Bright galaxy number counts

Abstract

The value of a plot of galaxy number counts per unit area of sky against magnitude as a probe of the early universe is well known, being influenced by both the curvature of expanding space and the deceleration parameter. The observations are complicated by changes which almost certainly have taken place in the intrinsic brightness of the distant galaxies over the long time periods involved, through both evolution and possible galaxy mergers over this time. In the blue band surveys fainter than ~ 22ͫ, the counts exceed the theoretical non-evolving models by a factor of 2-4, which at present is accounted for by requiring strong evolution in the models. An additional observational problem in determining the extent of such possible evolution is the observed under-density of counts over the brighter end of the curve (~ 12 — 18ͫ) compared to the numbers at fainter magnitudes. This thesis details the theoretical models on which the predictions are based, re-examines several recent surveys to evaluate how well these can be fitted to assumed luminosity functions of the Schechter type, then examines number counts over this brighter range to look for possible causes for the observed discrepancy. Four possibilities are considered to account for this: (1) Photometric anomalies and technical problems, (2) Incomplete surveys over this range, (3) The presence of voids or strong inhomogeneities in this region, (4) The effect of evolutionary changes on the shape of the theoretical models. It is concluded that the survey data is reasonably robust, and the number count models may require merging evolution to account for the bright- end discrepancy as well as the faint-end anomalies.