Measuring the star formation rate density from CNOC2

Abstract

This thesis charts the evolution of the Universal Star Formation Rate Density (SFRD) across the redshift range 0.2 < z < 0.6, using the 2nd Canadian Network for Observational Cosmology Survey (CN0C2). The redshift range covered by the Survey marks an important transitional period in the history of the Universe, over which the SFRD is thought to undergo rapid evolution. The various Star Formation Rate (SFR) indicators from the literature are reviewed and it is shown that the [OII] forbidden emission line can provide a very similar SFR to that reported by the Ha recombination line, if reddening is accounted for. We explain the CN0C2 Survey in detail, including the observation strategy and the instrumentation used. The Survey weighting scheme is also discussed. The purpose written code, used to measure the strengths of the [OII] emission line in each of the Survey spectra, is explained and tested, against both another automated code and by-eye measurements from IRAF. The procedure of re-normalising the on equivalent width errors that are reported by the code is also detailed. This leaves the line measurement errors very well quantified. We investigate the accuracy of the photometric redshift codes ImpZ and Hyperz in returning dust estimates, A(_v) from photometry alone. The benchmark for this comparison is set by Balmer Decrement measurements of a sub-set of the CN0C2 dataset. Combining the [OII] line strength measurements with the dust correction from the photometric redshift code ImpZ, we compute SFRs for individual galaxies in the CN0C2 Survey. These are combined with a weighting scheme to chart the evolution of SFRD with redshift and luminosity. It is found that giant galaxies undergo a strong evolution in terms of the fraction of those galaxies that are rapidly star forming, but that the absolute SFRD from giants does not change as quickly as that from lower luminosity galaxies. At all redshifts and luminosities covered by the Survey, galaxies with [OII] emission <30Å contribute roughly half the total SFRD. This means narrow band imaging surveys may miss a large fraction of star formation activity at those epochs. At z ~ 0.25 it is found that dwarf galaxies dominate the overall SFRD, in-keeping with results from the local Universe. We produce a best estimate Lilly-Madau plot based on our findings and compare it to several studies from the literature. It is shown that although the overall rise found by previous studies is confirmed, the shape of the rise is not consistent with other works, showing a sharper increase at low (0.2 < z < 0.4) redshift followed by a slowing at z ~0.6. The significance of this result remains unquantified however.