Browsing by Author "Lambas, DG"

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    Shapes of clusters and groups of galaxies
    (2006) Paz, DJ; Lambas, DG; Padilla, N; Merchán, M
    We study the properties of the three-dimensional and projected shapes of haloes using high-resolution numerical simulations and observational data where the latter comes from the 2PIGG [2dFGRS (2-degree Field Galaxy Redshift Survey) Percolation Inferred Galaxy Groups] and Data Release 3 of the Sloan Digital Sky Survey (SDSS-DR3GC) group catalogues. We investigate the dependence of the halo shape on characteristics such as mass and number of members. In the three-dimensional case, we find a significant correlation between the mass and the halo shape; massive systems are more prolate than small haloes. We detect a source of strong systematics in estimates of the triaxiality of a halo, which is found to be a strong function of the number of members; Lambda cold dark matter haloes usually characterized by triaxial shapes, slightly bent towards prolate forms, appear more oblate when taking only a small subset of the halo particles.
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    Spatial and dynamical properties of voids in a Λ cold dark matter universe
    (2005) Padilla, ND; Ceccarelli, L; Lambas, DG
    We study the statistical properties of voids in the distribution of mass, dark-matter haloes and galaxies ( B-J < -16) in a Lambda cold dark matter ( Lambda CDM) numerical simulation populated with galaxies using a semi-analytic galaxy formation model ( GALFORM, Cole et al.). We find that the properties of voids selected from GALFORM galaxies are compatible with those of voids identified from a population of haloes with mass M > 10(11.5) h(-1) M-circle dot, similar to the median halo mass, M-med = 10(11.3) h(-1) M-circle dot. We also find that the number density of galaxy- and halo-defined voids can be up to two orders of magnitude higher than mass-defined voids for large void radii, however, we observe that this difference is reduced to about half an order of magnitude when the positions are considered in redshift space. As expected, there are outflow velocities that show their maximum at larger void-centric distances for larger voids. We find a linear relation for the maximum outflow velocity, v(max) = v(0)r(void). The void-centric distance where this maximum occurs follows a suitable power-law fit of the form log( dv(max)) = ( r(void)/A)(B). At sufficiently large distances, we find mild infall motions on to the subdense regions. The galaxy velocity field around galaxy- defined voids is consistent with the results of haloes with masses above the median, showing milder outflows than the mass around mass-defined voids. We find that a similar analysis in redshift space would make both outflows and infalls appear with a lower amplitude. We also find that the velocity dispersion of galaxies and haloes is larger in the direction parallel to the void walls by similar or equal to 10-20 per cent. Given that voids are by definition subdense regions, the cross-correlation function between galaxy- defined voids and galaxies is close to xi = -1 out to separations comparable to the void size, and at larger separations the correlation function level increases, approaching the values of the auto-correlation function of galaxies. The cross-correlation amplitude of mass-defined voids versus mass has a more gentle behaviour remaining negative at larger distances. The cross-to auto-correlation function ratio as a function of the distance normalized to the void radius shows a small scatter around a relation that depends only on the object used to define the voids ( galaxies or haloes for instance). The distortion pattern observed in xi( sigma, pi) is that of an elongation along the line of sight that extends out to large separations. Positive xi contours evidence finger-of-god motions at the void walls. Elongations along the line of sight are roughly comparable between galaxy-, halo-and mass-defined voids.
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    The faint-end of the galaxy luminosity function in groups
    (2006) González, RE; Lares, M; Lambas, DG; Valotto, C
    We compute the galaxy luminosity function in spectroscopically selected nearby groups and clusters. Our sample comprises 728 systems extracted from the third release of the Sloan Digital Sky Survey in the redshift range 0.03 < z < 0.06 with virial mass range 10(11) M-circle dot < M-vir < 2 x 10(14) M-circle dot. To compute the galaxy luminosity function, we apply a statistical background subtraction method following usually adopted techniques. In the r band, the composite galaxy luminosity function shows a slope alpha = - 1.3 in the bright-end, and an upturn of the slope in the faint-end, M-r greater than or similar to -18 + 5 log (h), to slopes -1.9 < a < -1.6. We find that this feature is present also in the i, g and z bands, and for all explored group subsamples, irrespective of the group mass, number of members, integrated color or the presence of a hot intra-cluster gas associated to X-ray emission.
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    Velocity dispersions and cluster properties in the Southern Abell Redshift Survey clusters. II.
    (2002) Muriel, H; Quintana, H; Infante, L; Lambas, DG; Way, MJ
    We report an analysis of the dynamical structure of clusters of galaxies from a survey of photometric and spectroscopic observations in the fields of southern Abell clusters. We analyze the galaxy velocity field in extended regions up to 7 h(-1) Mpc from cluster centers, and we estimate mean velocity dispersions and their radial dependence. Only one from a total of 41 Abell clusters does not correspond to a dynamically bound system. However, four of these bound objects are double clusters. We estimate that 20% (seven clusters) of the 35 remaining are subject to serious projection effects. Normalizing the clustercentric distances by means of the overdensity radius r(200), and the velocity dispersion profiles (VDPs) by the corresponding mean cluster velocity dispersion, we computed the average VDP. Our results indicate a at behavior of the mean VDP at large distances from the cluster center. Nevertheless, we found that for the inner part of the clusters (r/r(200)less than or equal to1) the VDP is up to 10% smaller than at larger radii.