SURFACE BRIGHTNESS BIAS IN GALAXY CATALOGS - DISTANCE EFFECTS

We have investigated the apparent variation of the surface brightness distribution of disc galaxies with distance within three different sam ples 1) a diameter limited sample of ESO catalogue galaxies in the dir ection of the cluster A3574 in Centaurus, 2) all ESO catalogue disc ga laxies with redshifts, and 3) a sample of fainter galaxies from our su rveys of the Fornax Cluster area. In each case we find, as predicted f or a sample dominated by surface brightness selection effects, that th e distribution narrows with distance. Both high and low surface bright ness galaxies are under represented in galaxy catalogues. Not because they are rare, but because the volume over which they are sampled is c onsiderably smaller than that of their normal surface brightness count erparts. The question of how many galaxies there are in the Universe r emains un-answered. In addition, since selection is by apparent surfac e brightness, the most distant sample (where cosmological dimming beco mes important) contains galaxies of higher intrinsic surface brightnes s than do the nearby samples, again confirming a previous theoretical prediction. The galaxies we observe in the distant Universe are very d ifferent to those we observe close by because of observational selecti on.