A new approach to photoionization modelling and the physical conditions of HII regions and Star Forming Galaxies

Metallicity is one of the most prominent physical quantities in the study of galaxy evolution and specifically of star forming regions. There are two main methods to determine gas phase metallicity: empirical and theoretical approaches. The empirical method measures abundances using the information directly inferred from the emission lines in spectra. There are different ways to determine metallicity in the framework of the empirical method. The most commonly used one, called direct method, includes measurement of temperature-sensitive line ratios such as [OIII]λ4363/5007, [NII]λ5755/6584, and [SII]λ6312/9532. The lines at the numerator of the ratio are called auroral lines, and are used to directly measure the electron temperature (Te) of the nebular gas. However, when these weak auroral lines are not available, a method involving the ratios of strong emission lines must instead be used as an indirect metallicity indicator. In this case, the strong line ratios are calibrated against metallicity using direct measurements or theoretical models. The theoretical method uses multi-parametric photoionization models to reproduce the observed emission lines. These photoionisation codes are useful tools commonly used by the astronomical community to model the properties of gas irradiated by an ionizing source (e.g., in planetary nebulae, HII regions, and AGNs), which take into account the different spectral components involved. These codes solve numerically the ionization and thermal structures of an ionized cloud and output the nebular continuum and emission line spectra. Over the past decades, a wide range of studies has been developed to establish computational models of single-component ionized gas to reproduce the emission lines in galaxies. Single-zone models reproduce the ionization, caused by a continuum source, of a single, ionization- bounded, gas cloud of hydrogen with number density NH. The investigations that utilize a single cloud to analyse the expected spectral changes with variations in gas abundances or spectral energy distribution (SED) may derive misleading results. In this thesis, we will use a novel approach, where we treat the spectrum emerging from a galaxy or an HII region not as the product of a single emitting gas cloud, but as a combination of the emission of multiple clouds with different properties such as density and ionisation. In this way, it is possible to significantly improve the agreement between observations and simulated data. The spectrum predicted by these models depends on global integration, over gas density at a specific location, and over radius. Results depend weakly on the density distribution and they slightly depend on the radial distribution. This is in contrast to single-cloud models, which often are described by an ionization parameter and whose predicted spectrum has a strong dependence on this parameter. The locally optimally emitting clouds model (where locally means that each line could be formed efficiently only at a specific location in the density–flux plane and optimally that we consider only emission from a cloud with an optimal flux and density for each line) fit the observed spectrum with fewer free parameters than do single-cloud models. The locally optimally emitting clouds approach is a more physical model because, unless galaxy clouds have a remarkably restricted range of properties, we will observe the a variety of emitting clouds for most lines. The emission line spectrum from clouds distributed in gas density and radius is much less sensitive to changes in the gas abundances and SED than that emitted by a single cloud. Single cloud model-based studies have tried to use a wide range of physical parameters to reproduce complex emission lines observed in galaxies but they were not fully successful. In comparison with observations, single-based models predict too weak high ionization lines and too low predicted electronic temperature, which can generally infer from line ratio 7 [OIII]λ4363/[OIII]λ5007. In this thesis, we develop a new, more realistic approach based on the use of multiple components of photoionised gas to reproduce all observable emission lines simultaneously. To improve the agreement between the models predictions and the observations, we consider the fact that a star-forming galaxy is a collection of HII regions (or a HII region is a combination of different properties from different scales inside it). These regions have a wide range of properties. The galaxy’s emission lines are a combination of the emission of all these different regions. The model uses a set of the different gas clouds with different properties and combine them to reproduce all observed emission lines for one system. The organization of this thesis is as follows. In chapter1, we focus on gas-phase metallicity as a key parameter to determine the evolution of Gas in star-forming galaxy and HII regions. Also, we introduce both empirical and theoretical approaches to measure chemical properties of galaxies. In chapter2, we provide explanations of model spectra based on photoionisation simulations with CLOUDY. In chapter3, we describe a new novel, multi-cloud method, to model accurately the chemical properties of galaxies. We discuss how previous studies in the last 20-years tried to use a wide range of physical parameters to reproduce complex emission lines observed in HII regions and star-forming galaxies but all attempts have not been successful so far to comprehensively predict all observable emission lines and gas-phase metallicity. We tested our method using different samples of HII regions with high quality line flux measuraments. In chapter4, we present the reference sample we used to test the method on star forming galaxy spectra. This sample is based on SDSS stacked galaxy spectra, in order to detect faint emission lines, such as auroral line, that are not generally observable at high S/N in single galaxies. In chapter5 to show the performance of our new method, we demonstrate how our novel multiple clouds approach can reproduce the observed line fluxes on multiple species and ionisation states with an accuracy better than 10%. Moreover, we re-calibrate the new strong calibration relationship according to the new multi-cloud model, we present the relation between ionisation and metallicity, nitrogen and sulfur abundances, the relation between temperatures in different low and high ionisation parts. Additionally, we show which is the impact of excluding Te sensitive line ratios from the inputs of the model, especially at high metallicity.Finally, in section 6, we summarize our main results and discuss some future applications to investigate them by using our new method.