Relationship of galaxies from the second Byurakan survey to Zwicky clusters. II. Discussion

We analyze the data presented in a previous paper by Gyulzadyan and Petrosian, and discuss the results of a statistical investigation of the relationship between SBS galaxies and Zwicky clusters. The main results are that SBS galaxies follow the overall galaxy distribution in clusters and they do not avoid any type of Zwicky cluster. There is a significantly higher probability of finding SBS galaxies occurring in medium compact clusters than in open clusters. They also follow the well-established morphology-density relation. Earlier morphological type, higher luminosity, larger linear size, and redder SBS galaxies tend to be found in clusters with higher compactness, or in more compact regions of the clusters. The number distribution of SBS galaxies in Zwicky open clusters probably follows the distribution of normal galaxies. The number distribution of SBS galaxies in medium compact and compact clusters shows two-maxima structure. Published in Astrofizika, Vol. 52, No. 2, pp. 225–236 (May 2009).     

On structures and number-density distributions in clusters of galaxies

The structures of compact groups of galaxies are compared with the structures of rich clusters of galaxies. It was established that there are structural similarities between these two types of clusters of galaxies. It is imaginable that these structures are typical for all such complexes of galaxies and that the different structures are due to different initial conditions at the beginning of their evolution. For a final answer on this question it is necessary to accumulate more observational material with respect to the distribution of galaxies in such clusters. As part of a larger programme the present paper contains the derived number-density distributions and core radii for ten clusters.     

Two luminosity classes of quasars

Observations of quasars suggest that compact quasars with flat radio spectra belong to the class of dwarf galaxies and extended quasars with steep spectra to the one of bright galaxies. This can be seen also in the associations of quasars with clusters of galaxies, compact quasars occurring in nearby clusters and extended quasars in distant clusters.     

Dynamical evolution in clusters of galaxies with low-frequency radio emission

Clusters of galaxies in which radio emission at low frequencies (178 MHz) has been detected were classified on the Bautz-Morgan (BM) system according to the dominance of the brightest galaxy. Radio sources with steep low-frequency spectra occur in clusters of all BM types but more often in rich clusters; the distributions of BM types for clusters with high and low spectral indices between 38 and 178 MHz are similar. Glass copies of Mount Palomar Sky Survey plates were measured to determine the distribution of the ten brightest galaxies in clusters without dominant galaxies. Some clusters were found to have central cores of bright galaxies which may reflect mass segregation of galaxies due to dynamical friction. The bright galaxies in such cores may later merge to form dominant cD galaxies. The positions of the cD galaxies and cores of bright galaxies are often at projected distances <200 kpc from the low-frequency radio emission. The low-frequency spectrum of radio emission associated with a cD galaxy may be either steep or normal, but the low-frequency spectrum from a core of bright galaxies is usually steep. A steep spectrum may develop when a radio source is confined by hot gas in a cluster over a long period (10⁹ yr). Confinement would probably occur for radio sources associated with bright galaxies in the cores of clusters and cD galaxies in clusters. However, cD galaxies may have recurrent radio outbursts so that steep spectra are not always observed.     

星系团的分层效应

对星系团各类分层效应的有关问题做了概要的评述,包括成员星系在位置空间和(或)速度空间中的形态分层、光度(质量)分层和元素丰度分层的表现形式和探测途径,分层效应可能的形成机制及其对星系和星系团的结构和演化的影响。     

Globular clusters and galactic evolution (invited review)

Observations of the frequency with which globular clusters surround galaxies are reviewed. With a single exception the data do not favour the view that elliptical galaxies are formed from merging disc systems. Attention is also drawn to the unusually high frequency of globular clusters in central galaxies of rich clusters.     

Evolutionary synthesis models for galaxy transformation in clusters

The galaxy population in rich local galaxy clusters shows a ratio of one quarter elliptical galaxies, two quarters S0 galaxies, and one quarter spiral galaxies. Observations of clusters at redshift 0.5 show a perspicuously different ratio, the dominant galaxy type are spiral galaxies with a fraction of two quarters while the number of S0 galaxies decreases to a fraction of one quarter (Dressler et al. 1997). This shows an evolution of the galaxy population in clusters since redshift 0.5 and it has been suspected that galaxy transformation processes during the infall into a cluster are responsible for this change. These could be merging, starburst or ram-pressure stripping. We use our evolutionary synthesis models to describe various possible effects of those interactions on the star formation of spiral galaxies infalling into clusters. We study the effects of starbursts of various strengths as well as of the truncation of star formation at various epochs on the color and luminosity evolution of model galaxies of various spectral types. As a first application we present the comparison of our models with observed properties of the local S0 galaxy population to constrain possible S0 formation mechanisms in clusters. Application to other types of galaxies is planned for the future. This revised version was published online in August 2006 with corrections to the Cover Date.     

Clustering of galaxies on scales of ≤10<Emphasis Type="Italic">h</Emphasis><Superscript>−1</Superscript> based on the coma,bridge, and A1367 clusters

A correlation analysis is made of the spatial distribution of galaxies in the Coma, Bridge, and A1367 clusters, which form the Coma supercluster. The scale of the clustering of galaxies and the variation in their density distribution with the main parameters of the galaxies– luminosity, morphological type, and observed H I deficiency in the 21 cm line– are evaluated. The mass-to-luminosity ratios are computed for the spiral galaxies in the Coma, Bridge, and A1367 clusters. It is suggested that a larger fraction of hypothetical dark matter may be concentrated in the spiral galaxies which predominantly populate the subclusters previously identified by us within these clusters than in the spiral galaxies observed in the peripheral regions of the clusters.     

Influence of environment on the luminosity function of galaxies

The luminosity function (LF) of galaxies in different environments is studied. A method proposed by the author is used to determine the LF of galaxies. It is found that the luminosity functions of galaxies of different morphological types in single galaxies and small groups do not differ greatly. The luminosity functions of galactic clusters differ greatly from the analogous functions for other systems. A relatively large number of faint galaxies is observed in clusters. Groups with low dispersions in their radial velocities and with small average pairwise distances between the members contain relatively many faint galaxies and relatively few bright galaxies compared to groups with large dispersions in their radial velocities and large average pairwise distances between members. This applies to elliptical and lenticular galaxies, as well as to spiral and irregular galaxies.     

The evolution of [O ii] emission from cluster galaxies

We investigate the evolution of the star formation rate in cluster galaxies. We complement data from the Canadian Network for Observational Cosmology 1 (CNOC1) cluster survey  (0.15 < z < 0.6)  with measurements from galaxy clusters in the Two-degree Field (2dF) galaxy redshift survey  (0.05 < z < 0.1)  and measurements from recently published work on higher-redshift clusters, up to almost   z = 1  . We focus our attention on galaxies in the cluster core, i.e. galaxies with   r < 0.7  h ⁻¹₇₀ Mpc  . Averaging over clusters in redshift bins, we find that the fraction of galaxies with strong [O  ii ] emission is ≲20 per cent in cluster cores, and the fraction evolves little with redshift. In contrast, field galaxies from the survey show a very strong increase over the same redshift range. It thus appears that the environment in the cores of rich clusters is hostile to star formation at all the redshifts studied. We compare this result with the evolution of the colours of galaxies in cluster cores, first reported by Butcher and Oemler. Using the same galaxies for our analysis of the [O  ii ] emission, we confirm that the fraction of blue galaxies, which are defined as galaxies 0.2 mag bluer in the rest-frame B – V than the red sequence of each cluster, increases strongly with redshift. Because the colours of galaxies retain a memory of their recent star formation history, while emission from the [O  ii ] line does not, we suggest that these two results can best be reconciled if the rate at which the clusters are being assembled is higher in the past, and the galaxies from which it is being assembled are typically bluer.     

On the origin of the faint-end of the red sequence in high-density environments

With the advent of the new generation wide-field cameras it became possible to survey in an unbiased mode galaxies spanning a variety of local densities, from the core of rich clusters, to compact and loose groups, down to filaments and voids. The sensitivity reached by these instruments allowed to extend the observation to dwarf galaxies, the most “fragile” objects in the universe. At the same time models and simulations have been tailored to quantify the different effects of the environment on the evolution of galaxies. Simulations, models, and observations consistently indicate that star-forming dwarf galaxies entering high-density environments for the first time can be rapidly stripped from their interstellar medium. The lack of gas quenches the activity of star formation, producing on timescales of \({\sim }\)1 Gyr quiescent galaxies with spectro-photometric, chemical, structural, and kinematical properties similar to those observed in dwarf early-type galaxies inhabiting rich clusters and loose groups. Simulations and observations consistently identify ram pressure stripping as the major effect responsible for the quenching of the star-formation activity in rich clusters. Gravitational interactions (galaxy harassment) can also be important in groups or in clusters whenever galaxies have been members since early epochs. The observation of clusters at different redshifts combined with the present high infalling rate of galaxies onto clusters indicate that the quenching of the star-formation activity in dwarf systems and the formation of the faint end of the red sequence is a very recent phenomenon.     

Galaxy evolution in clusters

The galaxy populations in present-day clusters are distinctly different from those of the field, indicating that environment plays a strong role in galaxy evolution. This review discusses some of the recent observations of moderate to high redshift clusters. A consistent picture of galaxy evolution in clusters appears to be emerging, which includes a population of galaxies which formed early in the cluster history, as well as field galaxies which have had their star formation truncated upon falling into the cluster potential. Galaxy interactions probably play an important role in exhausting star formation in some of these galaxies. However, there is significant variation in the populations of different cluster samples, with substantial evidence that some galaxies have their star formation terminated more gradually. This suggests that different mechanisms may dominate in different clusters, perhaps because of the recent merging history of the clusters. We also present a recent analysis of population gradients in clusters which suggests that the observed evolution in cluster populations is consistent with a scenario where changing infall rates drive the fraction of star forming galaxies in clusters, rather than a changing physical mechanism within the cluster. Thus, galaxy populations may provide a fundamental measure of the growth of large scale structure. This revised version was published online in July 2006 with corrections to the Cover Date.     

不同光度星系大尺度分布的交叉相关分析

在对不同光度星系大尺度分布进行空间两点相关函数分析的基础上,仍以CfA红移巡天资料为样本,对不同光度星系分布进行了交叉相关分析。结果表明,不同光度星系间的交叉相关函数仍可近似地以幂函数表示,说明不同光度星系在空间是一起成团的。但在较小尺度上((?)4—6Mpc),光度较高的星系间相关更强,而在更大一些尺度上光度较高的星系间相关减弱更快,甚至变得比与光度较低星系间的相关更弱。结合前面对自相关函数分析的结果可以看到,统计上看来,星系分布形成群和团。群或团中亮的星系形成更致密的分布而较暗的星系则在这些群和团中分布较弥散。此结果表明星系光度和其环境(密度)有关,从而从观测上为Biased星系形成理论提供了一个可能的证据。     

高红移星系特征与星系环境

本文在星系有偏袒形成的冷暗物质模型框架下,假定星系都在红移～5时形成,研究了星系群和星系团的形成对星系演化的影响。由示冷暗物质模型中各种不同尺度扰动的幅度相近,星系群和星系团的维里化与星系的维里化在时间上比较接近,在星系群或星系团的维里化过程中获得巨大维里速度的星系仍然富有气体,因此,当它们因为频繁的互相碰撞,或者因为进入星系团中心区的稠密星系际介质受到巨大冲压,或者是被附近射电星系的喷流所产生的高压茧状体所包容,都可能激发活动性,造成恒星的爆发性形成.高红移星系团中观测到的B-O效应和高红移射电星系的光学像与射电喷流同轴向现象都可以据此得到解释.     

An attempt to test Ambartsumian's idea of the origin of the galaxy . II. location of galaxies within clusters and galaxy position angles

The location of galaxies in 377 rich Abell galaxy clusters is discussed. We compared the distributions of galaxies in the sample containing all galaxies with the sample compiled from the 20 brightest objects. Counts in circular sectors with angle width equal to 30î show isotropy. Only in the case of BM I clusters and the coordinate system related to the cluster major axis did we find anisotropic distributions. We investigated also the distributions of galaxy position angles within clusters exhibiting isotropy. The structure position angles for both samples have been studied as well. They appeared to be different. The difference in location of galaxies in the case of cD clusters, as well as the more elliptical shape and different position angles of samples containing bright galaxies is observed.     

Galaxy populations in the Fornax and Virgo clusters

In these first results from a photographic survey of nearby clusters and groups of galaxies, we compare the luminosity functions of galaxies of various Hubble types in the core of the Formax cluster to luminosity functions in the core of the Virgo cluster from Sandageet al. (1985). The galaxy classifications for the two clusters are based on identical plate material, and are hence directly comparable. The properties (galaxy density, velocity dispersion, X-ray luminosity) of the two clusters are quite different, yet we find few significant differences in the luminosity functions, or in the morphological mix of galaxies between the two clusters. In particular, while there is some indication that the ratios of giants to dwarfs and early to late-type galaxies in the two clusters differ, we cannot exclude the possibility that the ratios are identical. We discuss the selection limits of our survey and the completeness corrections that must be applied to the faint end of the luminosity function. The effective surface brightness of early-type galaxies in our sample decreases with decreasing luminosity. We show that this correlation is not an artifact of our selection criteria for dwarf spheroidal galaxies and use it to determine the relative distances to the Fornax and Virgo clusters. We also present evidence for a radial variation in the luminosity function of dwarf ellipticals within the two clusters in the sense that the faint end of the luminosity distribution is enhanced at the centre of each cluster.     

Searching for tidal tails – investigating galaxy harassment

Galaxy harassment has been proposed as a physical process that morphologically transforms low surface density disc galaxies into dwarf elliptical galaxies in clusters. It has been used to link the observed very different morphology of distant cluster galaxies (relatively more blue galaxies with 'disturbed' morphologies) with the relatively large numbers of dwarf elliptical galaxies found in nearby clusters. One prediction of the harassment model is that the remnant galaxies should lie on low surface brightness tidal streams or arcs. We demonstrate in this paper that we have an analysis method that is sensitive to the detection of arcs down to a surface brightness of 29 B μ and we then use this method to search for arcs around 46 Virgo cluster dwarf elliptical galaxies. We find no evidence for tidal streams or arcs and consequently no evidence for galaxy harassment as a viable explanation for the relatively large numbers of dwarf galaxies found in the Virgo cluster.     

X-ray emission from the galaxies in Abell 2634

It is difficult to detect X-ray emission associated with galaxies in rich clusters, because the X-ray images of the clusters are dominated by the emission from their hot intracluster media (ICM). Only the nearby Virgo cluster provides us with information about the X-ray properties of galaxies in clusters. Here we report on the analysis of a deep ROSAT HRI image of the moderately rich cluster Abell 2634, by which we have been able to detect the X-ray emission from the galaxies in the cluster. The ICM of Abell 2634 is an order of magnitude denser than that of the Virgo cluster, and so this analysis allows us to explore the X-ray properties of individual galaxies in the richest environment yet explored.
By stacking the X-ray images of the galaxies together, we show that the emission from the galaxies appears to be marginally resolved by the HRI. This extent is smaller than for galaxies in poorer environments, and is comparable to the size of the galaxies in optical light. These facts suggest that the detected X-ray emission originates from the stellar populations of the galaxies, rather than from extended hot interstellar media. Support for this hypothesis comes from placing the optical and X-ray luminosities of these galaxies in the L_B–L_X plane: the galaxies of Abell 2634 lie in the region of this plane where models indicate that all the X-ray emission can be explained by the usual population of X-ray binaries. It is therefore probable that ram pressure stripping has removed the hot gas component from these galaxies.     

Formation of Starburst Galaxies in Merging Clusters

A significant fraction of clusters of galaxies are observed to have substructure, which implies that merging between clusters and subclusters is a rather common physical process in cluster formation. It still remains unclear how cluster merging affects the evolution of cluster member galaxies. We report the results of numerical simulations that show the dynamical evolution of a gas-rich, late-type spiral in a merger between a small group of galaxies and a cluster. The simulations demonstrate that the time-dependent tidal gravitational field during merging excites non-axisymmetric structure of the galaxy, subsequently drives efficient transfer of gas to the central region and finally triggers a secondary starburst. This result provides a close physical relationship between the emergence of starburst galaxies and the formation of substructure in clusters. We accordingly interpret post-starburst galaxies located near substructure of the Coma Cluster as one observational example indicating the global tidal effects of group–cluster merging. Our numerical results further suggest a causal link between the observed excess of blue galaxies in distant clusters and the cluster virialization process through hierarchical merging of subclusters. This revised version was published online in July 2006 with corrections to the Cover Date.     

Analysis of the properties of clusters of galaxies in the region of the Ursa Major supercluster

We analyze the properties of the clusters of galaxies in the region of the Ursa Major (UMa) supercluster using observational data from SDSS and 2MASS catalogs. The region studied includes a supercluster (with a galaxy and cluster overdensity of 3 and 15, respectively) and field clusters inside the 150-Mpc diameter surrounding region. The total dynamical mass of 10 clusters of galaxies in UMa is equal to 2.25 × 10¹⁵ M _⊙, and the mass of 11 clusters of galaxies in the UMa neighborhood is equal to 1.70 × 10¹⁵ M _⊙. The fraction of early-type galaxies brighter than M _K ^* + 1 in the virialized regions of clusters is, on the average, equal to 70%, and it is virtually independent on the mass of the cluster. The fraction of these galaxies and their average photometric parameters are almost the same both for UMa clusters and for the clusters located in its surroundings. Parameters of the clusters of galaxies, such as infrared luminosities up to a fixed magnitude, the mass-to-luminosity ratio, and the number of galaxies have almost the same correlations with the cluster mass as in other samples of galaxies clusters. However, the scatter of these parameters for UMa member clusters is twice smaller than the corresponding scatter for field clusters, possibly, due to the common origin of UMa clusters and synchronized dynamical evolution of clusters in the supercluster.