A Panchromatic Spectral Population Synthesis Model for Young Star-Burst Galaxies

The study of the properties of galaxies to possibly map their formation and evolutionary paths is an active research field both from observational and theoretical point of view. Great observational efforts are continuously devoted to map the spectral energy distribution (SED) of galaxies in the local universe and at high-redshifts over all wavelength range, by surveying the sky with both ground and space-borne instruments. Surveys in different spectral bands have revealed galaxies with drastically different activity levels and energy budgets, symptoms of underlying disparate basic physical properties. It is these basic properties that one aims to uncover in order to get insights on the complex behaviour of mass assembly and recycling in galaxies. The fundamental tools necessary for this task are spectral synthesis models. These models are based on the availability of a complete library of stellar evolution models, that homogeneously cover the range of stellar masses, metallicities, and evolutionary phases that contribute to the integrated SED of a galaxy. From these and a library with a complete coverage of the stellar parameters, the integrated SED of simple stellar populations (SSP) can then be computed. To compare the theoretical SED with the observed one, it is important to account for, at very least, the main processes producing radiation and affecting the radiation path, i.e. stars, ionized gas nebulae for the main recombination lines and continuum emission, core-collapsed supernovae (CCSN) for the non-thermal radio emission, and the interaction between stellar radiation and dust grains in the interstellar medium (ISM). In this thesis, I focus on a major revision and extension of the SSP models based on the PAdova TRieste Stellar Evolution Code (PARSEC) that is used to compute stellar evolutionary tracks used for population synthesis purposes.. By means of PARSEC stellar tracks, I compute, at five different metallicities (0.0001 - 0.02) and four different IMF upper mass limits (40 - 350$M_{\odot}$), the integrated stellar light, the line and continuum emission (effects of ionized nebulae on this light) and of the non-thermal emission predicted by young SSPs. I then use the new SSP models in combination with the radiative transfer code GRASIL (GRAphites and SILicates) to include the stellar extinction by dust and the ensuing IR emission, therefore providing a panchromatic UV-to radio SED modelling. I calibrate and apply this new model to observations. Thereafter, I carry out a critical investigation of the effect of metallicity, IMF upper mass limit and star formation regime (star bursting versus normal star forming) on star formation rate (SFR) and dust attenuation properties of star forming galaxies. This allowed for the provision of a consistent set of SFR calibrations, that are explicitly dependent on metallicity and IMF upper mass limit and also on the age of the starburst, at wavelengths ranging from UV to radio. Finally, I considered the possible use of the radio slope to identify the the IMF upper mass limit where i showed that the method is well suited for high-redshift objects with a prolonged star formation