Astron. Astrophys. 325, 881-892 (1997)

4. The properties of the void galaxies

A natural question that would arise from a study that has the goal to search for galaxies in voids is whether the galaxies found in the low density regions have special properties in comparison with the characteristics of the total sample. We did not find a real void population, but we could still comment on the properties of the isolated galaxies found in some voids. Table 3 gives the main parameters of the void galaxies from the surveyed region analysed in this paper. The galaxies are ordered with respect of increasing the radial velocity, because the isolation of the galaxies also increases with distance. Column (1) contains the name of the galaxies and Column (2) gives the radial velocity. In the following columns we listed the separation between our galaxies and their nearest bright ZCAT neighbour, (HS-ZCAT) (Column (3)); between our galaxies and their nearest neighbour, (HS-HS-ZCAT) (which can be also one galaxy from our sample or a bright ZCAT) (Column (4)) and between our galaxies and their nearest late-type bright ZCAT neighbour, (HS-ZCAT(late)) (Column (5)). For one galaxy, HS1226+3719, which is a galaxy with absorption (see discussion below), only the separation to its nearest bright ZCAT is given. Then we give the apparent B magnitudes (Column (6)), the absolute magnitude (Column (7)), the flux of the [OIII] 5007 line (Column (8)) and the EW of the same line (Column (9)). For the galaxies for which the [OIII] 5007 line is not detected, the fluxes and EW are set to 0. For one galaxy, HS1310+3801, the fluxes and EW are not available. This galaxy was already known in the literature and it was not observed by us, and therefore was also not included in the statistical analyse.

The separations of the void galaxies show that 50 have the nearest neighbour among themself. This would suggest that the void galaxies are not uniformly distributed but also have the tendency to form fainter structures inside the bigger voids. Unfortunately, the low number of our isolated galaxies cannot allow us to draw any definitive conclusion. Nevertheless, some fainter ZCAT galaxies have been found to associate with some of our void galaxies. For example the Arch is followed by four faint ZCAT galaxies, and from these, all are late types. In Table 4 we list the ZCAT galaxies we found in Voids 1 and 2, respectively (even though they do not come from the complete sample), together with their main parameters (velocity, B magnitude and T morphological type, when available). The Arch seems also to divide Void 2 in three smaller voids. Lindner et al. (1996) and Szomoru et al. (1996b) also suggested that the galaxies found in voids have the tendency to cluster. In addition, 75 of our void galaxies have the nearest ZCAT neighbour among the late type galaxies. This should not come as a surprise since the late type galaxies have the tendency to be less clustered than the early types, which would preferentially form the skeleton of the clusters.

The absolute magnitudes of our void galaxies range between which means that all of them are dwarfs. These galaxies are therefore intrinsically faint objects, quite different from the galaxies that were found in the Bootes void (Weistrop et al. 1995), which were mainly galaxies or brighter. Nevertheless, the distribution of absolute magnitudes of our parent sample starts to drop around and only a few galaxies are found in the range . If a faint void population had typical luminosities around and below, we would just start to detect it, since we are very incomplete at the faint end. The few galaxies we found in the voids could constitute the tip of the iceberg of the void population.

The spectroscopic properties of our void galaxies are very different and they do not belong to only one class of objects. The EW and fluxes encompass mostly the whole range of values from the parent sample. Some void galaxies have extremely large EW and fluxes of the [OIII] 5007 line whether others are barely detectable. In Figure 6 we give some example of void galaxy spectra. The plots show that the galaxies we found in voids have different degrees of ionization, from very high ionization objects with very faint continuum, close to the extreme case of Searle-Sargent objects, up to very low ionization galaxies and strong continuum which indicate an underlying older stellar population. Amazingly, we found also one object that has no detectable [OIII] 5007 line; the only emission-line being H . This object was selected mainly because of the blue continuum and was considered a second priority candidate (see Paper 1 for a detailed description of the selection procedure) and was not included in the statistical analyse. But the most unexpected void galaxy is HS1226+3719, an object that entered in our sample as a failure of our selection procedure, being a galaxy with absorption. On the other hand this galaxy is one of our best void candidates, with an isolation of Mpc, and lying in the centre of Void 1. (Figure 2 a). This result fact makes us wonder whether a population of dwarf elliptical galaxies would not be in fact the hidden void population which would recover the biasing theories. But it is known (Binggeli 1989) that the dwarf ellipticals are the most clustered galaxies in the Universe, which populate mainly the clusters. This still do not exclude the possibility that the voids would be occupied by a population of dwarf red galaxies similar to the dwarf ellipticals we see now in clusters.

Fig. 6. Spectra of void galaxies. The y-axes contain the fluxes per unit of wavelength, (erg sec ) while the x-axes contain the wavelength, (Å).

© European Southern Observatory (ESO) 1997

Online publication: March 26, 1998

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