Clustering and the Search for Dim and Dark Galaxies

We re-investigate the question of whether there is a significant population of Dim (Low Surface Brightness) and/or Dark Galaxies (DDGs). We argue that if they are clustered with bright ones then a physical resolution of ~10 kpc. will be needed to to distinguish their 21-cm. and QSO Absorption Line(QSOAL) counterparts from their brighter neighbours, leading to a real possibility of confusion. But until now such a resolution has not been available in this context. New Very Large Array (VLA) observations reveal that the identifications with bright optical objects claimed in previous single-dish blind HI surveys are often unreliable. For instance 14/36 of our high-resolution (5 arc sec, 13kpc.) sample have no optical counterparts in the Digital Sky Survey. This suggests that DDGs might be commonplace after all, and we go on to re-examine the main arguments that have been used against them.. We find that the QSOAL argument founders on the aforementioned clustering while deep CCD surveys have so far covered too small an area to set set strong constraints on Low Surface Brightness Galaxies (LSBGs). A cosmos filled with low surface brightness galaxies, dark galaxies and intergalactic gas clouds is therefore possible.