Galaxy statistics

Having studied some properties of spiral and elliptical galaxies in detail, we now take a step back to examine the statistics of the different types. The two degree-field galaxy redshift survey (2dF), and the Sloan Digital Sky survey (SDSS), are two very recent galaxy redshift surveys. In a galaxy redshift survey, one first identifies galaxies in images of the sky, and then takes a spectrum for each of them. Because the Universe is expanding, spectral lines of stars in a distant galaxy are shifted from the laboratory toward longer wavelengths: they are `redshifted'. There is a relation between the distance to the galaxy and its redshift, and so a galaxy redshift survey provides us with the 3D position of a set of galaxies, about $2\times 10^5$ for the 2dF, about $10^6$ for the SDSS. So what kind of general trends do they uncover? We will discuss some of these here.

The distribution of galaxies probably traces to some extent the distribution of dark matter in the Universe. For example, we've seen that there is evidence for a lot of dark matter in the Universe as a whole (from the cluster argument), but also in individual galaxies, groups and clusters. And so, since we find that galaxies are distributed very inhomogeneous in groups and clusters, presumably also the dark matter is distributed in an inhomogeneous fashion. How? And what process determines this in the first place?

We have a good idea of how the dark matter is distributed on a cosmological scale, from some general considerations, backed-up by observations of the micro-wave background (i.e. the sea of photons left-over from when the Universe was still hot). Given this input, we can use numerical simulations to predict how dark matter is clustered on large scales. The nice part of the story is that this theory is highly predictive, and does predict structures in the galaxy distribution very similar to what we observe: a filamentary pattern delineating large regions of space devoid of galaxies, called ` voids', with regions of high galaxy density, clusters of galaxies, at the intersection of the filaments.