Dust and Nebular Emission in Star Forming Galaxies

In Chapter 2 we describe how we modelled the nebular emission produced by H II regions. We start with an introduction on the ionized gas physics and on the basic assumptions of the library. Then we describe of the adopted ionizing sources, the role of the ionizing parameter and the geometry of gas. We also discuss the accuracy and the limitations of the method and compare the results 'Yith observed H II galaxies In Chapter 3 we firstly summarize the main features of the GRASIL code and how galaxies are schematized in it. Then we introduce the method that we propose to correctly compute the nebular emission for a galaxy using the library described in the previous chapter, and compare the method to other models in literature. In Chapter 4 we apply the model to discuss several methods to estimate the attenuation in normal star forming galaxies by considering the UV, optical emission lines and FIR properties. In particular, we compare the attenuation suffered by the ionized gas with the one suffered by the stellar continuum. We show that observations require the extinction of different stellar populations to vary with age. In Chapter 5 we study the attenuation in starburst galaxies. In particular we examine the correlation between the FIR/UV ratio and the UV spectral index in the Wu et al. (2002) sample of starbursts. We also discuss the observed attenuation law and, by using our model, we explore its origin, and the effect of the age selective extinction. Furthermore we apply our model to explain some UV, FIR and optical properties of Very Luminous IR galaxies. Chapter 6 is dedicated to the measure of the star formation rate. We compare the different SFR estimators, namely the UV luminosity, the dust SED, the emission lines and the radio continuum. In the case of normal spiral galaxies, we provide our best calibrations for them and underline the different robustness of each estimator. In Chapter 7 we discuss some applications of IR nebular lines. In particular, we present a method to derive the metallicity of galaxies from IR nitrogen lines and radio emission. We also used the model to interpret the excitation diagram of galactic H II regions and starburst galaxies. Finally, we discuss the possibility to study the formation phase of elliptical galaxies with spectroscopical observations carried with SIRTF. Appendix A collects some observational data extracted from the Wu et al. (2002) sample of starburst galaxies.