P. A. A. Lopes

The Voronoi Tessellation cluster finder in 2+1 dimensions

We present a detailed description of the Voronoi Tessellation (VT) cluster finder algorithm in 2+1 dimensions, which improves on past implementations of this technique. The need for cluster finder algorithms able to produce reliable cluster catalogs up to redshift 1 or beyond and down to 10 13.5 solar masses is paramount especially in light of upcoming surveys aiming at cosmological constraints from galaxy cluster number counts. We build the VT in photometric redshift shells and use the two-point correlation function of the galaxies in the field to both determine the density threshold for detection of cluster candidates and to establish their significance. This allows us to detect clusters in a self-consistent way without any assumptions about their astrophysical properties. We apply the VT to mock catalogs which extend to redshift 1.4 reproducing the ΛCDM cosmology and the clustering properties observed in the Sloan Digital Sky Survey data. An objective estimate of the cluster selection function in terms of the completeness and purity as a function of mass and redshift is as important as having a reliable cluster finder. We measure these quantities by matching the VT cluster catalog with the mock truth table. We show that the VT can produce a cluster catalog with completeness and purity > 80% for the redshift range up to ∼1 and mass range down to ∼10 13.5 solar masses.

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, using literature data from BAX and optically selected clusters in DPOSS. 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 (either poor or at high redshifts). We employ four different statistical tests to test for the presence of substructure using optical two-dimensional data. We find that approximately 35% of the clusters show strong signs of substructure in the optical. 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 (~750 clusters), while tests involving temperature use a similar number of objects as previous works (≾100). The results presented here are in good agreement with existing literature.

NoSOCS in SDSS – II. Mass calibration of low redshift galaxy clusters with optical and X-ray properties

We use SDSS data to investigate the scaling relations of 127 No SOCS and 56 CIRS galaxy clusters at low redshift (z ≤ 0.10). We show that richness and both optical and X-ray luminosities are reliable mass proxies. The scatter in mass at a fixed observable is ∼40 per cent, depending on the aperture, sample and observable considered. For example, for the massive CIRS systems σ InM500|500 = 0.33 ± 0.05 and σ InM500|Lx = 0.48 ± 0.06. For the full sample σ InM500|N500 = 0.43 ± 0.03 and σ /InM500|Lx = 0.56 ± 0.06. The scaling relations based only on the richer systems (CIRS) are slightly flatter than those based on the full sample, but the discrepancies are within 1σ. We estimate substructure using 2D and 3D optical data, verifying that substructure has no significant effect on the cluster scaling relations (intercepts and slopes), independent of which substructure test we use. For a subset of 21 clusters, we estimate masses from the M-T x relation using temperature measures from Base de Donnees Amas de Galaxies X. The scaling relations derived from the optical and X-ray masses are indeed very similar, indicating that our method consistently estimates the cluster mass and yields equivalent results regardless of the wavelength from which we measure mass. For massive systems, we represent the mass-richness relation by a function with the form In(M 200 ) = A + B x In(N 200 /60), with M 200 being expressed in units of 10 14 M ☉ . Using the virial mass, for CIRS clusters, we find A = (1.39 ± 0.07) and B = (1.00 ± 0.11). For the same sample, but using the masses obtained by the caustic method, we get A = (0.64 ± 0.14) and B = (1.35 ± 0.34). If we consider the mass as estimated from T x (for the subset of 21 clusters with T x available) we derive A = (0.90 ± 0.10) and B (0.92 ± 0.10). The relations based on the virial mass have a scatter of σ InM200|N200 = 0.37 ± 0.05, while σ InM200|N200 = 0.77 ± 0.22 for the caustic mass and σ InM200|N200 = 0.34 ± 0.08 for the temperature-based mass.

A New Sample of Distant Compact Groups from the Digitized Second Palomar Observatory Sky Survey

We have identified 84 small, high-density groups of galaxies out to z ~ 0.2 in a region of ~2000 deg2 around the north Galactic pole using the digitized Second Palomar Observatory Sky Survey. The groups have at least four galaxies satisfying more stringent criteria than those used by Hickson in his pioneering work in 1982: the adopted limiting surface brightness for each group is brighter (24 mag arcsec-2 instead of 26 mag arcsec-2), and the spread in magnitude among the member galaxies is narrower (2 mag instead of 3). We also adopt a slightly modified version of the isolation criterion used by Hickson, in order to avoid rejecting groups with projected nearby faint background galaxies. A 10% contamination rate due to projection effects is expected for this sample based on extensive simulations.

SPIDER – IX. Classifying galaxy groups according to their velocity distribution

We introduce a new method to study the velocity distribution of galaxy systems, the Hellinger Distance (HD) - designed for detecting departures from a Gaussian velocity distribution. We define a relaxed galactic system as the one with unimodal velocity distribution and a normality deviation below a critical value (HD = 20) systems are significantly larger than in low multiplicity ones (N ) and the gaussianity of the velocity distribution of the groups. Bright galaxies (M_r <=-20.7) residing in the inner and outer regions of groups, do not show significant differences in the listed quantities regardless if the group has a Gaussian (G) or a Non-Gaussian (NG) velocity distribution. However, the situation is significantly different when we examine the faint galaxies (-20.7<M_r<=-17.9). In G groups, there is a remarkable difference between the galaxy properties of the inner and outer galaxy populations, testifying how the environment is affecting the galaxies. Instead, in NG groups there is no segregation between the properties of galaxies in the inner and outer regions, showing that the properties of these galaxies still reflect the physical processes prevailing in the environment where they were found earlier.

ON THE RADIAL STELLAR CONTENT OF EARLY-TYPE GALAXIES AS A FUNCTION OF MASS AND ENVIRONMENT

Using optical-optical and optical-NIR colors, we analyze the radial dependence of age and metallicity inside massive (M{sub *} {approx}> 10{sup 10.5} M{sub sun}), low-redshift (z < 0.1), early-type galaxies (ETGs), residing in both high-density group regions and the field. On average, internal color gradients of ETGs are mainly driven by metallicity, consistent with previous studies. However, we find that group galaxies feature positive age gradients, {nabla} {sub t}, i.e., a younger stellar population in the galaxy center, and steeper metallicity gradients, compared to the field sample, whose {nabla} {sub t} ranges from negative in lower mass galaxies to positive gradients at higher mass. These dependencies yield new constraints on models of galaxy formation and evolution. We speculate that age and metallicity gradients of group ETGs result from (either gas-rich or minor-dry) mergers and/or cold-gas accretion, while field ETGs exhibit the characteristic flatter gradients expected from younger, more metal-rich stars formed inside-out by later gas cooling.

NoSOCS in SDSS - III. The interplay between galaxy evolution and the dynamical state of galaxy clusters

Context. We investigate relations between the color and luminosity distributions of cluster galaxies and the evolutionary state of their host clusters. Aims. Our aim is to explore some aspects of cluster galaxy evolution and the dynamical state of clusters as two sides of the same process. Methods. We used 10 721 member galaxies of 183 clusters extracted from the Sloan Digital Sky Survey using a list of NoSOCS and CIRS targets. First, we classified the clusters into two categories, Gaussian and non-Gaussian, according to their velocity distribution measurements, which we used as an indicator of their dynamical state. We then used objective criteria to split up galaxies according to their luminosities, colors, and photometric mean stellar age. This information was then used to evaluate how galaxies evolve in their host clusters. Results. Meaningful color gradients, i.e., the fraction of red galaxies as a function of radius from the center, are observed for both the Gaussian velocity distribution and the non-Gaussian velocity distribution cluster subsamples, which suggests that member galaxy colors change on a shorter timescale than the time needed for the cluster to reach dynamical equilibrium. We also found that larger portions of fainter red galaxies are found, on average, in smaller radii. The luminosity function in Gaussian clusters has a brighter characteristic absolute magnitude and a steeper faint-end slope than it does in the non-Gaussian velocity distribution clusters. Conclusions. Our findings suggest that cluster galaxies experience intense color evolution before virialization, while the formation of faint galaxies through dynamical interactions probably takes place on a longer timescale, possibly longer than the virialization time.

Non-Gaussian velocity distributions – the effect on virial mass estimates of galaxy groups

We present a study of nine galaxy groups with evidence for non-Gaussianity in their velocity distributions out to 4R200. This sample is taken from the 57 groups selected from the 2dF Percolation-Inferred Galaxy Groups (2PIGG) catalogue of galaxy groups. Statistical analysis indicates that the non-Gaussian groups have masses significantly higher than that of the Gaussian groups. We also have found that all non-Gaussian systems seem to be composed of multiple velocity modes. Besides, our results indicate that multimodal groups should be considered as a set of individual units with their own properties. In particular, we have found that the mass distributions of such units are similar to that of the Gaussian groups. Our results reinforce the idea of non-Gaussian systems as complex structures in the phase space, likely corresponding to secondary infall aggregations at a stage before virialization. The understanding of these objects is relevant for cosmological studies using groups and clusters through the mass function evolution.

Segregation effects according to the evolutionary stage of galaxy groups

We study segregation phenomena in 57 groups selected from the 2dF Percolation-Inferred Galaxy Groups (2PIGG) catalogue of galaxy groups. The sample corresponds to those systems located in areas of at least 80 per cent redshift coverage out to 10 times the radius of the groups. The dynamical state of the galaxy systems was determined after studying their velocity distributions. We have used the Anderson–Darling test to distinguish relaxed and non-relaxed systems. This analysis indicates that 84 per cent of groups have galaxy velocities consistent with the normal distribution, while 16 per cent of them have more complex underlying distributions. Properties of the member galaxies are investigated taking into account this classification. Our results indicate that galaxies in Gaussian groups are significantly more evolved than galaxies in non-relaxed systems out to distances of ∼4R200, presenting significantly redder (B−R) colours. We also find evidence that galaxies with MR≤−21.5 in Gaussian groups are closer to the condition of energy equipartition.

Distribution and evolution of galaxy groups in the Ursa Major supercluster

Context. We study an SDSS sample of galaxies within 50 Mpc of the nominal center of the Ursa Major supercluster. Aims. Our aim is to study galaxy distribution around groups in the supercluster and the link between the distribution of relaxed and nonrelaxed galaxy systems with respect to the supercluster environment. Methods. Using the FoF algorithm, 40 galaxy groups were identified in this region. Velocity distributions for these groups were studied after applying a 3 -clipping routine for outlier removal. We classified the systems according to the results of normality and substructure tests applied to member galaxies. Then, we studied the relative distribution of relaxed and nonrelaxed systems across the supercluster. Results. We find that 68% of galaxy groups are Gaussian and that all the non-Gaussian systems have substructures and probably correspond to multimodal systems in redshift space. We also find that the Gaussian systems inhabit the denser regions of the supercluster, with higher densities of both red and blue galaxies within 2.5 h 1 Mpc, and have smaller group-group pairwise separations. Conclusions. Our results suggest a spatial segregation of dynamical states, where relaxed systems may have formed and evolved earlier and faster around high-density peaks, while nonrelaxed systems may be growing slower on the peripheries of lower density peaks. In this picture, galaxy clustering seems to be prompting a continuous internal evolution in the supercluster, with several groups collapsing into the more evolved and contracted regions.