arXiv subjects

R. R. de Carvalho

Publications and source records attributed to R. R. de Carvalho.

At least 55 records · Page 3Linked to original sources

arXiv2008

The Northern Sky Optical Cluster Survey III: A Cluster Catalog Covering Pi Steradians

We present the complete galaxy cluster catalog from the Northern Sky Optical Cluster Survey, a new, objectively defined catalog of candidate galaxy clusters at z<0.25 drawn from the Digitized Second Palomar Observatory Sky Survey (DPOSS). The data presented here cover the Southern Galactic Cap, as well as the less-well calibrated regions of the Northern Galactic Cap. In addition, due to improvements in our cluster finder and measurement methods, we provide an updated catalog for the well-calibrated Northern Galactic Cap region previously published in Paper II. The complete survey covers 11,411 square degrees, with over 15,000 candidate clusters. We discuss improved photometric redshifts, richnesses and optical luminosities which are provided for each cluster. A variety of substructure measures are computed for a subset of over 11,000 clusters. We also discuss the derivation of dynamical radii r_200 and its relation to cluster richness. A number of consistency checks between the three areas of the survey are also presented, demonstrating the homogeneity of the catalog over disjoint sky areas. We perform extensive comparisons to existing optically and X-ray selected cluster catalogs, and derive new X-ray luminosities and temperatures for a subset of our clusters. We find that the optical and X-ray luminosities are well correlated, even using relatively shallow ROSAT All Sky Survey and DPOSS data. This survey provides a good comparison sample to the MaxBCG catalog based on Sloan Digital Sky Survey Data, and complements that survey at low redshifts 0.07<z<0.1.

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arXiv2008

On the Nature of Fossil Galaxy Groups: Are they really fossils ?

We use SDSS-DR4 photometric and spectroscopic data out to redshift z~0.1 combined with ROSAT All Sky Survey X-ray data to produce a sample of twenty-five fossil groups (FGs), defined as bound systems dominated by a single, luminous elliptical galaxy with extended X-ray emission. We examine possible biases introduced by varying the parameters used to define the sample and the main pitfalls are discussed. The spatial density of FGs, estimated via the V/V_ MAX} test, is 2.83 x 10^{-6} h_{75}^3 Mpc^{-3} for L_x > 0.89 x 10^42 h_{75}^-2 erg/s consistent with Vikhlinin et al. (1999), who examined an X-ray overluminous elliptical galaxy sample (OLEG). We compare the general properties of FGs identified here with a sample of bright field ellipticals generated from the same dataset. These two samples show no differences in the distribution of neighboring faint galaxy density excess, distance from the red sequence in the color-magnitude diagram, and structural parameters such as a$_{4}$ and internal color gradients. Furthermore, examination of stellar populations shows that our twenty-five FGs have similar ages, metallicities, and $α$-enhancement as the bright field ellipticals, undermining the idea that these systems represent fossils of a physical mechanism that occurred at high redshift. Our study reveals no difference between FGs and field ellipticals, suggesting that FGs might not be a distinct family of true fossils, but rather the final stage of mass assembly in the Universe.

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arXiv2008

Cosmography with Galaxy Clusters

In the present work we focus on future experiments using cluster abundance observations to constraint the Dark Energy equation of state parameter, w. To obtain tight constraints from this kind of experiment, a reliable sample of galaxy clusters must be obtained from deep and wide-field images. We therefore present the computational environment (2DPHOT) that allow us to build the galaxy catalog from the images and the Voronoi Tessellation cluster finding algorithm that we use to identify the galaxy clusters on those catalogs. To test our pipeline with data similar in quality to what will be gathered by future wide field surveys, we process images from the Deep fields obtained as part of the LEGACY Survey (four fields of one square degree each, in five bands, with depth up to r'=25). We test our cluster finder by determining the completeness and purity of the finder when applied to mock galaxy catalogs made for the Dark Energy Survey cluster finder comparison project by Risa Wechsler and Michael Busha. This procedure aims to understand the selection function of the underlying dark matter halos.

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arXiv2008

NoSOCS in SDSS. I. Sample Definition and Comparison of Mass Estimates

We use Sloan Digital Sky Survey (SDSS) data to investigate galaxy cluster properties of systems first detected within DPOSS. With the high quality photometry of SDSS we derived new photometric redshifts and estimated richness and optical luminosity. For a subset of low redshift ($z \le 0.1$) clusters, we have used SDSS spectroscopic data to identify groups in redshift space in the region of each cluster, complemented with massive systems from the literature to assure the continuous mass sampling. A method to remove interlopers is applied, and a virial analysis is performed resulting in estimates of velocity dispersion, mass, and a physical radius for each low-$z$ system. We discuss the choice of maximum radius and luminosity range in the dynamical analysis, showing that a spectroscopic survey must be complete to at least M$^*+1$ if one wishes to obtain accurate and unbiased estimates of velocity dispersion and mass. We have measured X-ray luminosity for all clusters using archival data from RASS. For a smaller subset (twenty-one clusters) we selected temperature measures from the literature and estimated mass from the M-T$_X$ relation, finding that they show good agreement with the virial estimate. However, these two mass estimates tend to disagree with the caustic results. We measured the presence of substructure in all clusters of the sample and found that clusters with substructure have virial masses higher than those derived from T$_X$. This trend is not seen when comparing the caustic and X-ray masses. That happens because the caustic mass is estimated directly from the mass profile, so it is less affected by substructure.

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arXiv2008

2DPHOT: A Multi-purpose Environment for the Two-dimensional Analysis of Wide-field Images

We describe 2DPHOT, a general purpose analysis environment for source detection and analysis in deep wide-field images. 2DPHOT is an automated tool to obtain both integrated and surface photometry of galaxies in an image, to perform reliable star-galaxy separation with accurate estimates of contamination at faint flux levels, and to estimate completeness of the image catalog. We describe the analysis strategy on which 2DPHOT is based, and provide a detailed description of the different algorithms implemented in the package. This new environment is intended as a dedicated tool to process the wealth of data from wide-field imaging surveys. To this end, the package is complemented by 2DGUI, an environment that allows multiple processing of data using a range of computing architectures.

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arXiv2006

Truncated Star Formation in Compact Groups of Galaxies: A Stellar Population Study

We present the results of a study comparing the stellar populations in the elliptical galaxies of Hickson Compact Groups (HCGs) with those in low density environments. Three different population synthesis models and stellar population analyses are used to make the results more robust. The low-sigma galaxies in HCGs show an enhanced [Mg/Fe] ratio and a depleted metallicity [Z/H] with respect to their counterparts in the field. This behavior is interpreted as evidence for the action of a mechanism which truncated the star formation (SF). Hydrodynamical simulations of galaxy mergers (Di Matteo et al. 2005) support this interpretation by predicting the quenching of star formation soon after the merger event. Combining this scenario and the evidence presented here, the HCGs, generally considered to be ideal environments for galaxy-galaxy interactions, become ideal places for SF truncation.

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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

A Principal Component Analysis approach to the Star Formation History of elliptical galaxies in Compact Groups

(Abridged) Environmental differences in the stellar populations of early-type galaxies are explored using principal component analysis (PCA), focusing on differences between elliptical galaxies in Hickson Compact Groups (HCGs) and in the field. The method is model-independent and relies on variations between the observed spectra. The projections (PC1,PC2) reveal a difference with respect to environment, with a wider range in PC1 and PC2 in the group sample. We define a spectral parameter (zeta=0.36PC1-PC2) which simplifies this result to a single number: field galaxies have a very similar value of zeta, whereas HCG galaxies span a wide range in this parameter. We obtain a strong correlation between the values of zeta and the mass fraction in younger stars, so that some group galaxies present a higher fraction of them. PCA is more sensitive than other methods based on a direct analysis of observables such as the structure of the surface brightness profile or the equivalent width of absorption lines. The latter do not reveal any significant variation between field and compact group galaxies. Our results imply that the presence of young stars only amounts to a fraction of a percent in its contribution to the total variance.

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arXiv2006

The DPOSS II compact group survey: first spectroscopically confirmed candidates

This paper presents the results of a pilot redshift survey of 18 candidate compact groups from the distant DPOSS survey that extends the available surveys of compact groups of galaxies to z ~0.2 in redshift, mainly Hickson Compact Groups and Southern Compact Groups. The goal of our survey was to confirm group membership via redshift information and to measure the characteristic parameters of a representative, albeit small, sample of DPOSS survey groups. Of the 18 candidates observed, seven are found to be indeed isolated compact groups, i.e. groups with 3 or more concordant members and with no neighbouring known cluster, while 7 are chance projection configurations on the sky. Three remaining candidates, despite having 3 or more concordant member galaxies, are located in the neighbourhood of known clusters, while another candidate turned out to be a dense sub-condensation within Abell 0952. The median redshift of our 7 confirmed groups is z ~0.12, to be compared with a median redshift of 0.03 for the local sample of compact groups by Hickson. The typical group size is r~ 50 kpc, and the median radial velocity dispersion is 167 km/s, while typical crossing times range from 0.005 H$_{0}^{-1}$ to 0.03 H$_{0}^{-1}$ with a median value of 0.018 H$_{0}^{-1}$, all similar to the values usually found in the literature for such structures in the local universe. The average mass-to-light ratio for our groups, M/L(B), is 92h, higher than the value found for nearby Hickson compact groups but lower than that found for loose groups. Our results suggest that, once full redshift information for its members becomes available, the DPOSS sample will provide a reference sample to study the properties of compact groups beyond the local universe.

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arXiv2005

Color Gradients in Early-type Galaxies: Dependence on Environment and Redshift

Color gradients in early-type galaxies contain valuable clues about their formation and evolutionary histories and mechanisms. We examine color gradients in 1,700 early-type galaxies in 159 galaxy clusters spanning a redshift range of 0.05 to 0.2. We find that color gradients strongly depend on the environment where galaxies reside, with steeper color gradients in poor rather than rich clusters. No dependence of color gradients on galaxy luminosity is found in either rich or poor clusters. The difference in color gradients can be explained by a change in the internal metallicity and/or an age gradient in these galaxies. Our results support a reasonable picture whereby young early-type galaxies form in a dissipative collapse process, and then undergo increased (either major or minor) merging activity in richer rather than in poor clusters.

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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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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

Merging of Low-Mass Systems and the Origin of the Fundamental Plane

We present a new set of dissipationless N-body simulations to examine the feasibility of creating bright ellipticals (following the Kormendy relation) by hierarchically merging present-day early-type dwarf galaxies, and to study how the encounter parameters affect the location of the end-product in the $<μ_{e} > - R_{e}$ plane. We investigate the merging of one-component galaxies of both equal and different masses, the merging of two-component galaxy models to explore the effect of dark halos on the final galaxy characteristics, and the merging of ultracompact dwarf galaxies. We find that the increase of $<μ_{e}>$ with $R_e$ is attributable to an increase in the initial orbital energy. The merger remnants shift down in the $<μ_{e} > - R_{e}$ plane and fail to reach the KR. Thus, the KR is not reproducable by mergers of dwarf early-type systems, rendering untenable the theory that present-day dwarfs are responsible for even a small fraction of the present-day ellipticals, unless a considerable amount of dissipation is invoked. However, we do find that present-day dwarfs can be formed by the merger of ultra-compact dwarfs.

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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 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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arXiv2001

The Fundamental Plane of Elliptical Galaxies in Compact Groups

We present a study focusing on the nature of compact groups through the study of their elliptical galaxies. We determine velocity dispersions ($σ$) for 18 18 bright elliptical galaxies located in the core of Hickson compact groups and a control sample of 12 bright bona fide ellipticals located in the field or very loose groups. Several tests are carried out to avoid sources of systematic effects in $σ$ measurements. We use these velocity dispersions to compare the position of 11 compact group galaxies in the Fundamental Plane to that of a large and homogeneous sample of elliptical galaxies (Burstein et al. 1987). We find that little or no significant difference exists, as far as the Fundamental Plane is concerned, between ellipticals in compact groups and their counterparts in other environments.

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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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