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using all available sources including some unpublished redshifts.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