-selected BL Lac objects observed by Jannuzi and coworkers. The ensemble of nine BL Lac objects spans the redshift range from z=0.19 to ~1. Host galaxies are clearly detected in seven cases, while the other two, at z~0.258 (redshift highly uncertain) and z=0.997, are not resolved. The HST images make up a homogeneous data set with unprecedented morphological information between a few tenths of an arcsecond and several arcseconds from the nucleus, allowing us to rule out definitively a pure disk light profile in six of the seven detected host galaxies. The host galaxies are luminous ellipticals with an average absolute magnitude of M~-24.6 mag (with dispersion 0.7 mag), more than a magnitude brighter than L and comparable to the brightest cluster galaxies. The morphologies are generally smooth and have small ellipticities (0.2). Given such roundness, there is no obvious alignment with the more linear radio structures. In the six cases for which we have HST WFPC2 images in two filters, the derived color profiles show no strong spatial gradients and are as expected for K-corrected passively evolving elliptical galaxies. The host galaxies of the radio-selected and X-ray-selected BL Lac objects for this very limited sample are comparable in both morphology and luminosity.

Quasar host galaxy images from the Hubble Space Telescope Archive

ABSTRA C T We systematically searched the non-proprietary data in the Hubble Space Telescope (HST) Archive to identify low-redshift quasars that have been imaged with the Wide Field Planetary Camera-2 (WFPC-2) but that have not yet been presented in the literature. We identified five such objects. Since these represent a significant increment to the number of WFPC-2 imaged quasars previously presented in the literature, we here present images of these objects and a discussion of their properties. Of the five objects, two are involved in spectacular ongoing gravitational interactions. This brings the total number of quasars seen on WFPC-2 images to be in such interactions to 11. A further two of the five quasars have close companion objects (i.e. within 30 kpc) suggestive of a recent interaction. No evidence for an interaction can be seen on the fifth quasar. All five host systems are brighter than L, in keeping with the results found for other quasars imaged with WFPC-2. Three of the five quasars are at least 2.5 mag brighter than L, and are among the brightest host galaxies imaged with WFPC-2. The results widen the derived difference in the mean absolute luminosity of the hosts of radio-quiet quasars compared with the hosts of radio-loud quasars.

Wide Field Planetary Camera 2 Observations of Proxima Centauri: No Evidence of the Possible Substellar Companion

Two-epoch observations of Proxima Centauri using the Hubble Space Telescope (HST) Wide Field Planetary Camera 2 (WFPC2) are reported. Exposures of 10 and 40 s were recorded through the F1042M filter (λc ≈ 1 μm), permitting examination of the circumstellar region beyond 009 from Proxima Cen. No evidence of a substellar companion within 085 of Proxima Cen is seen, which counters the recently reported detection of a faint feature by Schultz et al. using HST's Faint Object Spectrograph (FOS). A feature in the WFPC2 images having a relative brightness and a separation comparable to those of the FOS feature would have been detected with a signal-to-noise ratio of ~22. Moreover, if the FOS feature were a substellar companion, it should have appeared in our F1042M images to be about 3.7 mag fainter than Proxima Cen. Inspection of deep WFPC2 images of Proxima Cen through three filters indicates that the FOS feature is not a background object. Local enhancements of WFPC2's point-spread function suggest a possible instrumental origin for the FOS feature, but the singularity and apparent motion of the FOS feature complicate this notion.

The Globular Clusters and Halo of M31

This thesis presents the results of a study of the globular cluster (GC) system in the Andromeda galaxy (5M31 5 NGC 224). Deep Vand I-band images of 24 M31 GCs were taken with the High Resolution Camera (HRCam) at the CanadaFrance-Hawaii Telescope (CFHT) with typical seeing full widths at half-maxima of . These images are the FWHM ∼ 00.7 best ground-based data to date for determining structural parameters (such as core and tidal radii, ellipticities, and orientations) of M31’s GCs. Two-dimensional seeing-convolved Michie-King models were fitted to each GC, and the results were compared to the structures of the Galactic GCs. There is no evidence for any differences between the mean tidal radii and ellipticities of the GCs in the two galaxies. Core radii and half-mass radii are too strongly affected by seeing to allow such a comparison to be made with these images. Deep ( ) Vand I-band images of fields containing V ∼ 27 lim G302 and G312, two GCs in the M31 halo, were obtained using the Wide Field Planetary Camera 2 (WFPC2) aboard the Hubble Space Telescope (HST). These data were used to construct color-magnitude diagrams (CMDs) of the two GCs and of the M31 halo. Both GCs have old stellar populations similar to those found in Galactic GCs. G302 and G312 have iron abundances of [Fe/H] 5 21.85 5 0.12 and [Fe/H] 5 20.56 5 0.03, respectively. G302 shows no evidence for a color gradient, but the inner regions of G312 are bluer than the outer regions. The halo CMDs indicate that the M31 halo stars have a mean metallicity of with a spread of — [m/H] ∼ 20.6 2 . The shape of the halo [m/H] distribution 2.0 & [m/H] & 20.2 is similar to that in the halo of the Milky Way, but the M31 halo has times that of the Galactic halo. The mean — [m/H] ∼ 8 metallicity of stars in the halo of M31 is ∼4 times greater than that of the M31 GC system, unlike the situation in the halo of the Milky Way, where both the stars and the GC system have similar mean metallicities. Finally, no evidence is found for the presence of young stars, or of a second parameter problem, in either GC. Figure 1 shows the dereddened CMDs for the two GCs and the corresponding halo fields. Structural parameters for G302 and G312 were derived from the WFPC2 images and compared to those measured on the HRCam images. The tidal radii and ellipticities from the CFHT data agree with the more reliable results obtained using the HST data. Using the same methods, core radii are found to agree when the seeing FWHM is &4 times the core radius. Star counts reveal an extended elliptical stellar halo beyond the fitted King-Michie tidal radius of G302. The size of this extended halo is consistent with stellar evaporation rates that have been predicted for Galactic GCs. The orientation of this halo is combined with the GC’s published radial velocity to derive orbital parameters for G302.

Mass Functions &amp; Stellar Populations of Globular Clusters

I present a summary of results from various HST photometric studies of the dense central regions of Galactic globular clusters that my collaborators and I have carried out over the last 6 years. The dataset includes short exposures of 47 Tuc, M15, M3 and M13 obtained with the aberrated Planetary Camera-I (PC-I) and F555W (“V”) and F785LP (“I”) filters, as well as post-refurbishment Wide Field Planetary Camera 2 (WFPC2) snapshots of the post core collapse clusters M15, M30, and NGC 6624 in F336W (“U”), F439W (“B”), and V. Recently, a very deep, doubly oversampled PC-I U image of the core of 47 Tuc, and accompanying B and V images, have also been analyzed. In addition, we have carried out extensive checks of incompleteness and photometric error with the help of multiband image simulations that mimic the relevant characteristics of the HST PC-I and WFPC2 images: empirical point spread function, crowding effects based on a realistic density profile and stellar luminosity function (LF), noise, undersampling, A/D saturation, etc..

Hubble Space TelescopeWFPC2 Observations of the Binary Fraction among Pre–Main‐Sequence Cluster Stars in Orion

We present estimates of the binary frequency for optically visible (IHST < 19 mag) members of the dense (mean star separation ~0.05 pc) embedded stellar clusters associated with NGC 2024, NGC 2068, and NGC 2071 (distance = 460 pc) based on Hubble Space Telescope (HST) Wide Field Planetary Camera 2 (WFPC2) observations. Of 99 targets, 15 are found to be double stars with projected linear separations of 138-1050 AU (03 < θ < 23). The resulting multiplicity fraction is 15/99 or 0.15 ± 0.04 in this separation range, while the comparable values observed for solar neighborhood G and K stars are 0.11 and 0.09, respectively. We also examined the archival F547M WFPC2 images of the Trapezium cluster obtained by O'Dell & Wen and found a binary frequency of 7/50 or 0.14 ± 0.05 in the separation range 138-828 AU for stars with VHST < 17. Our results compare well with the value of 0.16 ± 0.03 determined recently by Reipurth & Zinnecker from observations of 238 pre-main-sequence (PMS) stars in nearby (d ~ 150 pc) low-density (mean star separation ~0.3 pc) star-forming regions over comparable separation ranges. Our results suggest an excess of PMS binaries over main-sequence binaries with the same separations. In addition, we find no evidence that the fraction of binaries formed in relatively dense clusters differs from that characterizing low-density star-forming regions.

Quantitative Morphology of Moderate‐Redshift Galaxies: How Many Peculiar Galaxies Are There?

The advent of the Hubble Space Telescope (HST) has provided images of galaxies at moderate and high redshifts and changed the scope of galaxy morphologies considerably. It is evident that the Hubble Sequence requires modifications in order to incorporate all the various morphologies one encounters at such redshifts. We investigate and compare different approaches to quantifying peculiar galaxy morphologies on images obtained from the Medium Deep Survey (MDS) and other surveys using the Wide Field Planetary Camera 2 (WFPC2) on board the HST, in the I band (filter F814W). We define criteria for peculiarity and put them to use on a sample of 978 galaxies, classifying them by eye as either normal or peculiar. Based on our criteria and on concepts borrowed from digital image processing we design a set of four purely morphological parameters, which comprise the overall texture (or ``blobbiness'') of the image; the distortion of isophotes; the filling-factor of isophotes; and the skeleta of detected structures. We also examine the parameters suggested by Abraham et al. (1995). An artificial neural network (ANN) is trained to distinguish between normal and peculiar galaxies. While the majority of peculiar galaxies are disk-dominated, we also find evidence for a significant population of bulge-dominated peculiars. Consequently, peculiar galaxies do not all form a ``natural'' continuation of the Hubble sequence beyond the late spirals and the irregulars. The trained neural network is applied to a second, larger sample of 1999 WFPC2 images and its probabilistic capabilities are used to estimate the frequency of peculiar galaxies at moderate redshifts as $35 \pm 15 %$.