Rich Abell Clusters Research Articles

RECENT GALAXY MERGERS AND RESIDUAL STAR FORMATION OF RED SEQUENCE GALAXIES IN GALAXY CLUSTERS

ABSTRACT This study explored the Galaxy Evolution Explorer ultraviolet (UV) properties of optical red sequence galaxies in four rich Abell clusters at . In particular, we tried to find a hint of merger-induced recent star formation (RSF) in red sequence galaxies. Using the NUV colors of the galaxies, RSF fractions were derived based on various criteria for post-merger galaxies and normal galaxies. Following k-correction, about 36% of the post-merger galaxies were classified as RSF galaxies with a conservative criterion (NUV ), and that number was doubled (∼72%) when using a generous criterion (NUV ). The trend was the same when we restricted the sample to galaxies within 0.5 × R 200. Post-merger galaxies with strong UV emission showed more violent, asymmetric features in the deep optical images. The RSF fractions did not show any trend along the clustocentric distance within R 200. We performed a Dressler–Shectman test to check whether the RSF galaxies had any correlation with the substructures in the galaxy clusters. Within R 200 of each cluster, the RSF galaxies did not appear to be preferentially related to the clusters’ substructures. Our results suggested that only 30% of RSF red sequence galaxies show morphological hints of recent galaxy mergers. This implies that internal processes (e.g., stellar mass loss or hot gas cooling) for the supply of cold gas to early-type galaxies may play a significant role in the residual star formation of early-type galaxies at a recent epoch.

Two new catalogues of superclusters of Abell/ACO galaxy clusters out to redshift 0.15

We present two new catalogues of superclusters of galaxies out to a redshit of z = 0.15, based on the Abell/ACO cluster redshift compilation maintained by one of us (HA). The first of these catalogues, the all-sky Main SuperCluster Catalogue (MSCC), is based on only the rich (A-) Abell clusters, and the second one, the Southern SuperCluster Catalogue (SSCC), covers declinations delta < -17 deg and includes the supplementary Abell S-clusters. A tunable Friends-of-Friends (FoF) algorithm was used to account for the cluster density decreasing with redshift and for different selection functions in distinct areas of the sky. We present the full list of Abell clusters used, together with their redshifts and supercluster memberships and including the isolated clusters. The SSCC contains about twice the number of superclusters than MSCC for delta < -17 deg, which we found to be due to: (1) new superclusters formed by A-clusters in their cores and surrounded by S-clusters (50%), (2) new superclusters formed by S-clusters only (40%), (3) redistribution of member clusters by fragmentation of rich (multiplicity m > 15) superclusters (8%), and (4) new superclusters formed by the connection of A-clusters through bridges of S-clusters (2%). Power-law fits to the cumulative supercluster multiplicity function yield slopes of alpha = -2.0 and alpha = -1.9 for MSCC and SSCC respectively. This power-law behavior is in agreement with the findings for other observational samples of superclusters, but not with that of catalogues based on cosmological simulations.

Locating bound structure in an accelerating universe

Given the overwhelming evidence that the universe is currently undergoing an accelerated expansion, the question of what are the largest gravitationally bound structures remains. A couple of groups, Busha et al. 2003 (B03) and Dunner et al. 2006 (D06), have attempted to analytically define these limits, arriving at substantially different estimates due to differences in their assumptions about the velocities at the present epoch. In an effort to locate the largest bound structures in the universe, we selected the Aquarius (ASC), Microscopium (MSC), Corona Borealis (CBSC), and Shapley (SSC) superclusters for study, due to their high number density of rich Abell clusters. Simple N-body simulations, which assumed negligible intercluster mass, were used to assess the likelihood of these structures being gravitationally bound, and the predictions of the models of B03 and D06 were compared with those results. We find that ASC, and MSC contain pairs of clusters which are gravitationally bound, A2541/A2546 and A3695/A3696 respectively, with no other structures having a significant chance of being bound. For SSC, we find a group of five clusters, A3554, A3556, A3558, A3560, and A3562 that are bound, with an additional pair, A1736/A3559, having a slight chance of being bound. We find that CBSC has no extended bound structure, contrary to the findings of Small et al. 1998, who claim that the entire supercluster is bound. In regards to the analytical models, we find that B03 will identify structure that is definitely bound, but tends to underestimate the true extent of the structure, while D06 will identify all structure that is bound while overestimating its extent. Combined, the two models can provide lower and upper limits to the extent of bound structures so long as there are no other significant structures nearby or no significant dark matter exterior to the clusters.

POST-MERGER SIGNATURES OF RED-SEQUENCE GALAXIES IN RICH ABELL CLUSTERS AT z ≲ 0.1

We have investigated the post-merger signatures of red-sequence galaxies in rich Abell clusters at $z \lesssim$ 0.1: A119, A2670, A3330 and A389. Deep images in u', g', r' and medium-resolution galaxy spectra were taken using MOSAIC 2 CCD and Hydra MOS mounted on a Blanco 4-m telescope at CTIO. Post-merger features are identified by visual inspection based on asymmetric disturbed features, faint structures, discontinuous halo structures, rings and dust lanes. We found that ~ 25% of bright (M_r < -20) cluster red-sequence galaxies show post-merger signatures in four clusters consistently. Most (~ 71%) of the featured galaxies were found to be bulge-dominated, and for the subsample of bulge-dominated red-sequence galaxies, the post-merger fraction rises to ~ 38%. We also found that roughly 4% of bulge-dominated red-sequence galaxies interact (on-going merger). A total of 42% (38% post-merger, 4% on-going merger) of galaxies show merger-related features. Compared to a field galaxy study with a similar limiting magnitude (van Dokkum 2005), our cluster study presents a similar post-merger fraction but a markedly lower on-going merger fraction. The merger fraction derived is surprisingly high for the high density of our clusters, where the fast internal motions of galaxies are thought to play a negative role in galaxy mergers. The fraction of post-merger and on-going merger galaxies can be explained as follows. Most of the post-merger galaxies may have carried over their merger features from their previous halo environment, whereas interacting galaxies interact in the current cluster in situ. According to our semi-analytic calculation, massive cluster haloes may very well have experienced tens of halo mergers over the last 4-5 Gyr; post-merger features last that long, allowing these features to be detected in our clusters today. (Abridged)

COMMENTS ON ENVIRONMENTAL EFFECTS IN THE ORIGIN OF ANGULAR MOMENTA IN GALAXIES

We examine the orientations of galaxies in 43 rich Abell galaxy clusters belonging to superclusters and containing at least 100 members in the considered area as a function of supercluster multiplicity. It is found that the orientation of galaxies in the analyzed clusters is not random and the alignment decreases with supercluster richness, although the effect is statistically significant only for azimuthal angles. The dependence of galaxy alignment on cluster location inside or outside a supercluster and on supercluster multiplicity clearly shows the importance of environmental effects on the origin of galaxy angular momenta. The comparison with alignment of galaxies in a sample of rich Abell clusters not belonging to superclusters is made too.

Some Properties of Galaxy Structures

Abstract We analysed some properties of galaxies structures based on the PF catalog of galaxy structures (Panko &amp; Flin 2006) and the Tully NBG catalog (Tully 1988). At first, we analyzed the orientation of galaxies in the 247 optically selected rich Abell clusters, having at least 100 members. The distribution of the position angles of galaxies as well as of two angles describing spatial orientation of the galaxy planes were tested for isotropy, applying three statistical tests. We found the relation between the anisotropy and the cluster richness. The relation between the galaxy alignment and the Bautz-Morgan morphological type of the parent cluster is not present. A statistically marginal relation between the velocity dispersion and cluster richness is observed. We also analyzed ellipticities for 6188 low redshift (z &lt; 0.18) poor and rich galaxy structures which have been examined along with their evolution. Finally, we analyzed the Binggeli effect and found that the orientation of galaxy groups in the Local Supercluster (LSC), is strongly correlated with the distribution of neighbouring groups in the scale up to about 20 Mpc. Analysis of galaxy structures from the PF catalog shows quite different situation - the efect is observed only for more elongated structures (e ≤ 0.3). The effect is present in a distance range of about 60 h−1 Mpc.

The environmental effects in the origin of angular momenta of galaxies

We study the galaxy alignment in the sample of very rich Abell clusters located in and outside superclusters. The statistically significant difference among investigated samples exists. We found that in contrast to whole sample of cluster, where alignment increase with the cluster richness, the cluster belonging to the superclusters does not show this effect. Moreover, the alignment decreased with the supercluster richness. One should note however that orientations of galaxies in analyzed clusters are not random, both in the case when we analyzed whole sample of the clusters and only clusters belonging to the superclusters. The observed trend, dependence of galaxy alignment on both cluster location and supercluster richness clearly supports the influence of environmental effects to the origin of galaxy angular momenta.

THE ORIENTATION OF GALAXIES IN GALAXY CLUSTERS

We present an analysis of the spatial orientations of galaxies in 247 optically selected rich Abell clusters which have at least 100 members in the considered area. We investigated the relation between angles that give information about galaxy angular momenta and the number of members in each structure. The position angles of the galaxies' major axes, as well as two angles describing the spatial orientation of the galaxy plane, were tested for isotropy by applying three different statistical tests. It is found that the values of the statistics increase with the amount of the galaxies' members, which is equivalent to the existence of a relation between anisotropy and the number of galaxies in a cluster. The search for connection between the galaxies' alignments and Bautz–Morgan (BM) morphological types of examined clusters showed a weak dependence. A statistically marginal relation between velocity dispersion and cluster richness was observed. In addition, it was found that the velocity dispersion decreases with BM type at almost 3σ level. These results show the dependence of alignments with respect to clusters' richness, which can be regarded as an environmental effect.

ON THE ORIGIN OF COOL CORE GALAXY CLUSTERS: COMPARING X-RAY OBSERVATIONS WITH NUMERICAL SIMULATIONS

To better constrain models of cool core galaxy cluster formation, we have used X-ray observations taken from the Chandra and ROSAT archives to examine the properties of cool core and non-cool core clusters, especially beyond the cluster cores. We produced X-ray images, surface brightness profiles, and hardness ratio maps of 30 nearby rich Abell clusters (17 cool cores and 13 non-cool cores). We show that the use of double beta-models with cool core surface brightness profiles and single beta-models for non-cool core profiles yield statistically significant differences in the slopes (i.e., beta values) of the outer surface brightness profiles, but similar cluster core radii, for the two types of clusters. Hardness ratio profiles as well as spectroscopically-fit temperatures suggest that non-cool core clusters are warmer than cool core clusters of comparable mass beyond the cluster cores. We compared the properties of these clusters with the results from analogously reduced simulations of 88 numerical clusters created by the AMR Enzo code. The simulated surface brightness profiles have steeper beta-model fits in the outer cluster regions for both cool cores and non-cool cores, suggesting additional ICM heating is required compared to observed cluster ICMs. Temperature and surface brightness profiles reveal that the simulated clusters are over-cooled in their cores. As in the observations, however, simulated hardness ratio and temperature profiles indicate that non-cool core clusters are warmer than cool core clusters of comparable mass far beyond the cluster cores. The general similarities between observations and simulations support a model described in Burns et al. 2008 suggesting that non-cool core clusters suffered early major mergers destroying nascent cool cores.

An attempt to test Ambartsumian's idea of galaxy origin. I Galaxy cluster shape

We discuss the shape of 377 rich Abell clusters obtained through the application of the FOCAS package to DSS. The covariance ellipse method is used for description of the cluster shape. We discuss the problem for two samples. The first one contains all galaxies in the considered magnitude range m3, m3 + 3. The second sample contains only the 20 brightest galaxies. The Kolmogorov-Smirnov test is applied to show that these samples are taken from different populations. Such difference can result due to the different epoch origins of brighter and fainter galaxies in clusters, in accordance with Ambartsumians idea of galaxy formation through violent energetic phenomena in the nuclei of galaxies.

Supernovae in Low‐Redshift Galaxy Clusters: The Type Ia Supernova Rate

Supernova (SN) rates are a potentially powerful diagnostic of star formation history (SFH), metal enrichment, and SN physics, particularly in galaxy clusters with their deep, metal-retaining potentials, and simple SFH. However, a low-redshift cluster SN rate has never been published. We derive the SN rate in galaxy clusters at 0.06<z<0.19, based on type Ia supernovae (SNe Ia) that were discovered by the Wise Observatory Optical Transient Survey. As described in a separate paper, a sample of 140 rich Abell clusters was monitored, in which six cluster SNe Ia were found and confirmed spectroscopically. Here, we determine the SN detection efficiencies of the individual survey images, and combine the efficiencies with the known spectral properties of SNe Ia to calculate the effective visibility time of the survey. The cluster stellar luminosities are measured from the Sloan Digital Sky Survey (SDSS) database in the griz SDSS bands. Uncertainties are estimated using Monte-Carlo simulations in which all input parameters are allowed to vary over their known distributions. We derive SN rates normalized by stellar luminosity, in SNU units (SNe per century per 10^10 L_sun) in five photometric bandpasses, of 0.36+/-(0.22,0.14)+/-0.02 (B), 0.351+/-(0.210,0.139)+/-0.020 (g), 0.288+/-(0.172,0.114)+/-0.018 (r), 0.229+/-(0.137,0.091)+/-0.014 (i), 0.186+/-(0.111,0.074)+/-0.010 (z), where the quoted errors are statistical and systematic, respectively. The SN rate per stellar mass unit, derived using a color-luminosity-mass relation, is 0.098+/-(0.059,0.039)+/-0.009 SNe (century 10^10 M_sun)^-1. The low cluster SN rates we find are similar to, and consistent with, the SN Ia rate in local elliptical galaxies.

The radio luminosity function of the NEP distant cluster radio galaxies

A complete sample of 18 X-ray selected clusters of galaxies belonging to the ROSAT North Ecliptic Pole (NEP) survey has been observed with the Very Large Array at 1.4 GHz in B configuration. These are the most distant clusters in the X-ray survey with redshift in the range . Seventy-nine radio sources are detected within half an Abell radius with an observed peak brightness 0.17 mJy beam-1 , except for three sources, belonging to the same cluster, which have a higher peak brightness limit of 0.26 mJy beam-1 . The NEP field source counts are in good agreement with the source counts of a comparison survey, the VLA-VIRMOS deep field survey, indicating that the NEP sample is statistically complete. Thirty-two out of the 79 sources are within 0.2 Abell radii, twenty-two of them are considered cluster members based on spectroscopic redshifts or their optical magnitude and morphological classification. The cluster radio galaxies are used to construct the Radio Luminosity Function (RLF) of distant X-ray selected clusters. A comparison with two nearby cluster RLFs shows that the NEP RLF lies above the local ones, has a steeper slope at low radio powers (≤ 1024 W Hz-1 ) and shows no evidence for a break at W Hz-1 which is observed in the nearby cluster RLFs. We discuss briefly the origin and possible explanations of the differences observed in the radio properties of nearby and distant clusters of galaxies. The main result of this study is that the RLF of the distant X-ray clusters is very different from that of the local rich Abell clusters.

The Shapley super-cluster

The largest and the deepest super-structure known today is the Shapley super-cluster. This is the sky area with the highest over-density of galaxy clusters and therefore also an ideal region to test the effects of a high density environment on galaxies and on clusters. We performed an X-ray survey of a wide region surrounding the Shapley super-structure. Additionally to previously known super-cluster X-ray members, we identified diffuse X-ray emission from 35 cluster candidates without previous X-ray detection. 21 of them were previously known, optically selected super-cluster members, while the other candidates had not been previously detected in any wavelength range. Optical follow-up observations revealed that at least four of these new candidates also have optical cluster counterparts. The super-cluster shows a slightly flattened and elongated morphology. Clusters outside the central dense core are preferentially located in four perpendicular filaments in a similar way to what is seen in simulations of Large Scale Structure. We measure the cluster number density in the region to be more than one order of magnitude higher than the mean density of rich Abell clusters previously observed at similar Galactic latitudes; this over-density, in the super-cluster outskirts, is mainly due to an excess of low X-ray luminous clusters (with respect to an average population), which leads us to think that the whole region is still accreting low luminosity, small objects from the outskirts. Pushing our total X-ray mass estimate to fainter clusters would drastically increase the total super-cluster mass measure, because of the presence of the rich X-ray low luminosity population.

Transformations of galaxies in the environments of the cluster ABCG 209 atz$\mathsf{\sim}$ 0.2

Received; accepted Abstract. We analyse the properties of galaxy populations in the rich Abell cluster ABCG 209 at redshift z∼0.21, on the basis of spectral classification of 102 member galaxies. We take ad vantage of available structural parameters to study separa tely the properties of bulge-dominated and disk-dominated galaxies. The star formation histories of the cluster galaxy popula tions are investigated by using line strengths and the 4000 A break, through a comparison to stellar population synthesis models. The dynamical properties of different spectral classes are examined in order to infer the pas t merging history of ABCG 209. The cluster is characterized by the presence of two components: an old galaxy population, formed very early (z f ∼ 3.5), and a younger (z f ∼ 1.2) population of infalling galaxies. We find evidence of a m erger with an infalling group of galaxies occurred 3.5-4.5 Gyr ago. The correlation between the position of the young Hδ-strong galaxies and the X-ray flux shows that the hot intracluster medium triggered a starburst in this galaxy po pulation∼ 3 Gyr ago.

Spatial Distributions of A3558 in the Core Region of the Shapley Supercluster

The core region of the Shapley supercluster is dominated by three rich Abell clusters and two poor clusters. Since these member clusters are expected to be evolving rapidly in comparison to nonmember clusters because of the high merging rate, it is important to study the member clusters for understanding of the cluster evolution. Since the spatial distributions of gas temperature and metal abundance in each member cluster provide us with information on the interactions and motions of member clusters, they are useful for understanding their dynamics. From the results of eight ASCA pointing observations (total ~300 ks) of the core region, we obtained parameters of gas temperature, metal abundance, and X-ray luminosity for five member clusters and found that they are similar to the other field clusters not belonging to superclusters observed with ASCA. This result and the mean gravitational mass density of the core region indicate that the members are growing in the same way as the nonmember clusters, and the core of the supercluster is just on the way to contraction. Based on analyses of detailed spatial structures with a 4' × 4' scale, the two poor clusters show nearly isotropic temperature distributions, while the three Abell clusters are asymmetric. A3558 was analyzed with a 2' × 2' scale, owing to the statistical advantage, and it was revealed that A3558 has clear asymmetric distributions of gas temperature and X-ray surface brightness. This is thought to be caused by cluster-cluster mergings and/or group infallings. A metal-rich region with the size of ~320 h kpc was also found to the southeast, ~12' away from the cluster center of A3558. It is expected that either a remnant of a merged core has been left after a major merging or a group of galaxies has been recently infalling. Thus, the high dynamical activity of A3558 is proved.

Nature and environment of Very Luminous Galaxies

The most luminous galaxies in the blue passband have a larger correlation amplitude than L * galaxies. They do not appear to be preferentially located in rich clusters or groups, but a significant fraction of them seem to be in systems which include fainter members. We present an analysis of fields centered on 18 Very Luminous Galaxies ($M_B \le -21$) selected from the Southern Sky Redshift Survey 2, based on new observations and public data of the 2dF Galaxy Redshift Survey; we present also additional data on a CfA VLG and on Arp 127. We find that all the selected VLGs are physically associated with fainter companions. Moreover, there is a relation between the VLG morphology (early or late) and the dynamical properties of the system, which reflects the morphology–density relation. 6 out of the 18 SSRS2 VLGs are early type galaxies: 2 are in the center of rich Abell clusters with velocity dispersion $\sigma \sim 600$ km s -1 , and the other 4 are in poor clusters or groups with $\sigma \sim 300$. The VLG extracted from the CfA catalog is also an elliptical in a Zwicky cluster. The remaining 2/3 of the sample are late–type VLGs, generally found in poorer systems with a larger spread in velocity dispersion, from ~100 up to ~750 km s -1 . The low velocity dispersion, late–type VLG dominated systems appear to be analogous to our own Local Group. The possibile association of VLG systems with dark matter halos with mass comparable to rich groups or clusters, as suggested by the comparable correlation amplitude, would imply significant differences in the galaxy formation process. This work also shows that observing fields around VLGs represents an effective way of identifying galaxy systems which are not selected through other traditional techniques.

The nature and space density of fossil groups of galaxies

We describe the properties of a sample of galaxy groups with very unusual distributions of galaxy luminosities. The most extreme example has an X-ray luminosity similar to that of the Virgo cluster but has a very low richness, with only one galaxy brighter than L*, compared with six in Virgo. That one galaxy, however, is optically more luminous than any galaxy in Virgo and has an optical luminosity as bright as many of the central cD galaxies in rich Abell clusters. The characteristic feature of the fossil groups we study is that most of the light arises from one dominant, central galaxy. We define a fossil system and, based on this definition, construct a small X-ray-selected, flux-limited sample of fossil groups with well-known selection criteria. We confirm that these systems are indeed groups of galaxies, but dominated by one central luminous giant elliptical galaxy and with few, or no, L* galaxies. We find that fossil systems represent 8–20 per cent of all systems of the same X-ray luminosity. Fossil groups are at least as numerous as all poor and rich clusters combined, and are thus a possible site for the formation of luminous central cluster galaxies before infall into clusters occurs. The fossil systems in our sample have significantly higher X-ray luminosities than normal groups of similar total optical luminosities (or similar X-ray temperature, where the latter can be measured). These enhanced X-ray luminosities may be due to relatively cool gas in the innermost regions or due to a low central gas entropy. We interpret fossil groups as old, undisturbed systems which have avoided infall into clusters, but where galaxy merging of most of the L* galaxies has occurred. An early formation epoch, before that of most groups, could explain low central gas entropies and high X-ray luminosities.

Sensitive Radio and Optical Observations of z ∼ 0.2 Rich Abell Clusters

We present optical observations and radio continuum imaging data for a sample of rich, X-ray–bright Abell clusters at intermediate (z ~ 0.2) redshifts. We find that the radio galaxy population varies substantially from cluster to cluster within this homogeneous sample. The spatial distribution of the high-luminosity radio galaxies (HLRGs; L1.4 > 1023 W Hz-1) is very different from the low-luminosity radio galaxies (LLRGs; L1.4 ≤ 1022.75 W Hz-1), with the LLRGs displaying a flat spatial distribution in contrast to the centrally peaked HLRGs. A color-morphology classification shows that the HLRGs are composed primarily of galaxies with old stellar populations, whereas the LLRGs have a much more diverse composition. We do not see a correlation between the cluster radio fraction and cluster blue fraction. However, there is a moderate anticorrelation with richness, suggesting that a rich cluster is less likely to have radio-bright galaxies, whether the radio emission is due to active galactic nuclei or star formation.

Radio‐selected Galaxies in Very Rich Clusters at z ≤ 0.25. I. Multiwavelength Observations and Data Reduction Techniques

Radio observations were used to detect the "active" galaxy population within rich clusters of galaxies in a nonbiased manner that is not plagued by dust extinction or the K-correction. We present wide-field radio, optical (imaging and spectroscopy), and ROSAT All-Sky Survey (RASS) X-ray data for a sample of 30 very rich Abell (R ≥ 2) clusters with z ≤ 0.25. The VLA radio data samples the ultrafaint radio (L1.4 ≥ 2 × 1022 W Hz-1) galaxy population within these extremely rich clusters for galaxies with MR ≤ -21. This is the largest sample of low-luminosity 20 cm radio galaxies within rich Abell clusters collected to date.

The MX Northern Abell Cluster Survey II: The Abell/ACO Cluster Redshifts and Spatial Analyses

The MX Northern Abell Cluster Survey II is the final stage of a program designed to observe rich Abell clusters with m10 ≤ 17.0. We present 1542 new galaxy redshifts within 117 R ≥ 1 Abell cluster fields that have 16.9 ≤ m10 ≤ 17.0, 0h ≤ α ≤ 24h, -17° ≤ δ ≤ 90°, and |b| ≥ 30°. Of the 117 clusters observed for the MX Survey II, 105 new cluster redshifts were obtained with an average of eight cluster member galaxy redshifts. We update the m10-z estimator for Abell clusters and calculate the spatial number density of Abell clusters to be c = (8.25 ± 1.9) × 10-6 h3 Mpc-3 within z = 0.10. All R ≥ 1 Abell and ACO clusters with measured redshifts were combined to calculate the two-point spatial correlation function ξ(r) = (r/ro)γ for the largest complete sample of rich clusters to date. We find 18.6 ≤ ro ≤ 21.0 h-1 Mpc and -2.03 ≤ γ ≤ -1.95. The slope and correlation length are very robust and remain unaffected by the exclusion of compact and dense superclusters or by accounting for line-of-sight anisotropies, which are explained by the existence of three observationally confirmed and extended superclusters.