Galaxy number counts: a blend of evolutive stages of the Universe
Galaxy number counts (GNCs) is one of the most immediate, measurable quantities in Astrophysics. They are defined as the number of galaxies per magnitude interval and square degree in the sky, as a function of the apparent magnitude in a certain photometric band. Every model of galaxy formation and evolution should be able to reproduce the observed GNCs in all the photometric bands.
GNCs is one of the simplest cosmological quatities that a observer can measure. Originally considered by Hubble (1926) as a cosmological test, we know at the present that they are more sensitive to the galactic evolution than to the cosmological model, at least until intermediate-to-faint magnitudes (B>~24). At each magnitude, GNCs are a mixture of galaxies, exhibiting the same apparent magnitude, but that are located at very different redshifts, and at different evolutive stages (see the figure below).
This makes the interpretation of the observational GNCs in several photometric bands not easy, as it is a global parameter that involves galactic populations of different nature, in different evolutive stages, and at different redshifts. We are developing global models of galaxy formation and evolution, trying to reproduce the observed GNCs from blue to nIR bands, as well as other observable quantities.
You can find a summary of published galaxy counts data in several bands at the GNCs webpage of the Extragalactic Astronomy and Cosmology Research Group of the Durham University.
Figure 1: Explanatory schemme showing the mixture of galactic populations that shapes the GNCs. Suppose that the galactic sample shown in the top panel is the whole galactic population which is detected in a deep image, ordered according to the apparent magnitude (x-axis) and redshift (y-axis). If we count the number of galaxies in each apparent magnitude interval and normalize this number to the observed sky area and size of the magnitude interval, we will obtain the differential GNCs in that magnitude. As you can see in the schemme, at each magnitude we are going to detect a different mixture of galaxies, of different morphological types, at different redshifts, and at different evolutive stages. The top image is taken from Driver et al. (1998, ApJ, 496, L93).