6.2. The Space Density of Galaxies

In the preceding discussion we have established that there is no strong evidence for a significant amount of hydrogen or helium distributed in a diffuse background. Similarly, there does not seem to be solid evidence for a decaying particle background. Hence we are driven to the conclusion that other than the QSO absorption line systems, it would seem that most all of the baryons in the Universe are confined to galactic potentials. There is no significant population of intergalactic baryons. If this is indeed the case, then our baryon census is equivalent to determining the luminosity function of galaxies. Recall however, that the luminous component of easily visible galaxies contributes only 0.5% to the closure density. This is well below the limits on _b derived from primordial nucleosynthesis and suggests that many of the baryons in galactic potential wells are "dark" or there may exist "dark" galaxies that have not yet been properly accounted for in the census of nearby galaxies. Hence, we really have a "missing" baryon problem to deal with.

There are basically two approaches to determining the space density of galaxies. The first is to construct the galaxy luminosity function (GLF) from what is considered to be a fair sample of redshifts. The determination of the GLF is one of the fundamental cosmological observations that can be made. With the GLF one can estimate the mean luminosity density of the Universe and the mean M / L ratio of galaxies. This in turn provides a direct indication of the contribution of non-luminous matter in galaxies to . The shape of the luminosity function also can provide an important constraint for structure formation theories. As a consequence of its fundamental nature, the GLF has been worked on by many, many groups. Until recently, most determinations of the GLF have been similar but recent advances in galaxy sampling and redshift survey data suggest that a proper determination of the GLF still remains.