Maret Einasto

A 120-Mpc periodicity in the three-dimensional distribution of galaxy superclusters

ACCORDING to the favoured models for the formation of large-scale structure in the Universe (in which the dynamics of the Universe is dominated by cold dark matter), the distribution of galaxies and clusters of galaxies should be random on large scales. It therefore came as a surprise when a periodicity was reported1 in the distribution of high-density regions of galaxies in the directions of the Galactic poles, although the apparent lack of periodicity in other directions led to the initial report being regarded as a statistical anomaly2. A subsequent study3–6 also claimed evidence for periodicity on the same scale, but the statistical significance of this result was uncertain due to the small number of clusters used. Here, using a new compilation7 of available data on galaxy clusters, we present evidence for a quasi-regular three-dimensional network of rich superclusters and voids, with the regions of high density separated by ∼120 Mpc. If this reflects the distribution of all matter (luminous and dark), then there must exist some hitherto unknown process that produces regular structure on large scales.

Optical and X-Ray Clusters as Tracers of the Supercluster-Void Network. I. Superclusters of Abell and X-Ray Clusters

We study the distribution of Abell and X-ray–selected clusters of galaxies and derive selection functions of these cluster samples. We find that selection functions and percolation properties of Abell clusters of richness class R = 0 and R ≥ 1 are similar; the percolation of X-ray clusters occurs at the same dimensionless radius as the percolation of Abell clusters. We present a new catalog of superclusters of Abell clusters out to a redshift of zlim = 0.13, a catalog of X-ray clusters located in superclusters determined by Abell clusters, and a list of additional superclusters of X-ray clusters. We investigate the distribution of X-ray–selected clusters of galaxies with respect to superclusters determined by Abell clusters of galaxies and show that the distribution of X-ray clusters follows the supercluster-void network determined by Abell clusters. We find that X-ray clusters are more strongly clustered than other clusters: the fraction of X-ray clusters is higher in rich superclusters, and the fraction of isolated X-ray clusters is lower than the fraction of isolated Abell clusters. Poor, non-Abell X-ray clusters follow the supercluster-void network as well: these clusters are embedded in superclusters determined by Abell clusters and populate filaments between them.

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).

Groups of galaxies in the SDSS Data Release 7: Flux- and volume-limited samples

Aims. We extract groups of galaxies as flux-limited and volume-limited samples from the SDSS Data Release 7 (DR7) to study the supercluster-void network and environmental properties of groups therein. Volume-limited catalogues are particularly useful for a comparison of numerical simulations of dark matter halos and the large-scale structure with observations. Methods. The extraction of a volume-limited sample of galaxies and groups requires special care to avoid excluding too much observational data. We use a modified friends-of-friends (FoF) method with a slightly variable linking length to obtain a preliminary flux-limited sample. We take the flux-limited groups as the basic sample to include as many galaxies as possible in the volume-limited samples. To determine the scaling of the linking length we calibrated group sizes and mean galaxy number densities within groups by magnitude dilution of a nearby group sub-sample to follow the properties of groups with higher luminosity limits. Results. Our final flux-limited sample contains 78800 groups and volume-limited subsamples with absolute magnitude limits M r = -18, -19, -20, and -21 contain 5463, 12590, 18 973, and 9139 groups, respectively, in the DR7 main galaxy main area survey. The group catalogue is available at the CDS. Conclusions. The spatial number densities of our groups within the subsamples, as well as the mean sizes and rms velocities of our groups practically do not change from sub-sample to sub-sample. This means that the catalogues are homogeneous and well suited for a comparison with simulations.

Superclusters of galaxies from the 2dF redshift survey - I. The catalogue

We use the 2dF Galaxy Redshift Survey data to compile catalogues of superclusters for the Northern and Southern regions of the 2dFGRS, altogether 543 superclusters at redshifts 0.009 {le} z {le} 0.2. We analyze methods of compiling supercluster catalogues and use results of the Millennium Simulation to investigate possible selection effects and errors. We find that the most effective method is the density field method using smoothing with an Epanechnikov kernel of radius 8 h{sup -1} Mpc. We derive positions of the highest luminosity density peaks and find the most luminous cluster in the vicinity of the peak, this cluster is considered as the main cluster and its brightest galaxy the main galaxy of the supercluster. In catalogues we give equatorial coordinates and distances of superclusters as determined by positions of their main clusters. We also calculate the expected total luminosities of the superclusters.

Galaxy morphology, luminosity, and environment in the SDSS DR7

Aims. We study the influence of the environment on the evolution of galaxies by investigating the luminosity function (LF) of galaxies of different morphological types and colours at different environmental density levels. Methods. We construct the LFs separately for galaxies of different morphology (spiral and elliptical) and of different colours (red and blue) using data from the Sloan Digital Sky Survey (SDSS), correcting the luminosities for the intrinsic absorption. We use the global luminosity density field to define different environments, and analyse the environmental dependence of galaxy morphology and colour. The smoothed bootstrap method is used to calculate confidence regions of the derived luminosity functions. Results. We find a strong environmental dependency for the LF of elliptical galaxies. The LF of spiral galaxies is almost environment independent, suggesting that spiral galaxy formation mechanisms are similar in different environments. Absorption by the intrinsic dust influences the bright-end of the LF of spiral galaxies. After attenuation correction, the brightest spiral galaxies are still about 0.5 mag less luminous than the brightest elliptical galaxies, except in the least dense environment, where spiral galaxies dominate the LF at every luminosity. Despite the extent of the SDSS survey, the influence of single rich superclusters is present in the galactic LF of the densest environment.

Groups of galaxies in the SDSS Data Release 5 - A group-finder and a catalogue

Aims. We extract groups of galaxies from the SDSS Data Release 5 with the purpose of studying the supercluster-void network and environmental properties of groups therein. Groups of galaxies as density enhancements can be used to determine the luminosity density field of the network. Methods. We use a modified friends-of-friends (FoF) method with adopt ed variable linking length in transverse and radial directi on to eliminate selection effects and to find reliably as many groups as possible to track th e supercluster network. Results. We take into account various selection effects due to the use of a magnitude limited sample. To determine linking length scaling we study the luminosity-density relation in observed groups. We follow the changes in group sizes and mean galaxy number densities within groups when shifting nearby groups to larger distances. As a result we show that the linking length should be a slowly growing function with distance. Our final sample contains 17 143 groups in the equatorial, and 33219 groups in the northern part of the DR5 survey with membership Ng≥ 2. The group catalogue is available at our web-site (http://www.obs.ee/∼erik/index.html). Conclusions. Due to a narrow magnitude window in the SDSS the group catalogue based on this survey has been obtained by moderately growing linking length scaling law up to redshift z = 0.12. Above this redshift the scaling law turns down. In the redshift range z=0.12 - 0.2 only the cores are detected. Along with applying weights when calculating luminosities it is possible to use gr oups for determination of the large-scale luminosity-density fi eld.

Clusters and superclusters in the Sloan Digital Sky Survey

We apply the 2-dimensional high-resolution density field of galaxies of the Early Data Release of the Sloan Digital Sky Survey with a smoothing lengths 0.8 h −1 Mpc to extract clusters and groups of galaxies, and a low-resolution field with smoothing lengths 10 h −1 Mpc to extract superclusters of galaxies. We investigate properties of density field clusters and su- perclusters and compare properties of these clusters and superclusters with Abell clusters, and superclusters found on the basis of Abell clusters. We found that clusters in high-density environment have a luminosity a factor of 5−10 higher than in low- density environment. There exists a large anisotropy between the SDSS Northern and Southern sample in the properties of clusters and superclusters: most luminous clusters and superclusters in the Northern sample are a factor of 2 more luminous than the respective systems in the Southern sample.

Multimodality in galaxy clusters from SDSS DR8: substructure and velocity distribution

Context. The study of the signatures of multimodality in groups and clusters of galaxies, an environment for most of the galaxies in the Universe, gives us information about the dynamical state of clusters and about merging processes, which affect the formation and evolution of galaxies, groups and clusters, and larger structures – superclusters of galaxies and the whole cosmic web. Aims. We search for the presence of substructure, a non-Gaussian, asymmetrical velocity distribution of galaxies, and large peculiar velocities of the main galaxies in clusters with at least 50 member galaxies, drawn from the SDSS DR8. Methods. We employ a number of 3D, 2D, and 1D tests to analyse the distribution of galaxies in clusters: 3D normal mixture modelling, the Dressler-Shectman test, the Anderson-Darling and Shapiro-Wilk tests, as well as the Anscombe-Glynn and the D’Agostino tests. We find the peculiar velocities of the main galaxies, and use principal component analysis to characterise our results. Results. More than 80% of the clusters in our sample have substructure according to 3D normal mixture modelling, and the DresslerShectman (DS) test shows substructure in about 70% of the clusters. The median value of the peculiar velocities of the main galaxies in clusters is 206 km s −1 (41% of the rms velocity). The velocities of galaxies in more than 20% of the clusters show significant non-Gaussianity. While multidimensional normal mixture modelling is more sensitive than the DS test in resolving substructure in the sky distribution of cluster galaxies, the DS test determines better substructure expressed as tails in the velocity distribution of galaxies (possible line-of-sight mergers). Richer, larger, and more luminous clusters have larger amount of substructure and larger (compared to the rms velocity) peculiar velocities of the main galaxies. Principal component analysis of both the substructure indicators and the physical parametres of clusters shows that galaxy clusters are complicated objects, the properties of which cannot be explained with a small number of parametres or delimited by one single test. Conclusions. The presence of substructure, the non-Gaussian velocity distributions, as well as the large peculiar velocities of the main galaxies, shows that most of the clusters in our sample are dynamically young.

Superclusters of galaxies from the 2dF redshift survey. II. Comparison with simulations

We investigate properties of superclusters of galaxies found on the basis of the 2dF Galaxy Redshift Survey, and compare them with properties of superclusters from the Millennium Simulation.We study the dependence of various characteristics of superclusters on their distance from the observer, on their total luminosity, and on their multiplicity. The multiplicity is defined by the number of Density Field (DF) clusters in superclusters. Using the multiplicity we divide superclusters into four richness classes: poor, medium, rich and extremely rich.We show that superclusters are asymmetrical and have multi-branching filamentary structure, with the degree of asymmetry and filamentarity being higher for the more luminous and richer superclusters. The comparison of real superclusters with Millennium superclusters shows that most properties of simulated superclusters agree very well with real data, the main differences being in the luminosity and multiplicity distributions.