Galaxy surveys 327Figure 11.4.Measurement of the LF at different redshifts from the CFRS survey (Lilly
et al1995). The redshift bin and number of objects for each LF are given in the label
in each panel. The dividing line between ‘red’ and ‘blue’ samples corresponds to the
rest-frame colour of an Sbc galaxy. A clear evolution is visible in the blue sample, whereas
no significant evolution is observed in the red sample out toz 0 .7.
any of these models. It is believed that most of the morphological evolution
of these irregular and peculiar galaxies occurs at 1. z. 2, as a result
of interactions or merging, to lead to the assembling of the familiar Hubble
sequence. In general, fairly complex luminosity evolution models, which also
include a prescription for dust obscuration, fail to predict number counts at the
faintest magnitudes or the number density of galaxies atz&2. This is a clear
indication that a much deeper physical understanding of the galaxy formation
processes is needed (‘active’ versus simple passive evolution). Central, unsolved
key questions are how the star formation activity is modulated by merging and