arXiv subjects

R. R. Gal

Publications and source records attributed to R. R. Gal.

At least 37 records · Page 2Linked to original sources

arXiv2006

X-ray Galaxy Clusters in NoSOCS: Substructure and the Correlation of Optical and X-ray Properties

We present a comparison of optical and X-ray properties of galaxy clusters in the northern sky. We determine the recovery rate of X-ray detected clusters in the optical as a function of richness, redshift and X-ray luminosity, showing that the missed clusters are typically low contrast systems when observed optically. We employ four different statistical tests to test for the presence of substructure using optical two-dimensional data, finding that approximately 35% of the clusters show strong signs of substructure. However, the results are test-dependent, with variations also due to the magnitude range and radius utilized.We have also performed a comparison of X-ray luminosity and temperature with optical galaxy counts (richness). We find that the slope and scatter of the relations between richness and the X-ray properties are heavily dependent on the density contrast of the clusters. The selection of substructure-free systems does not improve the correlation between X-ray luminosity and richness, but this comparison also shows much larger scatter than one obtained using the X-ray temperature. In the latter case, the sample is significantly reduced because temperature measurements are available only for the most massive (and thus high contrast) systems. However, the comparison between temperature and richness is very sensitive to the exclusion of clusters showing signs of substructure. The correlation of X-ray luminosity and richness is based on the largest sample to date ($\sim$ 750 clusters), while tests involving temperature use a similar number of objects as previous works ($\lsim$100). The results presented here are in good agreement with existing literature.

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arXiv2006

Mass Along the Line of Sight to the Gravitational Lens B1608+656: Galaxy Groups and Implications for H_0

We report the discovery of four groups of galaxies along the line of sight to the B1608+656 gravitational lens system. One group is at the redshift of the primary lensing galaxy (z = 0.631) and appears to have a low mass, with eight spectroscopically-confirmed members and an estimated velocity dispersion of 150+/-60 km/s. The three other groups are in the foreground of the lens. These groups contain ~10 confirmed members each, and are located at redshifts of 0.265, 0.426, and 0.520. Two of the three additional groups are centered roughly on the lens system, while the third is centered ~1 arcminute south of the lens. We find that each group provides an external convergence (kappa_ext) of ~0.005--0.060, depending on the assumptions made in calculating the convergence. For lens systems where no additional observables that can break the mass-sheet degeneracy exist, the determination of H_0 will be biased high by a factor of (1-kappa_ext)^{-1} if the external convergence is not properly included in the model. For the B1608+656 system, the stellar velocity dispersion of the lensing galaxy has been measured, thus breaking the mass-sheet degeneracy due to the group that is physically associated with the lens. The effect of the other groups along the line of sight can be folded into the overall uncertainties due to large-scale structure (LSS) along the line of sight. Because the B1608+656 system appears to lie along an overdense line of sight, the LSS will cause the measurement of H_0 to be biased high for this system. The systematic bias introduced by LSS could be 5% or greater. (Abridged)

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arXiv2005

Closing in on a Short-Hard Burst Progenitor: Constraints from Early-Time Optical Imaging and Spectroscopy of a Possible Host Galaxy of GRB 050509b

The localization of the short-duration, hard-spectrum GRB 050509b was a watershed event. Thanks to the nearly immediate relay of the GRB position by Swift, we began imaging the GRB field 8 minutes after the burst and continued for the following 8 days. No convincing optical/infrared candidate afterglow or supernova was found for the object. We present a re-analysis of the XRT afterglow and find an absolute position that is ~4" to the west of the XRT position reported previously. Close to this position is a bright elliptical galaxy with redshift z=0.2248, about 1' from the center of a rich cluster of galaxies. Based on positional coincidences, the GRB and the bright elliptical are likely to be physically related. We thus have discovered evidence that at least some short-duration, hard-spectra GRBs arise at cosmological distances. However, while GRB 050509b was underluminous compared to long-duration GRBs, we demonstrate that the ratio of the blast-wave energy to the gamma-ray energy is consistent with that of long-duration GRBs. Based on this analysis, on the location of the GRB (40 +- 13 kpc from a bright galaxy), on the galaxy type (elliptical), and the lack of a coincident supernova, we suggest that there is now observational consistency with the hypothesis that short-hard bursts arise during the merger of a compact binary. We limit the properties of a Li-Paczynski ''mini-supernova.'' Other progenitor models are still viable, and additional rapidly localized bursts from the Swift mission will undoubtedly help to further clarify the progenitor picture. (abridged)

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arXiv2005

A Catalog of Distant Compact Groups Using DPOSS

In this paper we present an objectively defined catalog of 459 small, high density groups of galaxies out to z~0.2 in a region of 6260 square degrees in the northern sky derived from the Digitized Second Palomar Observatory Sky Survey. Our catalog extends down to r = 19.0 and has a median redshift of z_med = 0.12, making it complementary to Hickson's catalog for the nearby universe (z_med = 0.03). The depth and angular coverage of this catalog makes it valuable for studies of the general characteristics of small groups of galaxies and how galaxies evolve in and around them. We also examine the relationship between compact groups and large scale structure.

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arXiv2005

Morphology of low-redshift compact galaxy clusters I. Shapes and radial profiles

The morphology of clusters of galaxies may be described with a set of parameters which contain information about the formation and evolutionary history of these systems. In this paper we present a preliminary study of the morphological parameters of a sample of 28 compact Abell clusters extracted from DPOSS data. The morphology of galaxy clusters is parameterized by their apparent ellipticity, position angle of the major axis, centre coordinates, core radius and beta-model power law index. Our procedure provides estimates of these parameters by simultaneously fitting them all, overcoming some of the difficulties induced by sparse data and low number statistics typical of this kind of analysis. The cluster parameters were fitted in a 3 x 3 h^-2 sqMpc region, measuring the background in a 2 <R< 2.5 h^-1Mpc annulus. We also explore the correlations between shape and profile parameters and other cluster properties. One third of this compact cluster sample has core radii smaller than 50 h^-1 kpc, i.e. near the limit that our data allow us to resolve, possibly consistent with cusped models. The remaining clusters span a broad range of core radii up to 750 h^-1 kpc. More than 80 per cent of this sample has ellipticity higher than 0.2. The alignment between the cluster and the major axis of the dominant galaxy is confirmed, while no correlation is observed with other bright cluster members. No significant correlation is found between cluster richness and ellipticity. Instead, cluster richness is found to correlate, albeit with large scatter, with the cluster core radius.[abridged]

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arXiv2005

Pushing the Boundaries of the Cl 1604 Supercluster at z~0.9

The Cl 1604 supercluster at z~0.9 is known to contain at least four distinct member clusters, separated in both projection and redshift. In this paper we present deep, multicolor wide-field imaging of a region spanning ~45' on a side, corresponding to 21 h^{-1}_{70} Mpc (physical) at the supercluster redshift. We select galaxies whose colors correspond to those of spectroscopically confirmed cluster members in the r' vs. (r'-i') color-magnitude diagram. Using an adaptive kernel, we generate a map of the projected red galaxy density and identify numerous new candidate clusters which are likely supercluster members. Assuming that all of the density peaks are associated with the supercluster, its transverse size is ~10 h^{-1}_{70} Mpc, which is still significantly smaller than the nearly 93 h^{-1}_{70} Mpc depth in redshift space.

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arXiv2004

Massive Star Populations in Wolf-Rayet Galaxies

We analyze longslit spectral observations of fourteen Wolf-Rayet galaxies from the sample of Schaerer, Contini & Pindao (1999). All 14 galaxies show broad Wolf-Rayet emission in the blue region of the spectrum, consisting of a blend of NIII4640, CIII4650, CIV4658, and HeII4686 emission lines, which is a spectral characteristic of WN stars. Broad CIV5808 emission, termed the red bump, is detected in 9 galaxies and CIII5996 is detected in 6 galaxies. These emission features are due to WC stars. We derive the numbers of late WN and early WC stars from the luminosity of the blue and red bumps, respectively. The number of O stars is estimated from the luminosity of the Hbeta emission line, after subtracting the contribution of WR stars. The Schaerer & Vacca 1998 (hereafter SV98) models predict that the number of WR stars relative to O stars, N_WR/N_O, increases with metallicity. For low metallicity galaxies, the results agree with predictions of evolutionary synthesis models for galaxies with a burst of star formation, and indicates an IMF slope -2 < Gamma < -2.35 in the low metallicity regime. For high metallicity galaxies our observations suggest a Salpeter IMF (Gamma = -2.35) and an extended short burst. The main possible sources of error are the adopted luminosities for single WCE and WNL stars. We also report, for the first time, NGC 450 as a galaxy with WR characteristics.

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arXiv2004

The Northern Sky Optical Cluster Survey IV: An Intermediate Redshift Galaxy Cluster Catalog and the Comparison of Two Detection Algorithms

We present an optically selected galaxy cluster catalog from ~ 2,700 square degrees of the Digitized Second Palomar Observatory Sky Survey (DPOSS), spanning the redshift range 0.1 < z < 0.5, providing an intermediate redshift supplement to the previous DPOSS cluster survey. This new catalog contains 9,956 cluster candidates and is the largest resource of rich clusters in this redshift range to date. The candidates are detected using the best DPOSS plates based on seeing and limiting magnitude. The search is further restricted to high galactic latitude (|b| > 50), where stellar contamination is modest and nearly uniform. We also present a performance comparison of two different detection methods applied to this data, the Adaptive Kernel and Voronoi Tessellation techniques. In the regime where both catalogs are expected to be complete, we find excellent agreement, as well as with the most recent surveys in the literature. Extensive simulations are performed and applied to the two different methods, indicating a contamination rate of ~ 5%. These simulations are also used to optimize the algorithms and evaluate the selection function for the final cluster catalog. Redshift and richness estimates are also provided, making possible the selection of subsamples for future studies.

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arXiv2004

Spectroscopic Confirmation of the Cl 1604 Supercluster at z~0.9

We present spectroscopic confirmation of the Cl 1604 supercluster at z~0.9. Originally detected as two individual clusters, Cl 1604+4304 at z = 0.90 and Cl 1604+4321 at z = 0.92, which are closely separated in both redshift and sky position, subsequent imaging revealed a complex of red galaxies bridging the two clusters, suggesting that the region contained a large scale structure. We have carried out extensive multi-object spectroscopy, which, combined with previous measurements, provides ~600 redshifts in this area, including 230 confirmed supercluster members. We detect two additional clusters that are part of this structure, Cl 1604+4314 at z = 0.87 and Cl 1604+4316 at z = 0.94. All four have properties typical of local clusters, with line-of-sight velocity dispersions between 489 and 962 km/s. The structure is significantly extended in redshift space, which, if interpreted as a true elongation in real space, implies a depth of 93 Mpc. We examine the spatial and redshift distribution of the supercluster members.

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arXiv2003

Peculiar Broad Absorption Line Quasars found in DPOSS

With the recent release of large (i.e., > hundred million objects), well-calibrated photometric surveys, such as DPOSS, 2MASS, and SDSS, spectroscopic identification of important targets is no longer a simple issue. In order to enhance the returns from a spectroscopic survey, candidate sources are often preferentially selected to be of interest, such as brown dwarfs or high redshift quasars. This approach, while useful for targeted projects, risks missing new or unusual species. We have, as a result, taken the alternative path of spectroscopically identifying interesting sources with the sole criterion being that they are in low density areas of the g - r and r - i color-space defined by the DPOSS survey. In this paper, we present three peculiar broad absorption line quasars that were discovered during this spectroscopic survey, demonstrating the efficacy of this approach. PSS J0052+2405 is an Iron LoBAL quasar at a redshift z = 2.4512 with very broad absorption from many species. PSS J0141+3334 is a reddened LoBAL quasar at z = 3.005 with no obvious emission lines. PSS J1537+1227 is a Iron LoBAL at a redshift of z = 1.212 with strong narrow Mgii and Feii emission. Follow-up high resolution spectroscopy of these three quasars promises to improve our understanding of BAL quasars. The sensitivity of particular parameter spaces, in this case a two-color space, to the redshift of these three sources is dramatic, raising questions about traditional techniques of defining quasar populations for statistical analysis.

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arXiv2003

The Multiplicity Function of Galaxies

The multiplicity function (MF) of groups and clusters of galaxies is determined using galaxy catalogues extracted from a set of Digitized Palomar Sky Survey (DPOSS) plates. The two different types of structures (of low and high richness) were identified using two different algorithms: a modified version of the van Albada method for groups, and a peak finding algorithm for larger structures. In a 300 sq. deg. area up to z<0.2, we find 2944 groups and 179 clusters. Our MF covers a wide range of richnesses, from 2 to 200, and the two MF's derived by the two algorithms match smoothly without the need for additional conditions or normalisations. The resulting multiplicity function, of slope alpha= -2.08 +- 0.07, strongly resembles a power law.

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arXiv2003

The Northern Sky Optical Cluster Survey II: An Objective Cluster Catalog for 5800 Square Degrees

We present a new, objectively defined catalog of candidate galaxy clusters based on the galaxy catalogs from the Digitized Second Palomar Observatory Sky Survey (DPOSS). This cluster catalog, derived from the best calibrated plates in the high latitude (|b|>30) Northern Galactic Cap region, covers 5,800 square degrees, and contains 8,155 candidate clusters. A simple adaptive kernel density mapping technique, combined with the SExtractor object detection algorithm, is used to detect galaxy overdensities, which we identify as clusters. Simulations of the background galaxy distribution and clusters of varying richnesses and redshifts allow us to optimize detection parameters, and measure the completeness and contamination rates for our catalog. Cluster richnesses and photometric redshifts are measured, using integrated colors and magnitudes for each cluster. An extensive spectroscopic survey is used to confirm the photometric results. This catalog, with well-characterized sample properties, provides a sound basis for future studies of cluster physics and large scale structure.

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arXiv2002

The Digitized Second Palomar Observatory Sky Survey (DPOSS) II: Photometric Calibration

We present the photometric calibration technique for the Digitized Second Palomar Observatory Sky Survey (DPOSS), used to create seamless catalogs of calibrated objects over large sky areas. After applying a correction for telescope vignetting, the extensive plate overlap regions are used to transform sets of plates onto a common instrumental photometric system. Photometric transformations to the Gunn gri system for each plate, for stars and galaxies, are derived using these contiguous stitched areas and an extensive CCD imaging library obtained for this purpose. We discuss the resulting photometric accuracy, survey depth, and possible systematic errors.

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arXiv2000

Exploration of Large Digital Sky Surveys

We review some of the scientific opportunities and technical challenges posed by the exploration of the large digital sky surveys, in the context of a Virtual Observatory (VO). The VO paradigm will profoundly change the way observational astronomy is done. Clustering analysis techniques can be used to discover samples of rare, unusual, or even previously unknown types of astronomical objects and phenomena. Exploration of the previously poorly probed portions of the observable parameter space are especially promising. We illustrate some of the possible types of studies with examples drawn from DPOSS; much more complex and interesting applications are forthcoming. Development of the new tools needed for an efficient exploration of these vast data sets requires a synergy between astronomy and information sciences, with great potential returns for both fields.

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arXiv2000

Searches for Rare and New Types of Objects

Systematic exploration of the observable parameter space, covered by large digital sky surveys spanning a range of wavelengths, will be one of the primary modes of research with a Virtual Observatory (VO). This will include searches for rare, unusual, or even previously unknown types of astronomical objects and phenomena, e.g. as outliers in some parameter space of measured properties, both in the catalog and image domains. Examples from current surveys include high-redshift quasars, type-2 quasars, brown dwarfs, and a small number of objects with puzzling spectra. Opening of the time domain will be especially interesting in this regard. Data-mining tools such as unsupervised clustering techniques will be essential in this task, and should become an important part of the VO toolkit.

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arXiv2000

Luminosity function of clusters of galaxies

The composite galaxy luminosity function (hereafter LF) of 39 Abell clusters of galaxies is derived by computing the statistical excess of galaxy counts in the cluster direction with respect to control fields. Due to the wide field coverage of the digitised POSS-II plates, we can measure field counts around each cluster in a fully homogeneous way. Furthermore, the availability of virtually unlimited sky coverage allows us to directly compute the LF errors without having to rely on the estimated variance of the background. The wide field coverage also allows us to derive the LF of the whole cluster, including galaxies located in the cluster outskirts. The global composite LF has a slope alpha ~ -1.1+/-0.2 with minor variations from blue to red filters, and M* ~ -21.7,-22.2,-22.4 mag (H_0=50 km/s/Mpc) in g, r and i filters, respectively. These results are in quite good agreement with several previous determinations and in particular with the LF determined for the inner region of a largely overlapping set of clusters, but derived making use of a completely different method for background subtraction. The similarity of the two LFs suggests the existence of minor differences between the LF in the cluster outskirts and in the central region, or a negligible contribution of galaxies in the cluster outskirts to the global LF.

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arXiv2000

The Butcher-Oemler Effect in 295 Clusters: Strong Redshift Evolution and Cluster Richness Dependence

We examine the Butcher-Oemler effect and its cluster richness dependence in the largest sample studied to date: 295 Abell clusters. We find a strong correlation between cluster richness and the fraction of blue galaxies, f_B, at every redshift. The slope of the f_B(z) relation is similar for all richnesses, but at a given redshift, f_B is systematically higher for poor clusters. This is the chief cause of scatter in the f_B vs. z diagram: the spread caused by the richness dependence is comparable to the trend in f_B over a typical redshift baseline, so that conclusions drawn from smaller samples have varied widely. The two parameters, z, and a consistently defined projected galaxy number density, N, together account for all of the observed variation in f_B within the measurement errors. The redshift evolution of f_B is real, and occurs at approximately the same rate for clusters of all richness classes.

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arXiv2000

Exploring the Multi-Wavelength, Low Surface Brightness Universe

Our current understanding of the low surface brightness universe is quite incomplete, not only in the optical, but also in other wavelength regimes. As a demonstration of the type of science which is facilitated by a virtual observatory, we have undertaken a project utilizing both images and catalogs to explore the multi-wavelength, low surface brightness universe. Here, we present some initial results of this project. Our techniques are complimentary to normal data reduction pipeline techniques in that we focus on the diffuse emission that is ignored or removed by more traditional algorithms. This requires a spatial filtering which must account for objects of interest, in addition to observational artifacts (e.g., bright stellar halos). With this work we are exploring the intersection of the catalog and image domains in order to maximize the scientific information we can extract from the federation of large survey data.

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