Modeling the interstellar medium of high redshift galaxies

The nature of the galaxies that formed within the first billion year of the Cosmic history is still poorly constrained, despite observations performed with increasingly sensitive telescopes and sophisticated cosmological simulations running on powerful computers. Even if we are living in a golden era for the extragalactic astrophysics, several fundamental questions are only partially answered: 1. Are the first galaxies that appeared in the Universe similar to those observed at later epochs? 2. What is the metal and dust content of these galaxies? 3. How much the metal abundance affects the physical conditions of their interstellar medium (ISM)? 4. What are the thermal and dynamical properties of various gas phases of their ISM? In the near future, an huge breakthrough in solving these crucial issues will be achieved with the Atacama Large Millimeter/submillmeter Array (ALMA), the most powerful millimeter/sub-millimeter interferometer on the Earth. One of the ALMA scientific goals is to observe the redshifted far-infrared (FIR) metal cooling and molecular lines arising from the interstellar medium of the galaxies that emerged from the cosmic Dark Ages billions of years ago. So far, the mere detection of these objects has been the main purpose of the observational campaigns, however, with its unprecedented sensitivity and spatial resolution, ALMA will be able to revolutionize the observational Cosmology revealing the cold gas and the dust within these galaxies. The aim of this work is to devise a self-consistent theoretical model describing the ISM properties of high redshift galaxies that allows to predict the luminosities of various metal and molecular lines observable with ALMA. We want to study the correlation between the line luminosities and fundamental galaxy properties, such as the star formation rate (SFR) and the metallicity (Z). We perform high resolution cosmological simulations of star forming galaxies at the end of the Epoch of Reionization (z '6), and we build on top of them a sub-grid model describing the cooling and the heating processes that take place in the neutral diffuse ISM. While the cosmological simulation allows to follow, on Mpc scales, the galaxy formation starting from the primordial quantum fluctuations, the sub-grid model provides a sub-kpc description of the thermodynamic equilibrium within the ISM. The Thesis is structured as follows: 1. In Chapter 1, we present the standard model for the structure formation in the Universe, we describe the main processes shaping the evolution of the Reionization, and the observational techniques adopted to detect high redshift galaxies. 2. In Chapter 2 we discuss in details the physics of the thermodynamic equilibrium of the ISM and that of the molecular clouds. 3. The analytical model developed to infer the abundance of molecular gas, and the luminosity of molecular tracers, in a sample of simulated high redshift galaxies is outlined in Chapter 3. 4. Chapter 4 is focused on the sub-grid model constructed during this Thesis to describe the thermodynamical equilibrium of the neutral diffuse gas in the ISM of the high redshifts galaxies. 5. The contribution of the photodissociation regions to the the far-infrared line emissivity is outlined in Chapter 5. 6. In the subsequent Chapter we discuss the relation between FIR line luminosity, the star formation rate, and the metallicity of high-z galaxies. 7. The predictions achieved in this work have been successfully tested against the few sub-millimeter data from high-z galaxies so far available and they have been used as a theoretical support in several ALMA proposals. The reader can find an extensive discussion about these points in Chapter 7. 8. We present the conclusions and the future prospects in the last Chapter.