Bumpy Power Spectra and Galaxy Clusters

The evolution of the abundance of galaxy clusters is not a reliable measure of Omega if there are features on scales of a few Mpc in the primordial power spectrum. Conversely, if we know the cosmological model parameters from other measurements, the cluster abundance evolution permits us to probe features in the power spectrum that are in the nonlinear regime at the present epoch, and hence difficult to discern directly from current epoch measurements. We have investigated the influence of an artificially introduced Gaussian feature on an otherwise unperturbed SCDM power spectrum. Using these modified spectra as an input to cosmological N-body simulations, we are able to show that in terms of the cluster abundance evolution, a SCDM model displays characteristics similar to an OCDM model. However, strong modifications would also be visible at a redshift z=0 in the dark matter power spectrum whereas minor alterations to the usual SCDM spectrum are washed away by non-linear evolution effects. We therefore conclude that features with characteristics such as discussed here might not be detectable using observations of the galaxy power spectrum, the local cluster abundance or the large-scale velocity field as measured by the velocity distribution of galaxy clusters. The only quantity that shows a pronounced difference at the present epoch between our models under investigation is the halo-halo correlation function which appears to be strongly biased with respect to an unmodified SCDM model. This is due to a lack of power on certain scales which subsequently modifies the relative amplitude of high-and low-k waves. Apart from observations of the evolution of cluster abundance, measurements of the Lyman-alpha forest at high redshift could put constraints on possible features in the power spectrum, too.