We live in an expanding universe, therfore for structures to form, the gravitational collapse of the object must overcome the expansion. Thus the evolution of structure in the universe depends upon the rate of expansion of the universe, the average density of the universe and the density of the object in question. These are parameterized by Hubbles constant, the density parameter and the overdensity of the structure. By examining how the structures we observe in the universe are evolving we can probe the cosmology of the universe, determining its eventual fate. Clusters of galaxies are the most massive, gravitationally bound objects in the universe. According to formation theories, this means they will be the most recent objects to have formed and so the evolution of massive clusters of galaxies will be the most apparent and the most sensitive to the underlying cosmology.
To measure the evolution of clusters of galaxies we would ideally want to know the mass of all the clusters, and measure how the number density of clusters varies with mass, and how this function changes with time.
The mass of clusters of galaxies is not trivial to measure. However the X-ray luminosity is relatively easy to determine, and it is expected to be positively correlated with mass. Thereby, if we measure the number density of the clusters of galaxies as a function of luminosity we will have an idea of the mass distribution of clusters. If we observe this at different epochs by measuring the X-ray luminosity function at different redshifts then we can measure how clusters are evolving. This will have implications for the cosmology of the universe.
I have reanalysed the XLF of the Einstein Extended Medium Sensitivity Survey using ROSAT PSPC data. It was found that the new XLF is consistent with the local XLF. This result, along with a comparison of the expected number of clusters predicted from local XLFs, indicate that that the space density of luminous massive clusters has either not evolved or has increased by a small factor ~2 since z=0.4. This implies that the universe is open.
| The EMSS XLF for clusters with 0.3 < z < 0.6 as recalculated using ROSAT PSPC data. The curve is the best fitting Schechter function to the local XLF from Boehringer et al. (2001). The lack of any strong evolution implies a low density for the universe. |
