Infalling groups and galaxy transformations in the cluster A2142

Einasto, Maret,Deshev, Boris,Lietzen, Heidi,Kipper, Rain,Tempel, Elmo,Park, Changbom,Gramann, Mirt,Heinä,mä,ki, Pekka,Saar, Enn,Einasto, Jaan Springer-Verlag 2018 Astronomy and astrophysics Vol.610 No.-

<P><I>Context.</I> Superclusters of galaxies provide dynamical environments for the study of the formation and evolution of structures in the cosmic web from galaxies, to the richest galaxy clusters, and superclusters themselves.</P><P><I>Aims.</I> We study galaxy populations and search for possible merging substructures in the rich galaxy cluster A2142 in the collapsing core of the supercluster SCl A2142, which may give rise to radio and X-ray structures in the cluster, and affect galaxy properties of this cluster.</P><P><I>Methods.</I> We used normal mixture modelling to select substructure of the cluster A2142. We compared alignments of the cluster, its brightest galaxies (hereafter BCGs), subclusters, and supercluster axes. The projected phase space (PPS) diagram and clustercentric distributions are used to analyse the dynamics of the cluster and study the distribution of various galaxy populations in the cluster and subclusters.</P><P><I>Results.</I> We find several infalling galaxy groups and subclusters. The cluster, supercluster, BCGs, and one infalling subcluster are all aligned. Their orientation is correlated with the alignment of the radio and X-ray haloes of the cluster. Galaxy populations in the main cluster and in the outskirts subclusters are different. Galaxies in the centre of the main cluster at the clustercentric distances 0.5 h<SUP>−1</SUP> Mpc (<I>D</I>c∕<I>R</I>vir < 0.5, <I>R</I>vir = 0.9 h<SUP>−1</SUP> Mpc) have older stellar populations (with the median age of 10−11 Gyr) than galaxies at larger clustercentric distances. Star-forming and recently quenched galaxies are located mostly at the clustercentric distances <I>D</I>c ≈ 1.8 h<SUP>−1</SUP> Mpc, where subclusters fall into the cluster and the properties of galaxies change rapidly. In this region the median age of stellar populations of galaxies is about 2 Gyr. Galaxies in A2142 on average have higher stellar masses, lower star formation rates, and redder colours than galaxies in rich groups. The total mass in infalling groups and subclusters is <I>M</I> ≈ 6 × 10<SUP>14</SUP> h<SUP>−1</SUP><I>M</I>⊙, that is approximately half of the mass of the cluster. This mass is sufficient for the mass growth of the cluster from redshift <I>z</I> = 0.5 (half-mass epoch) to the present.</P><P><I>Conclusions.</I> Our analysis suggests that the cluster A2142 has formed as a result of past and present mergers and infallen groups, predominantly along the supercluster axis. Mergers cause complex radio and X-ray structure of the cluster and affect the properties of galaxies in the cluster, especially at the boundaries of the cluster in the infall region. Explaining the differences between galaxy populations, mass, and richness of A2142, and other groups and clusters may lead to better insight about the formation and evolution of rich galaxy clusters.</P>

QUASARS AS A TRACER OF LARGE-SCALE STRUCTURES IN THE DISTANT UNIVERSE

Song, Hyunmi,Park, Changbom,Lietzen, Heidi,Einasto, Maret American Astronomical Society 2016 The Astrophysical journal Vol.827 No.2

<P>We study the dependence of the number density and properties of quasars on the background galaxy density using the currently largest spectroscopic data sets of quasars and galaxies. We construct a galaxy number density field smoothed over the variable smoothing scale of between approximately 10 and 20 h(-1) Mpc over the redshift range 0.46 < z < 0.59 using the Sloan Digital Sky Survey (SDSS) Data Release 12 (DR12) Constant MASS galaxies. The quasar sample is prepared from the SDSS-I/II DR7. We examine the correlation of incidence of quasars with the large-scale background density and the dependence of quasar properties such as bolometric luminosity, black hole mass, and Eddington ratio on the large-scale density. We find a monotonic correlation between the quasar number density and large-scale galaxy number density, which is fitted well with a power-law relation, nQ proportional to rho(0.618)(G). We detect weak dependences of quasar properties on the large-scale density such as a positive correlation between black hole mass and density, and a negative correlation between luminosity and density. We discuss the possibility of using quasars as a tracer of large-scale structures at high redshifts, which may be useful for studies of the growth of structures in the high-redshift universe.</P>

Tracing a high redshift cosmic web with quasar systems

Einasto, Maret,Tago, Erik,Lietzen, Heidi,Park, Changbom,Heinä,mä,ki, Pekka,Saar, Enn,Song, Hyunmi,Liivamä,gi, Lauri Juhan,Einasto, Jaan EDP Sciences 2014 Astronomy and astrophysics Vol.568 No.1

<P>Context. To understand the formation, evolution, and present-day properties of the cosmic web we need to study it at low and high redshifts. Aims. We trace the cosmic web at redshifts that range from 1.0 ≤ z ≤1.8 by using the quasar (QSO) data from the SDSSDR7QSO catalogue. Methods. We apply a friend-of-friend algorithm to the quasar and random catalogues to determine systems at a series of linking length and analyse richness and sizes of these systems. Results. At the linking lengths l ≤ 30 h<SUP>−1</SUP> Mpc, the number of quasar systems is larger than the number of systems detected in random catalogues, and the systems themselves have smaller diameters than random systems. The diameters of quasar systems are comparable to the sizes of poor galaxy superclusters in the local Universe. The richest quasar systems have four members. The mean space density of quasar systems, ≈10<SUP>−7</SUP> (h<SUP>−1</SUP> Mpc)<SUP>−3</SUP>, is close to the mean space density of local rich superclusters. At intermediate linking lengths (40 ≤ l ≤70 h<SUP>−1</SUP> Mpc), the richness and length of quasar systems are similar to those derived from random catalogues. Quasar system diameters are similar to the sizes of rich superclusters and supercluster chains in the local Universe. The percolating system, which penetrate the whole sample volume appears in a quasar sample at a smaller linking length than in random samples (85 h<SUP>−1</SUP> Mpc). At the linking length 70 h<SUP>−1</SUP> Mpc, the richest systems of quasars have diameters exceeding 500 h<SUP>−1</SUP> Mpc. Quasar luminosities in systems are not correlated with the system richness. Conclusions. Quasar system catalogues in our web pages and at the Strasbourg Astronomical Data Center (CDS) serve as a database for searching superclusters of galaxies and for tracing the cosmic web at high redshifts.</P>

LARGE SDSS QUASAR GROUPS AND THEIR STATISTICAL SIGNIFICANCE

박창범,송현미,Maret Einasto,Heidi Lietzen,Pekka Heinamaki 한국천문학회 2015 Journal of The Korean Astronomical Society Vol.48 No.1

We use a volume-limited sample of quasars in the Sloan Digital Sky Survey (SDSS) DR7quasar catalog to identify quasar groups and address their statistical significance. This quasar samplehas a uniform selection function on the sky and nearly a maximum possible contiguous volume that canbe drawn from the DR7 catalog. Quasar groups are identified by using the Friend-of-Friend algorithmwith a set of fixed comoving linking lengths. We find that the richness distribution of the richest 100quasar groups or the size distribution of the largest 100 groups are statistically equivalent with those ofrandomly-distributed points with the same number density and sky coverage when groups are identifiedwith the linking length of 70 h−1Mpc. It is shown that the large-scale structures like the huge LargeQuasar Group (U1.27) reported by Clowes et al. (2013) can be found with high probability even if quasarshave no physical clustering, and does not challenge the initially homogeneous cosmological models. Ourresults are statistically more reliable than those of Nadathur (2013), where the test was made only for thelargest quasar group. It is shown that the linking length should be smaller than 50 h−1Mpc in order forthe quasar groups identified in the DR7 catalog not to be dominated by associations of quasars groupedby chance. We present 20 richest quasar groups identified with the linking length of 70 h−1Mpc for furtheranalyses.

Supercluster A2142 and collapse in action: infalling and merging groups and galaxy transformations

Einasto, Maret,Gramann, Mirt,Park, Changbom,Kim, Juhan,Deshev, Boris,Tempel, Elmo,Heinä,mä,ki, Pekka,Lietzen, Heidi,Lä,hteenmä,ki, Anne,Einasto, Jaan,Saar, Enn EDP Sciences 2018 Astronomy and astrophysics Vol.620 No.-

<P><I>Context</I>. Superclusters with collapsing cores represent dynamically evolving environments for galaxies, galaxy groups, and clusters.</P><P><I>Aims</I>. We study the dynamical state and properties of galaxies and groups in the supercluster SCl A2142 that has a collapsing core, to understand its possible formation and evolution.</P><P><I>Methods</I>. We find the substructure of galaxy groups using normal mixture modelling. We have used the projected phase space (PPS) diagram, spherical collapse model, clustercentric distances, and magnitude gap between the brightest galaxies in groups to study the dynamical state of groups and to analyse group and galaxy properties. We compared the alignments of groups and their brightest galaxies with the supercluster axis.</P><P><I>Results</I>. The supercluster core has a radius of about 8 <I>h</I><SUP>−1</SUP> Mpc and total mass <I>M</I>tot ≈ 2.3 × 10<SUP>15</SUP> h<SUP>−1</SUP><I>M</I>⊙ and is collapsing. Galaxies in groups on the supercluster axis have older stellar populations than off-axis groups, with median stellar ages 4 − 6 and < 4 Gyr, correspondingly. The cluster A2142 and the group Gr8 both host galaxies with the oldest stellar populations among groups in SCl A2142 having the median stellar age <I>t</I> > 8 Gyr. Recently quenched galaxies and active galactic nuclei (AGNs) are mostly located at virial radii or in merging regions of groups, and at clustercentric distances <I>D</I>c ≈ 6 <I>h</I><SUP>−1</SUP> Mpc. The most elongated groups lie along the supercluster axis and are aligned with it. Magnitude gaps between the brightest galaxies of groups are less than one magnitude, suggesting that groups in SCl A2142 are dynamically young.</P><P><I>Conclusions</I>. The collapsing core of the supercluster, infall of galaxies and groups, and possible merging groups, which affect galaxy properties and may trigger the activity of AGNs, show how the whole supercluster is evolving.</P>