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Astron. Astrophys. 318, 405-415 (1997)

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4. Spatial distribution

Fig. 4-7 show in the form of [FORMULA] - and [FORMULA] -cone diagrams how our objects are distributed in space in comparison to the ZCAT galaxies (l.c.). Here we use again the ZCAT since its greater quantity of data allows better mapping of the voids (Fairall et al. 1991). The figures comprise galaxies up to 15000 km/s velocity distance. Of ours 105 are plotted in total, the rest lies beyond this limit. The dashed lines indicate the approximate boundaries of the voids considered.

4.1. Void VN2 (I, II, III)

In the region surveyed by us, three voids, nearly empty of ZCAT galaxies, with roughly 2500 km/s diameter are located behind each other at central velocities of about 3500, 7000 and 10000 km/s (Fig. 4a and b). The pattern of concentrations and filaments envelopping and separating these voids vanishes at least beyond 11000 km/s due to the growing incompleteness of the ZCAT sample. Our observations cross the first void (I) and touch the two others (II, III). None of our galaxies is situated in void I or in front of it. The majority of them belongs to the already known accumulations whereas about a dozen lie at the outskirts of the voids II and III. Only two galaxies are clearly in the void II: They appear in both cone diagrams projected into this void. But one has to notice that in particular void II is not well defined. Of the 74 galaxies observed by us in this region 30 are at distances beyond 15000 km/s.

4.2. Void VN8 (I, II)

The dominant feature in the region of VN8 (Fig. 5a and b) is the filament of the Perseus-Pisces supercluster at about 5000 km/s velocity distance. With the central velocity of 3000 km/s a clearly defined void (I) is located in front of it. Another less populated area of about 4000 km/s extension appears behind it between 6000 and 10000 km/s distance (II). Our observations cross both of these regions. Again none of our galaxies is to be found within void I. Apart from a few exceptions all are situated in the surrounding sheets or further away than 15000 km/s (26 of 55 observed ones within the marked cones of Fig. 5a and b). Only one object appears in both cone diagrams projected into VN8 II and can therefore be considered as a real void galaxy.

4.3. Void VN4

We limited our observations of this region to one field of 0.79 [FORMULA] size (Table 6, Paper I) and have measured the radial velocities of 23 galaxies. The interesting void is situated at about 3500 km/s (Fig. 6a and b). Behind it, between 6000 and 10000 km/s, one hits parts of the "Great Wall". In the [FORMULA] -diagram the Coma-cluster appears at [FORMULA] between 5000 and 9000 km/s. Ten of the galaxies found by us form a small cluster with [FORMULA] km/s. Their absolute magnitudes are [FORMULA]. Remarkably enough none of our objects is located at smaller distances. 13 of the 23 observed ones are at 15000 km/s or farther away. Again we did not detect one single new galaxy within the near void.

4.4. Region A2151

Figs. 7a and b show the region comprising the cluster A2151 at [FORMULA] km/s. This field was originally chosen to test our selection criteria in the environment of a rich cluster where luminosities down to [FORMULA] at least are to be expected. Furthermore, a rather extended void seems to be located in front of the cluster around 6500 km/s. With [FORMULA] of our sample we should be able to identify objects until [FORMULA] at the cluster distance. But besides one single galaxy in the foreground no one was found in the empty area. Of the 30 objects observed 23 belong to the A2151 cluster or the Hercules Supercluster. They have absolute magnitudes [FORMULA] which demonstrates that we did not quite reach the expected brightness limit. Six galaxies are beyond 15000 km/s.

4.5. Nearest neighbour analysis

The cone diagrams demonstrate the galaxy distribution by two-dimensional projections. Another way to characterize the spatial correlations is by calculating the distance [FORMULA] of a galaxy to its nearest neighbour (e.g. Salzer 1989). First, the separation between each of our galaxies and its nearest ZCAT neighbour was calculated taking into account as well such ones a little outside the observed cone. Next, [FORMULA] was determined for the ZCAT galaxies in the observed cone to ZCAT galaxies inside and outside the cone. We did this for the 4 regions observed by us.

Fig. 8 shows the result in the case of VN2. Here the velocity range considered was restricted from 4500 to 12000 km/s: Our sample contains no objects nearer than this lower limit while beyond the upper one the ZCAT selection is severely incomplete. The accumulations of the ZCAT galaxies are reflected by small [FORMULA] values down to 0.5 Mpc at 5500 and 11500 km/s. A clear separation of our objects from the others is indicated by values above 6 Mpc at 7000 and around 10500 km/s. These galaxies are situated in sparsely populated regions as Figs. 4a and b have already demonstrated. They could be considered as void galaxies but we should again point out that the borders of those depleted areas are not well defined.

[FIGURE] Fig. 8. Nearest neighbour distances of the 44 ZCAT galaxies brighter than 15.5 magn. (points) and of the 40 sample galaxies (symbols as in Fig. 3) in a [FORMULA] [FORMULA] field between 4500 and 12000 km/s in the region VN2.

The [FORMULA] diagrams of the VN8, VN4 and A2151 fields give no further insight in addition to what the cone diagrams are already demonstrating.

4.6. Discussion

The presented results are of a mixed nature. On the one hand, we found several objects near the edges of voids as in the cases of VN2 II and III as well as in VN8 II. Moreover, two galaxies are clearly situated in the midst of VN2 II and one galaxy in VN8 II. One has to take into account though that at velocity distances approaching 10000 km/s the boundaries of the voids are not very clear since the ZCAT sample is thinning out and the structures are defined by less and less galaxies of growing luminosity.

On the other hand, and this result we consider as the most remarkable one of our search, not a single hitherto unknown galaxy did show up within the nearby voids VN2 I, VN8 I and VN4. Situated at about 3000 km/s velocity distance these voids are rather well defined by the field of the surrounding ZCAT galaxies and more or less completely empty. Another interesting case is the apparently huge void located in front of A2151 at about 6000 km/s where we did not make a strike as well. In these places the density contrast between inside and outside the void boundaries appears to be considerable, but our findings do not allow to quantify this ratio.

Evidently the majority of our objects is similarly distributed as the ZCAT galaxies. In this respect our results differ not fundamentally from these of the surveys quoted in the introduction. This means, as other authors had already pointed out, too, that the empirical findings are not in favour of the biased galaxy formation scenario.

4.7. Summary

  • On deep direct images, taken in the direction of previously known nearby voids, as well as from POSS plates of the same regions we tried to select intrinsically faint galaxies for follow-up spectroscopy. More than 1000 candidates have been identified. The intention was to search for a suspected void-population.
  • Redshifts and other properties were finally measured of about 200 objects.
  • The percentage of luminous background-galaxies proved to be still rather high demonstrating the difficulty of the pre-selection procedure. More than 40 [FORMULA] of our sample galaxies have radial velocities above 15000 km/s.
  • The majority of the closer objects lies in areas where the already known galaxies are concentrated. Some are located within voids, but we did not find indications of a widespread void-population.
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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998
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