Vasta, M.; (2010) Modelling ionised and photodissociated regions. Doctoral thesis , UCL (University College London).

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Abstract

The bulk of the mass in star-forming environments is contained in photodissociated regions (PDRs) where the chemistry is controlled by the far ultraviolet radiation field coming from nearby ionizing sources. H II regions can be found throughout active star forming galaxies as they are also controlled by the presence of a nearby ionizing source. In fact, H II regions, PDRs, and molecular clouds are not distinct physical regions, but rather a series of layers that are unified by the transport of radiation from hot stars into cold gas and the flow of cold molecular gas into hot ionized regions. The relative contributions of the ultraviolet radiation field from different gas phases can be estimated from observations of several far infrared forbidden lines such as [C II]158\mum, [O I]145\mum and 63\mum. Fine structure emission line studies provide valuable information on H II and PDR regions and it is well known that they originate in the outer part of both H II and PDR regions. The only exception are the nitrogen lines that clearly come exclusively from ionized regions. With the launch of ISO the far infrared properties of galaxies were observed with greater sensitivity than ever before. This thesis deals with the study of [C II]158\mum, [O I]145\mum, 63\mum and partly CO in external galaxies. In Chapter 3 we investigate the [C II]158\mum, [O I]145\mum and 63\mum emission lines in a sample of external galaxies in order to use them as diagnostics to infer the physical conditions in the gas, such as temperatures, densities and radiation fields. This study was carried out using the photodissociation UCL PDR code, testing PDR model results against ISO LWS observations. In addition, a detailed study of the oxygen self absorption, computed by modelling the emission line profiles with the radiative transfer SMMOL code, was conducted. In Chapter 4 a detailed study of the starburst NGC 4038 has been performed by using both MOCASSIN and UCL PDR. The contribution of [C II] 158\mum, [O I] 63\mum and 145\mum lines coming from the H II region was studied using the 3D ionization MOCASSIN code. We then used the computed radiation field at the ionization front as an input for the photodissociation UCL PDR code and an examination of the oxygen self absorption and a comparison of the CO rotational emission lines with CSO observations, were also presented. Finally, to improve the interpretation of future observations such as those from ALMA and HERSCHEL, a feasibility test on the coupling of the two codes (MOCASSIN and UCL PDR) was performed and the results are presented in Chapter 5. Conclusion and ideas for future work are presented in Chapter 6.

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