The Cluster Abundance in Flat and Open Cosmologies

We use the galaxy cluster X-ray temperature distribution function to constrain the amplitude of the power spectrum of density inhomogeneities on the scale corresponding to clusters. We carry out the analysis for critical density universes, for low density universes with a cosmological constant included to restore spatial flatness and for genuinely open universes. That clusters with the same present temperature but different formation times have different virial masses is included. We model cluster mergers using two completely different approaches, and show that the final results from each are extremely similar. We give careful consideration to the uncertainties involved, carrying out a Monte Carlo analysis to determine the cumulative errors. For critical density our result agrees with previous papers, but we believe the result carries a larger uncertainty. For low density universes, either flat or open, the required amplitude of the power spectrum increases as the density is decreased. If all the dark matter is taken to be cold, then the cluster abundance constraint remains compatible with both galaxy correlation data and the {\it COBE} measurement of microwave background anisotropies for any reasonable density.Comment: Uuencoded package containing LaTeX file (uses mn.sty) plus 7 postscript figures incorporated using epsf. Total length 10 pages. Final version, to appear MNRAS. COBE comparison changed to 4yr data. No change to results or conclusion

Early Enrichment of the Intergalactic Medium and its Feedback on Galaxy Formation

Supernova-driven outflows from early galaxies may have had a large impact on the kinetic and chemical structure of the intergalactic medium (IGM). We use three-dimensional Monte Carlo cosmological realizations of a simple linear peaks model to track the time evolution of such metal-enriched outflows and their feedback on galaxy formation. We find that at most 30% of the IGM by volume is enriched to values above 10^-3 solar in models that only include objects that cool by atomic transitions. The majority of enrichment occurs relatively early (5 < z < 12) and resulting in a mass-averaged cosmological metallicity between 10^-3 and 10^-1.5 solar. The inclusion of Population III objects that cool through H2 line emission has only a minor impact on these results: increasing the mean metallicity and filling factor by at most a factor of 1.4, and moving the dawn of the enrichment epoch to a redshift of approximately 14 at the earliest. Thus enrichment by outflowing galaxies is likely to have been incomplete and inhomogeneous, biased to the areas near the starbursting galaxies themselves. Models with a 10% star formation efficiency can satisfactorily reproduce the nearly constant (2 < z < 5, Z approximately 3.5 x 10^-4 solar) metallicity of the low column density Ly-alpha forest derived by Songaila (2001), an effect of the decreasing efficiency of metal loss from larger galaxies. Finally, we show that IGM enrichment is intimately tied to the ram-pressure stripping of baryons from neighboring perturbations. This results in the suppression of at least 20% of the dwarf galaxies in the mass range 10^8.5 to 10^9.5 solar, in all models with filling factors greater than 2%, and an overall suppression of approximately 50% of dwarf galaxies in the most observationally-favored model.Comment: 8 pages, 5 figures, accepted to Ap

Cosmological Implications of the Fundamental Relations of X-ray Clusters

Based on the two-parameter family nature of X-ray clusters of galaxies obtained in a separate paper, we discuss the formation history of clusters and cosmological parameters of the universe. Utilizing the spherical collapse model of cluster formation, and assuming that the cluster X-ray core radius is proportional to the virial radius at the time of the cluster collapse, the observed relations among the density, radius, and temperature of clusters imply that cluster formation occurs in a wide range of redshift. The observed relations favor the low-density universe. Moreover, we find that the model of $n\sim -1$ is preferable.Comment: 7 pages, 4 figures. To be published in ApJ Letter

Cold dark matter models with high baryon content

Recent results have suggested that the density of baryons in the Universe, OmegaB, is much more uncertain than previously thought, and may be significantly higher. We demonstrate that a higher OmegaB increases the viability of critical-density cold dark matter (CDM) models. High baryon fraction offers the twin benefits of boosting the first peak in the microwave anisotropy power spectrum and of suppressing short-scale power in the matter power spectrum. These enable viable CDM models to have a larger Hubble constant than otherwise possible. We carry out a general exploration of high OmegaB CDM models, varying the Hubble constant h and the spectral index n. We confront a variety of observational constraints and discuss specific predictions. Although some observational evidence may favour baryon fractions as high as 20 per cent, we find that values around 10 to 15 per cent provide a reasonable fit to a wide range of data. We suggest that models with OmegaB in this range, with h about 0.5 and n about 0.8, are currently the best critical-density CDM models.Comment: 14 pages, LaTeX, with 9 included figures, to appear in MNRAS. Revised version includes updated references, some changes to section 4. Conclusions unchange