Lyman Continuum Radiation from Intermediate-Mass Black Holes : An Estimation of the Contribution of LyC Radiation from IMBHs in z~3 Galaxies

Sammanfattning: This thesis concerns the cosmic reionization of the intergalactic medium in the high redshift universe and, in particular, is a first approach to examine what role intermediate mass black holes (IMBHs) play in this cosmic age. Unexpectedly high values of hydrogen ionizing radiation, compared to current models, have been reported from galaxies at redshift ~3 (used as stand-ins for galaxies at the age of reionization), and thus these models need revision, potentially by adding effects of IMBH content in the galaxies. The thesis consists of two parts: a literature study of the cosmic reionization, galaxies at high redshift and accreting black holes, and a research part where spectral colours of observed galaxies at redshift ~3, modelled IMBH accretion discs and a stacked quasar spectrum are compared, to examine if black hole physics might play a part in producing the extra radiation observed, and if so, how many IMBHs (of varying mass and accretion rate) would be needed to produce this radiation. Results show that the IMBH model spectra have colours that agree with them being possible sources of the ionizing radiation from the observed galaxies. However, the colours of the quasar spectrum were found to be too red for a typical quasar to be a plausible such source. We find that for several combinations of the parameters mass and accretion rate, our model leads to reasonable numbers of IMBHs, whereas there are cases in which parameters lead to non-physical numbers (e.g. <1). However, the model for IMBH spectra used is simplified and does not incorporate emission lines, feedback effects or viewing angle dependency of the observed luminosity. Despite this, our results are consistent with galaxies hosting one or several IMBHs, and further research should be conducted with more precision to establish the exact way they should be accounted for in models. The thesis is conducted at the Division of Astronomy and Space Physics at Uppsala University.