The Abell-ACO catalogue includes 2712 northern clusters originally published by Abell (1958), 1364 rich southern clusters that are counterparts to the Abell clusters and 1174 supplementary poor southern clusters (Abell et al. 1989). Some rich clusters are duplications, therefore the combined Abell-ACO catalogue includes at most 4069 rich clusters. In this paper we use only these rich clusters of the Abell-ACO catalogue and call them simply as clusters.
We are updating redshift data for Abell-ACO clusters continuously using all available sources including some unpublished redshifts. The present discussion reflects our dataset as of May 1995. A catalogue of published redshifts and velocity dispersions for Abell-ACO clusters, including supplementary clusters, is in preparation (cf. Andernach et al. 1995). For clusters without observed redshift a photometric estimate of the distance is given using the correlation between redshifts and magnitudes of cluster galaxies (Peacock & West 1992). The errors of estimated redshifts are about 27% for the northern (Abell) and 18% for the southern (ACO) clusters which are considerably higher than errors for spectroscopically measured redshifts. The redshifts have been corrected to the rest frame of the Local Group () and for the expansion effects. The expansion correction depends on the adopted model and density parameter of the universe. We have used a correction which corresponds to a closed universe () and a value of the density parameter, . Results depend on the particular value of the density parameter only very weakly.
For a number of clusters published redshifts obviously belong to a foreground or background galaxy (some of them are marked by ACO and Struble & Rood 1991, and also by Dalton et al. 1994). We have used estimated redshifts instead of poorly observed ones if and if the number of measured galaxy redshifts per clusters was . The influence of such clusters on our catalogue will be discussed later.
To compile the supercluster catalogue we extracted from the whole Abell-ACO catalogue a spatially limited sample up to a distance z = 0.12. This sample contains 1304 clusters, and includes clusters of all richness classes. Of these clusters 2/3 have measured redshifts. We have included in our study clusters of richness class 0. Arguments for this were already discussed by EETDA. Possible projection effects discussed by Sutherland (1988), Dekel et al. (1989) and others are not crucial for the present study as we are mostly interested in the distribution of clusters on much larger scales (cf. EETDA).
Superclusters have been determined by the clustering (or friends-of-friends) algorithm (Huchra & Geller 1982; Press & Davis 1982; Zeldovich et al. 1982). Clusters are searched for neighbours at a fixed neighbourhood radius; objects having distances between each other less than this radius are collected to a system. We use the same neighbourhood radius as in EETDA, 24 h Mpc. EETDA showed that at neighbourhood radii up to about 16 h Mpc the cores of individual superclusters start to form; at radii larger than 30 h Mpc superclusters begin to join into percolating agglomerates. At the radius of about 24 h Mpc\ superclusters are the largest still relatively isolated density enhancements in the Universe. Our analysis shows that the main results do not change if we use the neighbourhood radius in the interval of 20 - 28 h Mpc.
In some cases the clustering radius used here is too large, and forces clusters to join into large aggregates which probably cannot be considered as single superclusters. One example for this is the Shapley supercluster that will be discussed by Jaaniste et al. (1997).
We include in the catalogue of superclusters all systems with at least two member clusters. We shall use the term multiplicity k for the number of member clusters in a supercluster. The distance limit is set at z = 0.12; in this volume there are in total 220 superclusters (for the neighbourhood radius 24 h Mpc). The distribution of multiplicities of the superclusters in our catalogue is shown in Fig. 1 (click here). Here we plot also isolated clusters. Complete data on superclusters having at least four members (multiplicity, centre coordinates, list of member clusters and identifications with previous catalogues) are given in Table A1 in the Appendix, the whole catalogue is presented in electronic form in Table A2. Clusters for which only estimated redshifts are available are appended by a letter e.
A number of superclusters have well-known previous identifications. These are given in Col. (7) of Table A1. Their designations are usually based on the constellation on which the supercluster members are projected. In the case of rich, well-determined superclusters without previous identifications we assigned new identifications using the same system. If there were more than one supercluster projected on the same constellation, we added the letters A, B, and so on (in order of increasing z). Otherwise, if the supercluster members were projected on more than one constellation, we used a double name.
About 1/3 of the clusters in our sample have estimated redshifts only (437 of 1304 clusters). The median distance of clusters with measured redshifts (230 h Mpc) is smaller than that of clusters with estimated redshifts (300 h Mpc), which reflects the better completeness in redshift measurements for nearer clusters.
In order to see the influence of the use of clusters with estimated redshifts on our catalogue, we performed the cluster analysis using only clusters with measured redshifts. We searched for systems using the same neighbourhood radius as before, 24 h Mpc. As a result we obtained a test catalogue of superclusters with 136 systems. All the superclusters containing less than two members with measured redshifts disappeared, of course, after this procedure. However, the remaining superclusters appeared to be surprisingly stable: almost all systems with at least two clusters with measured redshifts were found also in this test catalogue, and only a few clusters with measured redshifts were excluded from systems. One supercluster, the Aquarius supercluster (SCL 205), was split up into two subsystems.
Thus we consider all the superclusters with less than two members with measured redshifts as supercluster candidates. These superclusters have a letter c to its catalogue number. We also marked those clusters with measured redshifts that were eliminated from systems determined by clusters with measured redshifts only, as described above.
Figure 1: The distribution of supercluster multiplicities for the neighbourhood radius R = 24 h Mpc. Isolated clusters (k = 1) are included for comparison
Of the 220 systems in the new catalogue, 50 superclusters are identical with superclusters in the previous catalogue, 80 have changed the multiplicity (in most cases these superclusters have gained or lost 1 - 2 members due to newly measured redshifts). The catalogue contains 25 previously unreported superclusters within the distance of d < 300 h Mpc; all 65 superclusters beyond 300 h Mpc are reported here for the first time. As seen from these numbers, our regular updating of the catalog has lead to a considerable improvement. In addition, our analysis showed that the large scale structures delineated by superclusters from the present and previous catalogues are almost identical in the nearby volume covered by both catalogues.
Figure 2: Mean number of galaxies in clusters belonging to superclusters of multiplicity k
We divide superclusters into several richness classes. We call superclusters with less than 4 members as poor, and those with 4 or more members as rich. Rich superclusters are divided into subclasses: superclusters with 4 - 7 members are called as medium rich, and those with 8 or more members as very rich. About half of the 220 superclusters of the catalogue are cluster pairs; the catalogue contains 53 medium rich superclusters, and 25 very rich superclusters. Very rich superclusters represent the regions of the highest density in the Universe. They contain 25% of all clusters and over 30% of all supercluster members. Of these very rich superclusters 4 have been catalogued for the first time. These are the Draco (SCL 114, k = 16), the Caelum (SCL , k = 11), the Bootes A (SCL 150, k = 10), and the Leo - Virgo (SCL 107, k = 8) superclusters. In the following sections we shall compare the spatial distribution of superclusters of different richness.
Figure 3: Selection functions for clusters. In the upper panel the density of clusters is shown as a function of the galactic latitude, , in the lower panel as a function of distance from the observer, r. Solid histograms are for all clusters, dashed histograms for clusters in very rich superclusters. Straight lines show the linear approximation of the selection function. The curves are normalised to 1 for the galactic poles and for zero distance to the observer
Supercluster masses are evidently larger when they contain more galaxies. To check the relationship between the supercluster richness and the number of galaxies contained in a supercluster we plot in Fig. 2 (click here) the mean number of galaxies in superclusters against supercluster multiplicity. We used the Abell count of galaxies (C in ACO) as the number of galaxies per cluster. Clearly, the mean number of galaxies in clusters located in superclusters of different multiplicity is practically constant. This test shows that the supercluster multiplicity is an indicator of the mass of the supercluster (see also Frisch et al. 1995). An example supported by actual observations is the Shapley supercluster, the richest supercluster in our catalogue. It contains the richest clusters in the volume under study and a large number of X-ray emitting clusters which indicate the presence of a deep potential well in this supercluster (Breen et al. 1994; EETDA).
First we give some notes on previously known superclusters.
The Shapley supercluster (SCL 124), first described by Shapley in (1930), is certainly the most prominent supercluster in the region under study (ZZSV). This supercluster contains the richest Abell clusters in the area studied, and a number of X-ray clusters (Quintana et al. 1995 and references therein). This supercluster is located approximately 140 h Mpc from us, bordering the farther side of the Northern Local void (EETDA, Lindner et al. 1995).
The Virgo-Coma supercluster (SCL 111) with 16 members forms a wall between two voids. Of these 16 clusters 6 have estimated redshifts about 1.5 times larger than are their (poorly) observed redshifts. Thus a possible alternative interpretation of the data is that some of the clusters are more distant, and the measured redshifts belong to foreground galaxies in the region of this supercluster. If we discard these clusters then the supercluster contains at least 8 members and still meets our criterion for very rich superclusters.
The Horologium-Reticulum supercluster (SCL 48), the longest and the second richest supercluster in the previous catalogue, has been split into subsystems containing now 26 members instead of 32 (EETDA) (see Table 1 (click here)), being still the second most rich supercluster in the new catalogue but not the longest one (Jaaniste et al. 1997).
Now we comment on those very rich superclusters () in our catalogue which were not previously reported.
The Draco supercluster (SCL 114) has 16 members, all with measured redshifts, being one of the richest superclusters in the region under study. The Draco supercluster lies at a distance of 300 h Mpc on a side of a void of diameter of about 130 h Mpc, the near side of which is determined by the Ursa Majoris supercluster. The Draco supercluster is one of the most isolated very rich superclusters in our catalog. However, being located near the distance limit of our sample this supercluster might have a neighbour farther away. The shape of this supercluster resembles a pancake with axis ratios 1:4:5 (Jaaniste et al. 1997).
The Bootes A supercluster (SCL 150) borders a giant void on the farther side of the Bootes supercluster which separates this void from the Bootes void. Nine of the ten members of this supercluster have measured redshifts.
The Leo-Virgo supercluster (SCL 107) has 8 members, six of them have measured redshifts. This supercluster borders the same void as SCL 111.
The Caelum supercluster candidate (SCL 59c) borders the same void as the Fornax-Eridanus supercluster and is seen in Fig. 3 (click here) by Tully et al. (1992) as a density enhancement. However, a word of caution is needed: only two of the 11 members of this supercluster have measured redshifts.
The Fornax-Eridanus supercluster candidate (SCL 53c) too consists mostly of clusters with estimated redshifts. The multiplicity of this supercluster may change when new redshift data for rich clusters in this region become available.
Figure 4: The distribution of clusters in supergalactic coordinates in slices of thickness d = 100 h Mpc in the supergalactic X direction. Clusters belonging to very rich superclusters are denoted with filled circles; clusters, belonging to medium rich superclusters - with empty circles, and isolated clusters and members of poor superclusters are plotted with small dots. The first and last slices are thicker since due to the use of the spherical volume outlying slices contain less clusters
Figure 5: The distribution of clusters belonging to very rich superclusters in supergalactic coordinates. Supercluster identifications are given