Clustering and the Search for Dim and Dark Galaxies

TL;DR

This study uses high-resolution VLA observations to challenge previous claims about the rarity of Dim and Dark Galaxies, suggesting they may be more common than previously thought due to clustering effects and observational limitations.

Contribution

It demonstrates that previous optical identifications of HI sources are often unreliable and re-evaluates the evidence against the existence of numerous Dim and Dark Galaxies.

Findings

Many HI sources lack optical counterparts in high-resolution images.

Clustering complicates the identification of Dim and Dark Galaxies.

Deep surveys have not yet covered enough area to rule out their abundance.

Abstract

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.