M. Gramann

Flux- and volume-limited groups/clusters for the SDSS galaxies: catalogues and mass estimation

We provide flux-limited and volume-limited galaxy group and cluster catalogues, based on the spectroscopic sample of the SDSS data release 10 galaxies. We used a modified friends-of-friends (FoF) method with a variable linking length in the transverse and radial directions to identify as many realistic groups as possible. The flux-limited catalogue incorporates galaxies down to m_r = 17.77 mag. It includes 588193 galaxies and 82458 groups. The volume-limited catalogues are complete for absolute magnitudes down to M_r = -18.0, -18.5, -19.0, -19.5, -20.0, -20.5, and -21.0; the completeness is achieved within different spatial volumes, respectively. Our analysis shows that flux-limited and volume-limited group samples are well compatible to each other, especially for the larger groups/clusters. Dynamical mass estimates, based on radial velocity dispersions and group extent in the sky, are added to the extracted groups. The catalogues can be accessed via this http URL and the Strasbourg Astronomical Data Center (CDS).

SDSS superclusters: morphology and galaxy content

Context. Understanding the formation, evolution and present-day properties of the cosmic web and objects forming it is an important task in cosmology. Aims. We compare the galaxy populations in superclusters of different morphology in the nearby Universe (180 h −1 Mpc≤ d ≤ 270 h −1 Mpc) to see whether the inner structure and overall morphology of superclusters are important in shaping galaxy properties in superclusters. Methods. We find supercluster morphology with Minkowski functionals and analyse the probability density distributions of colou rs, morphological types, stellar masses, star formation rate ( SFR) of galaxies, and the peculiar velocities of the main galaxies in groups in superclusters of filament and spider types, and in the field . We test the statistical significance of the results with the KS test. Results. The fraction of red, early-type, low SFR galaxies in filament -type superclusters is higher than in spider-type superclu sters; in low-density global environments their fraction is lower than in superclusters. In all environments the fraction of red , high stellar mass, and low SFR galaxies in rich groups is higher than in poor groups. In superclusters of spider morphology red, high SFR galaxies have higher stellar masses than in filament-type superclusters. Groups of equal richness host galaxies with larger stellar masses, a larger fraction of early-type and red galaxies, and a higher fracti on of low SFR galaxies, if they are located in superclusters of filament morphology. The peculiar velocities of the main galaxies in groups from superclusters of filament morphology are higher than in those of spider morphology. Groups with higher peculiar velocities of their main galaxies in filament-type supercluste rs are located in higher density environment than those with low peculiar velocities. There are significant di fferences between galaxy populations of the individual richest superclusters. Conclusions. Both local (group) and global (supercluster) environments and even supercluster morphology play an important role in the formation and evolution of galaxies. Differences in the inner structure of superclusters of filament a nd spider morphology and the dynamical state of galaxy groups in them may lead to the differences found in our study.

Multimodality of rich clusters from the SDSS DR8 within the supercluster-void network

Context. The study of the properties of galaxy clusters and their environment gives us information about the formation and evolution of galaxies, groups and clusters, and larger structures – superclusters of galaxies and the whole cosmic web. Aims. We study the relations between the multimodality of galaxy clusters drawn from the SDSS DR8 and the environment where they reside. As cluster environment we consider the global luminosity density field, supercluster membership, and supercluster morphology. Methods. We use 3D normal mixture modelling, the Dressler-Shectman test, and the peculiar velocity of cluster main galaxies as signatures of multimodality of clusters. We calculate the luminosity density field to study the environmental densities around clusters, and to find superclusters where clusters reside. We determine the morphology of superclusters with the Minkowski functionals and compare the properties of clusters in superclusters of different morphology. We apply principal component analysis to study the relations between the multimodality parameters of clusters and their environment simultaneously. Results. Multimodal clusters reside in higher density environment than unimodal clusters. Clusters in superclusters have higher probability to have substructure than isolated clusters. The superclusters can be divided into two main morphological types, spiders and filaments. Clusters in superclusters of spider morphology have higher probabilities to have substructure and larger peculiar velocities of their main galaxies than clusters in superclusters of filament morphology. The most luminous clusters are located in the high-density cores of rich superclusters. Five of seven most luminous clusters, and five of seven most multimodal clusters reside in spider-type superclusters; four of seven most unimodal clusters reside in filament-type superclusters. Conclusions. Our study shows the importance of the role of superclusters as high density environment, which affects the properties of galaxy systems in them.

Toward Understanding Rich Superclusters

We present a morphological study of the two richest superclusters from the 2dF Galaxy Redshift Survey (SCL126, the Sloan Great Wall, and SCL9, the Sculptor supercluster). We use Minkowski functionals, shapefinders, and galaxy group information to study the substructure of these superclusters as formed by different populations of galaxies. We compare the properties of grouped and isolated galaxies in the core region and in the outskirts of superclusters. The fourth Minkowski functional V3 and the morphological signature K1- K2 show a crossover from low-density morphology (outskirts of supercluster) to high-density morphology (core of supercluster) at mass fraction mf � 0.7. The galaxy content and the morphology of the galaxy populations in supercluster cores and outskirts is different. The core regions contain a larger fraction of early type, red galaxies, and richer groups than the outskirts of superclusters. In the core and outskirt regions the fine structure of the two prominent superclusters as delineated by galaxies from different populations also differs. The values of the fourth Minkowski functional V3 show that in the supercluster SCL126 the population of early type, red galaxies is more clumpy than the population of late type, blue galaxies, especially in the outskirts of the supercluster. In the contrary, in the supercluster SCL9, the clumpiness of the spatial distribution of galaxies of different type and color is quite similar in the outskirts of the supercluster, while in the core region the clumpiness of the late type, blue galaxy population is larger than the clumpiness of the early type, red galaxy population. Our results suggest that both local (group/cluster) and global (supercluster) environments are important in forming galaxy morphologies and colors (and determining the star formation activity). The differences between the superclusters indicate that these superclusters have different evolutional histories. Subject headings: cosmology: large-scale structure of the Universe – clusters of galaxies; cosmology: large-scale structure of the Universe – Galaxies; clusters: general

Environments of nearby quasars in Sloan Digital Sky Survey

Context. For the first time, spectroscopic galaxy redshift surveys are enabling galaxies to be studied with the nearest quasars. This allows the dependence of the activity of a quasar on its environment to be studied in a more extensive way than before. Aims. We study the spatial distribution of galaxies and properties of groups of galaxies in the environments of low redshift quasars in the Sloan Digital Sky Survey (SDSS). Our aim is to understand how the nearby quasars are embedded in the local and global density field of galaxies and how the environment affects quasar activity. Methods. We analyze the environments of nearby quasars using number counts of galaxies. We also study the dependence of group properties on their distance to the nearest quasar. The large-scale environments are studied by analyzing the locations of quasars in the luminosity density field. Results. Our study of the number counts of galaxies in quasar environments shows an underdensity of bright galaxies at a few Mpc from quasars. Groups of galaxies with a quasar closer than 2 Mpc are also poorer and less luminous than average. Our analysis of the luminosity density field shows that quasars clearly avoid rich superclusters. Nearby quasars seem to be located in the outskirts of superclusters or in the filaments connecting them. Conclusions. Our results suggest that quasar evolution may be affected by density variations both on supercluster scales and in the local environment.

Groups in the Millennium Simulation and in SDSS DR7

The Millennium N-body simulation and SDSS DR7 galaxy and galaxy group catalogues are compared to study the properties of galaxy groups and the distribution of galaxies in groups. We construct mock galaxy group catalogues for a Millennium semi-analytical galaxy catalogue by using the same friends-of-friends method, which was used by Tago et al. (2010) to analyse the SDSS data. We analyse in detail the group luminosities, group richnesses, virial radii, sizes of groups and their rms velocities for four volume-limited samples from observations and simulations. Our results show that the spatial densities of groups agree within one order of magnitude in all samples with a rather good agreement between the mock catalogues and observations. All group property distributions have similar shapes and amplitudes for richer groups. For galaxy pairs and small groups the group properties for observations and simulations are clearly different. In addition, the spatial distribution of galaxies in small groups is different: at the outskirts of the groups the galaxy number distributions do not agree, although the agreement is relatively good in the inner regions. Differences in the distributions are mainly due to the observational limitations in the SDSS sample and to the problems in the semi-analytical methods that produce too compact and luminous groups.

Dynamical state of superclusters of galaxies: do superclusters expand or have they started to collapse?

We investigate the dynamical state of superclusters in Lambda cold dark matter (�CDM) cosmological models, where the density parameter 0 = 0.2 0.4 and �8 (the rms fluctuation on the 8h 1 Mpc scale) is 0.7 0.9. To study the nonlinear regime, we use N-body simulations. We define superclusters as maxima of the density field smoothed on the scale R = 10h 1 Mpc. Smaller superclusters defined by the density field smoothed on the scale R = 5h 1 Mpc are also investigated. We find the relations between the radially averaged peculiar velocity and the density contrast in the superclusters for different cosmological models. These relations can be used to estimate the dynamical state of a supercluster on the basis of its density contrast. In the simulations studied, all the superclusters defined with the 10h 1 Mpc smoothing are expanding by the present epoch. Only a small fraction of the superclusters defined with R = 5h 1 Mpc has already reached their turnaround radius and these superclusters have started to collapse. In the model with 0 = 0.3 and �8 = 0.9, the number density of objects which have started to collapse is 5 × 10 6 h 3 Mpc 3 . The results for superclusters in the N-body simulations are compared with the spherical collapse model. We find that the radial peculiar velocities in N-body simulations are systematically smaller than those predicted by the spherical collapse model (� 25% for the R = 5h 1 Mpc superclusters).

Unusual A2142 supercluster with a collapsing core: distribution of light and mass

We study the distribution, masses, and dynamical properties of galaxy groups in the A2142 supercluster. We analyse the global luminosity density distribution in the supercluster and divide the supercluster into the high-density core and the low-density outskirts regions. We find galaxy groups and filaments in the regions of different global density, calculate their masses and mass-to-light ratios and analyse their dynamical state with several 1D and 3D statistics. We use the spherical collapse model to study the dynamical state of the supercluster. We show that in A2142 supercluster groups and clusters with at least ten member galaxies lie along an almost straight line forming a 50 Mpc/h long main body of the supercluster. The A2142 supercluster has a very high density core surrounded by lower-density outskirt regions. The total estimated mass of the supercluster is M_est = 6.2 10^{15}M_sun. More than a half of groups with at least ten member galaxies in the supercluster lie in the high-density core of the supercluster, centered at the rich X-ray cluster A2142. Most of the galaxy groups in the core region are multimodal. In the outskirts of the supercluster, the number of groups is larger than in the core, and groups are poorer. The orientation of the cluster A2142 axis follows the orientations of its X-ray substructures and radio halo, and is aligned along the supercluster axis. The high-density core of the supercluster with the global density D8 > 17 and perhaps with D8 > 13 may have reached the turnaround radius and started to collapse. A2142 supercluster with luminous, collapsing core and straight body is an unusual object among galaxy superclusters. In the course of the future evolution the supercluster may be split into several separate systems.

Features in the primordial power spectrum: constraints from the cosmic microwave background and the limitation of the 2dF and SDSS redshift surveys to detect them

We allow a more general (step-function) form of the primordial power spectrum than the usual featureless power-law Harrison-Zeldovich (with spectral index n = 1) power spectrum, and fit it to the latest cosmic microwave background data sets. Although the best-fitting initial power spectrum can differ significantly from the power-law shape, and contains a dip at scales k ∼ 0.003 h Mpc - 1 , we find that Ω m 0.24, consistent with previous analyses that assume power-law initial fluctuations. We also explore the feasibility of the early releases of the 2dF and Sloan Digital Sky Survey (SDSS) galaxy redshifts surveys to see these features, and we find that even if features exist in the primordial power spectrum, they are washed out by the window functions of the redshift surveys on scales k < 0.03 h Mpc - 1 .

Two cosmological models for clusters of galaxies

We investigate the properties of clusters of galaxies in two cosmological models using N-body simulations and the Press--Schecter (PS) theory. In the first model, the initial power spectrum of density fluctuations is in the form P(k) ∝ k-2 at wavelengths λ <120 h-1 Mpc. In the second model, the initial linear power spectrum of density fluctuations contains a feature (bump) at wavelengths λ∼ 30--60, h-1 Mpc, which correspond to the scale of superclusters of galaxies. We examine the mass function, peculiar velocities, the power spectrum and the correlation function of clusters in both models for different values of the density parameter Ο0 and σ8 (the rms fluctuation on the 8, h-1Mpc scale). The results are compared with observations. We find that in many aspects the power spectrum of density fluctuations in model (2) fits the observed data better than the simple power-law model (1). In the first model, the mass function and peculiar velocities of clusters are consistent with observations only if Ο0<0.6. In the second model, the permitted region in the (Ο0, σ8) plane is larger. In this model, the power spectrum of clusters is in good agreement with the observed power spectrum of the APM clusters. This model predicts that there is a bump in the correlation function of clusters at separations r ∼ 20--35, h-1 Mpc. In the future, accurate measurements of the cluster correlation function at these distances can serve as a discriminating test for this model. We examine the linear theory predictions for the peculiar velocities of peaks in the Gaussian field and compare these to the peculiar velocities of clusters in N-body simulations. We determine the clusters as the maxima of the density field smoothed on the scale R ∼ 1.5, h-1 Mpc, and define their peculiar velocities using the same smoothing scale as for the density field. The numerical results show that in this case the rms peculiar velocities of clusters increase with cluster richness. The rms peculiar velocity of small clusters is similar to the linear theory expectations, while the rms peculiar velocity of rich clusters is higher than that predicted in the linear theory (∼ 18 per cent for clusters with a mean intercluster separation d cl=30 h-1 Mpc).