The redshift one universe : star formation and galaxy assembly

• Main Author: Doherty, M.
• Published: University of Cambridge 2005
• Subjects: 520
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.598581

This thesis represents an exploration of the Universe at redshift one, using two distinct observational approaches, to investigate star formation and galaxy assembly at this epoch. I present an Hα survey of <i>z</i> ~ 1 galaxies, to determine star formation rates (SFRs), with the near-infrared multi-object spectrograph CIRPASS. I present robust detections of Hα in <i>z</i> ~ 1 galaxies, demonstrating the first successful application of this technique to observing high redshift galaxies. Stacking the spectra in the rest-frame to infer a total SFR for the field, I find a lower limit (uncorrected for dust reddening) on the star formation rate density at redshift <i>z </i>= 1 of 0.04M/yr/MPc³. This implies rapid evolution in the star formation rate density from <i>z</i> = 0 to <i>z</i> = 1 which is proportional to (1 + <i>z</i>)^3.1. Comparing to star formation rate estimates from the UV, it appears that star formation rates inferred from Hα are, on average, a factor of two higher than those based on the UV continuum alone. Extremely Red Objects (EROs) have been shown to be a mix of old, passively evolving massive galaxies and young dust-reddened starbursting galaxies at redshifts <i>z</i> ≈ 1 – 2, both of which place stringent constraints on galaxy formation models. I present optical spectroscopy of a large sample of EROs with (<i>I – H</i>) > 3.0, <i>H</i> ≤ 20.5 (Vega), in three fields. Spectroscopic redshifts are determined for 44/67 sources and I discuss the redshift distribution and spatial clustering of this population in the light of the significant field-to-field variation in number density and properties. I pinpoint a filamentary structure at <i>z</i> = 1.22 in the Chandra Deep Field South. A dominant old stellar population is inferred for 75% of the spectroscopic sample but only 28% have spectra with no evidence of recent star formation activity, such as would be expected for a strictly passively-evolving population. About ~30% of the absorption line spectra are of the ‘E+A’ type with prominent Balmer absorption, consistent, on average, with mass growth of 5015% in the past Gyr. Overall, the results suggest that the bulk of the ERO population is an established population of clustered massive galaxies undergoing intermittent activity consistent with continued growth over the redshift interval 0.8< <i>z</i> <1.6.