Faint blue galaxy

A faint blue galaxy is a distant, irregularly shaped galaxy in which star formation occurs at a high rate. From the late 1970s it became apparent in deep galaxy surveys that a population of these blue galaxies existed at vast distances.

Later investigation confirmed that going back in cosmological time suddenly a population of small, blue galaxies appears. The exact redshift at which this happens is still uncertain.

These systems are small and far away. Little is known about their structure. However, they form stars very efficiently, hence the blue colours. One possibility is that these galaxies contain Population III stars.

Two explanations have been put forward:

#Star formation in these systems may be a very transient phenomenon. They light up and fade very quickly. This explanation is problematic due to the remnants which have not been found in the local universe. The remnants should be old, red systems. The low surface brightness galaxies do not qualify as they still have lots of gas and are fairly blue themselves. It is unknown what could turn off efficient star formation suddenly. The remnants may still be discovered since most local surveys are done at fairly blue colours.
#These small galaxies are consumed by merger events and become part of larger galaxies. This is difficult to prove as the signs in the final galaxies would be hard to recognize. Support comes from the hierarchical structure formation picture as this predicts merging to happen far more frequently in the early universe than now.

The problem here is the sheer number of faint blue galaxies and their large separation.