Cosmology

Abstract:

The rate at which galaxy halos form is thought to play a key role in explaining many observable cosmological phenomena such as the initial epoch at which luminous matter forms and the distribution of active galaxies. Here we show how Press-Schechter theory can be used to provide a simple, completely analytic model of the halo formation rate. This model shows good agreement with both Monte-Carlo and N-body simulation results.

Abstract:

We estimate the evolution of the contribution of galaxies to the cosmic background flux at 912 Å by means of a semi-analytic model of galaxy formation and evolution. Such modelling has been quite successful in reproducing the optical properties of galaxies. We assume that high-redshift damped Lyman α systems are the progenitors of present-day galaxies, and we design a series of models that are consistent with the evolution of cosmic comoving emissivities in the available near-infrared, optical, ultraviolet and far-infrared bands along with the evolution of the neutral hydrogen content and average metallicity of damped Lyman α systems. We use these models to compute the galactic contribution to the Lyman-limit emissivity and background flux for 0 ≃ z ≤ 4. We take into account the absorption of Lyman-limit photons by HI and dust in the interstellar medium of the galaxies. We find that the background Lyman-limit flux due to galaxies might dominate (or be comparable to) the contribution from quasars at almost all redshifts if the absorption by HI in the interstellar medium is neglected. Such HI absorption would result in a severe diminishing of this flux - by almost three orders of magnitude at high redshifts and by one to two orders at z ≃ 0. Though the resulting galaxy flux is completely negligible at high redshifts, it is comparable to the quasar flux at z ≃ 0.

Abstract:

We describe the new, fast, high-precision microdensitometer SuperCOSMOS. Some aspects of hardware and software design that enable high-precision astrometry from photographic plates are explained. We show that the positioning repeatability of the measuring machine is less than 0.1 μm standard error in either coordinate, and the absolute positional accuracy is about 0.15 μm standard error. Furthermore, measurements of the same plate in different orientations show that the sampling errors are small (e.g. ∼0.2 μm, rising to ∼1.0 μm at the plate limit, for stellar images in a IIIaJ emulsion), thus allowing the extraction of relative positional information from Schmidt plates at accuracies less than 1 μm. We demonstrate that SuperCOSMOS is capable of measuring the positions of bright stars (i.e. those more than ∼4 mag above the plate limit) to a precision ∼0.5 μm with survey-grade photographic plates employing fine-grained emulsions.