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Title: On feedback and the formation of galaxies
Author: Johansson, P. H.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2006
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In this thesis, we study the effect of feedback on the formation and evolution of galaxies using a wide range of techniques, including observations, semi-analytic modelling and numerical simulations. There is now a good understanding of the gravitational evolution and clustering of dark matter haloes. In contrast the study of feedback processes, that regulate the formation and growth of the baryonic component in galaxies is still in its infancy. In this thesis, feedback is defined to be a process involving the baryonic component that invokes a back reaction of the process on itself or on the causes that produced it and examples include feedback from supernova and AGNs, radiative feedback and feedback induced by shocks. From an observational near-infrared survey we extract a sample of old elliptical Extremely Red Objects (EROs) at z ~ 1. We find a relatively high number density of massive galaxies at z ~ 1, in agreement with recent observations of cosmic downsizing. The observed space density of old elliptical galaxies is compared to predictions from galaxy formation models. We find that standard hierarchical models underpredict the number of old EROs at z ~ 1, suggesting that additional feedback processes not included in the models, such as AGN feedback might be required to reproduce the observations. Using our ERO sample we study a population of sources with extremely high mid-to-near infrared flux ratios and show that this sample broad band selection method selects primarily sources with extremely obscured AGN activity at z ≳ 1. Our results indicate that obscured AGN activity contribute ~ 20% of the observed cosmic IR background and that a significant fraction of obscured AGN activity is still missed by X-ray surveys. We have developed an analytic model of supernova feedback to study the fraction and metallicity of ejected gas, the relationship between outflow and star formation rates and the mass-metallicity relation of a galaxy population as a function of redshift. We find that our physically motivated, though simplified model of the interstellar medium and star formation produces results that are in good agreement with a range of observations. The model is applied to calculate the metallicity evolution and statistical properties of damped Lyman-α systems (DLAs). The combined effects of star formation, outflows and infall in our model produces a mild evolution in the NHI-weighed metallicity and a nearby constant evolution of ΩHI with redshift in the range of z = 0 – 5, consistent with observations.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available