New Structure In The Shapley Supercluster
M.J.Drinkwater, D.Proust, Q.A.Parker, H.Quintana, E.Slezak, PASA, 16 (2), in press.

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Results

Previous studies of the SSC have covered a very large region of sky, but we will limit our analysis in this paper to the region of sky we observed with FLAIR-II: the three UK Schmidt fields in Table 1. In some cases we will further restrict our analysis to the two Southern fields F382 and F383 where our observations were much more complete. The distribution of these fields and the galaxies we observed is shown in Fig. 1. We also show any previously observed galaxies and the known Abell Clusters.

**Figure 1:** Distribution of galaxies measured with FLAIR-II (squares) in the three UK Schmidt fields (large dashed squares) observed. The co-ordinate axes are for equinox B1950. Also shown are previously measured galaxies (dots) and known Abell clusters (labeled circles).
$\begin{figure} \begin{center} \vspace{-1.5cm} \hspace{1.0cm} \psfig{file=plot_sky1.eps,width=12.0cm}\end{center}\end{figure}$

We present the resulting distribution of galaxies towards the Shapley supercluster in Fig. 2 as cone diagrams and in Fig. 3 as the histogram of all velocities up to

$40000\hbox{$\rm\thinspace km s^{-1}$}$ . The importance of the SSC in this region of the sky is demonstrated by the fact the fully three quarters of the galaxies we measured belong to the SSC with velocities in the range 7580-18300

$\rm\thinspace km s^{-1}$ . In all the plots the new data are indicated by different symbols to emphasise their impact (this can also be seen by comparing these figures with the equivalent ones in Paper II). It can be seen that by probing large regions of the SSC away from the rich Abell clusters, we have revealed additional structure which we discuss in the following sections.

**Figure 2:** Cone diagrams of all known galaxy redshifts in the direction of the Shapley supercluster. Previously published galaxies are plotted as dots; the new measurements are plotted as open squares. The angular scale is enlarged by a factor of 3 for clarity. The distribution in Declination is repeated at the Right for comparison without the new measurements.
$\begin{figure} \begin{center} \vspace{-1cm} \hbox{ \hspace{-3.5cm} \psfig{file=p... ... \psfig{file=plot_dec0.eps,width=14.0cm}} \vspace{-1cm} \end{center}\end{figure}$

**Figure:** Histogram of galaxy redshifts in the direction of the Shapley supercluster in the region defined by the three Schmidt fields shown in Fig. 1 with a step size of 500
$\rm\thinspace km s^{-1}$ . The upper histogram is for all the data and the lower histogram just gives the previously published data.
$\begin{figure} \begin{center} \vspace{-2cm} \hspace{0.7cm} \psfig{file=plot_histo1.eps,width=14.0cm}\end{center}\end{figure}$

Foreground Galaxies

First in agreement with previous work we also note the presence of a foreground wall of 269 galaxies (Hydra-Centaurus region) at

$\overline{V}= 4242\hbox{$\rm\thinspace km s^{-1}$}$ with

$\sigma = 890\hbox{$\rm\thinspace km s^{-1}$}$ in the range

$2000-6000\hbox{$\rm\thinspace km s^{-1}$}$ . This distribution can be related with the nearby cluster A3627 associated with the ``Great Attractor'' (Kraan-Korteweg et al. 1996).

Clusters in the Shapley Supercluster

The previous observations reported in Papers I and II concentrated on the Abell clusters, clarifying the location of many of them. We reproduce a list of the main clusters in the SSC region in Table 2 for reference and plot their positions in Fig. 1. As noted above, our new measurements concentrate on galaxies outside the rich clusters in this field. In particular we observed virtually no galaxies in foreground or background clusters. We compare the distribution of the SSC galaxies to the Abell clusters in two velocity slices in Figs. 4 and 5.

In the near side of the SSC (

$7580<v<12700\hbox{$\rm\thinspace km s^{-1}$}$ : Fig. 4) we detected several new galaxies in the clusters A3571 and A3572. This region has a very extended velocity structure with several galaxies in the higher range (Fig. 5). At the velocity of the main part of the SSC (

$12700<v<18300\hbox{$\rm\thinspace km s^{-1}$}$ : Fig. 5) we have found additional galaxies in many of the clusters, especially the poorer ones like AS726, AS731 and A3564. The main conclusion however is that the clusters are seen as peaks in a sheet-like distribution rather than isolated objects.

**Figure:** Galaxies and clusters in the direction of the Shapley supercluster with velocities in the range
$7580<v<12700\hbox{$\rm\thinspace km s^{-1}$}$ (near side of the main supercluster). Previously published galaxies are plotted as dots and our new measurements are plotted as open squares. Abell clusters in this velocity range are plotted as labeled circles of radius 0.5 deg (approximately 1 Abell radius at this distance).
$\begin{figure} \begin{center} \vspace{-1.5cm} \hspace{1.0cm} \psfig{file=plot_skya.eps,width=12.0cm}\end{center}\end{figure}$

**Figure:** Galaxies and clusters in the direction of the Shapley supercluster with velocities in the range
$12700<v<18300\hbox{$\rm\thinspace km s^{-1}$}$ (main supercluster). Previously published galaxies are plotted as dots and our new measurements are plotted as open squares. Abell clusters in this velocity range are plotted as labeled circles of radius 0.5 deg (approximately 1 Abell radius at this distance).
$\begin{figure} \begin{center} \vspace{-1.5cm} \hspace{1.0cm} \psfig{file=plot_skyb.eps,width=12.0cm}\end{center}\end{figure}$

Structure of the Shapley Supercluster

The main impact of our new data is to revise our knowledge of the large-scale structure of the SSC by measuring a large number of galaxies away from the rich Abell clusters previously studied. The majority of the galaxies we observed were part of the SSC, so our principal result is to show that the SSC is bigger than previously thought with an additional 230 galaxies in the velocity range

$7580<v<18300\hbox{$\rm\thinspace km s^{-1}$}$ compared to 492 previously known in our survey area.

Looking at the cone diagrams (Fig. 2) and the velocity histogram in Fig. 3 our first new observation is that the SSC is clearly separated into two components in velocity space, the nearer one at

$\overline{v}=10800\hbox{$\rm\thinspace km s^{-1}$}$ (

$\sigma_v=1300\hbox{$\rm\thinspace km s^{-1}$}$ ) to the East of the main concentration at

$\overline{v}=14920\hbox{$\rm\thinspace km s^{-1}$}$ (

$\sigma_v=1100\hbox{$\rm\thinspace km s^{-1}$}$ ). The two regions contain 200 and 522 galaxies respectively. Some evidence for this separation was noted in the velocity distribution in Paper II, but it is much clearer with our new data.

Secondly, it can be see from the Declination cone diagram in Fig. 2 as well as the sky plots in Figs. 4 and 5 that the Southern part of the SSC consists of two large sheets of galaxies of which the previously measured Abell Clusters represent the peaks of maximum density.

To consider the significance of this extended distribution of galaxies it is helpful to define an inter-cluster sample consisting of galaxies in the Southern fields (F382 and F383) outside the known Abell clusters in the SSC velocity range. We eliminated all galaxies within a 0.5 degree radius (about 1 Abell radius) of all the clusters shown in Figs. 4 and 5. Very few of the previously-measured galaxies remain in the sample. In Fig. 6 we plot a histogram of the galaxy velocities in this inter-cluster sample compared to the predicted n(z) distribution of galaxies. The predicted distribution was based on the number counts of Metcalfe et al. (1991) normalised to the area of the Southern sample after removing clusters (44 deg²) and corrected for completeness (304 out of a possible 1194 galaxies measured in total). We also show the histogram (shaded) and predictions (dashed) for the previously-measured galaxies in the same field (128 out of a possible 1194). The histogram shows that even for the inter-cluster galaxies there is a large overdensity in the SSC region (7500<cz<18500

$\rm\thinspace km s^{-1}$ ): we measure 161 galaxies compared to 74 expected. This is an overdensity of $2.0\pm0.2$ detected at the 10 sigma level. This is averaged over the whole SSC velocity range; the overdensity in individual 1000

$\rm\thinspace km s^{-1}$ bins peaks at about 7. By comparison the previous data (42 galaxies, 33 expected) gave an overdensity of 1.3 detected at only 1.5 sigma. The overdensity for the whole SSC including the Abell clusters is, of course, much larger still.

**Figure:** Histogram of all galaxy redshifts in Southern inter-cluster region the Shapley supercluster with a bin size of 1000
$\rm\thinspace km s^{-1}$ . The upper histogram is for both our new data and the previously published data and the lower histogram just gives the old data. For each histogram a curve shows the predicted distribution allowing for the size of the region and the completeness of the samples based on a uniform galaxy distribution (Metcalfe et al. 1991).
$\begin{figure} \begin{center} \vspace{-2cm} \hspace{0.7cm} \psfig{file=plot_histo2.eps,width=14.0cm}\end{center}\end{figure}$

These new observations mean that we must modify the conclusions of Paper I about the overall shape of the SSC. In Paper I it was concluded from the velocity distribution of the clusters that the SSC was very elongated and either inclined towards us or rotating. The SSC extends as far as our measurements to the South, so we find it is not elongated or flattened. We now suggest that it is more complex still, being composed of the known Abell clusters embedded in two sheets of galaxies of much larger extent.

Next Section: Discussion
Title/Abstract Page: New Structure In The
Previous Section: Observations
Contents Page: Volume 16, Number 2

ASKAP

Public

Australia Telescope National Facility

Results

Foreground Galaxies

Clusters in the Shapley Supercluster

Structure of the Shapley Supercluster