Number-redshift distribution

Much attention has been paid recently to an apparent excess of faint blue galaxies observed in photometric surveys. When the models of the B band number counts are normalized at B=16, the data show an excess over the luminosity evolution models of a factor of 2 at B=22. ([Tyson, 1988], [Lilly et al., 1991]) However, the K band number counts do not show this same excess, ([Gardner et al., 1993]). The shape of the number-redshifr distribution of surveys conducted at 20 < B < 22.5 by [Broadhurst et al., 1988] and [CoUess et al., 1990] are fitted by the no-evolution model. The median redshifts of the data from these surveys, and deeper data of [Cowie et al., 1991] and [AUington-Smith et al., 1992] show no evolution as faint as B=24. Proposed explanations for the high B band number counts include massive amounts of merging at intermediate redshifts (z 0.4) ([Broadhurst et al., 1992]) and an excess population of dwarf galaxies which appears at these redshifts, but has dissipated or faded by the present epoch. ([Cowie et al., 1991])... 

At B = 22, the excess over the models seen in the B band number counts is a factor of 2. The redshift distribution at this level, both in the B and the K bands shows no sign that an3rthing other than luminosity evolution is needed. The problem resides in the normalization of the number counts. 

Figure 17. (Reproduced from Ellison 2000). C IV column density distribution in Q1422+231 at (z) = 3.15 f(N) is the number of C IV systems per column density interval and per unit redshift path. The filled circles are the data the straight line shows the best fitting power-law slope a = 1.44, assuming the distribution to be of the form f(N)dN = BN adN. The open circles show the values corrected for incompleteness at the low column density end with these correction factors there is no indication of a turnover in the column density distribution down to the lowest values of N(C IV) reached up to now. Earlier indications of a turnover shown by the grey (Petitjean Bergeron 1994) and dashed (Songaila 1997) curves are now seen to be due to the less sensitive detection limits of those studies, rather than to a real paucity of weak Lya lines.

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