Hubble Deep Fever: A faint galaxy diagnosis
  The longstanding faint blue galaxy problem is gradually subsiding as a result of technological advancement, most notably from high-resolution Hubble Space Telescope imaging. In particular two categorical facts have recently been established, these are:   1) The excess faint blue galaxies are of irregular morphologies, and,   2) the majority of these irregulars occur at redshifts 1 < z < 2.   These conclusions are based on the powerful combination of morphological and photometric redshift data for all galaxies in the Hubble Deep Field to I < 26. Our interpretation is that the faint blue galaxy excess, which incidentally coincides with the peak in the observed mean galaxy star formation rate, represents the final formation epoch of the familiar spiral galaxy population. This conclusion is corroborated by the low abundance of normal spirals at z > 2. Taking these facts together we favour a scenario where the faint blue excess is primarily due to the formation epoch of spiral systems via merging at redshifts 1 < z < 2. The final interpretation now awaits refinements in our understanding of the local galaxy population ! 
S. P. Driver (UNSW)