Chemical evolution of galaxies: from the Local Group to the Local Universe

The chemical composition of the galaxy interstellar medium continuously evolves as a function of time because of many environmental physical processes. Restitution of metals from dying stars, astration of metals due to the star formation activity, gas inflows and outflows, radial mixing of both gas and stars are all fundamental mechanisms in driving the chemical evolution of the interstellar medium of galaxies. In the first part of the Thesis, I introduce the basic concepts lying at the basis of the galaxy chemical evolution modeling and present my original contributions in the foundation of this field. In particular, I have explored, for the first time, the effect of the metallicity and initial mass function on the stellar yield of oxygen and metals per stellar generation. This quantity is usually assumed as a free parameter in the analytical models, adjusted to reproduce the data, without realizing that it is the outcome of stellar evolutionary models, depending on a variety of factors. Moreover, I present a new general method for solving the chemical evolution of galaxies, which I apply to reproduce the observed chemical abundance patterns in the Milky Way. This method might be very useful for other complementary stellar population synthesis models, to trace the evolution of chemical elements - such as iron - which are restored with a distribution of delay times from the star formation event. The second part of the Thesis is devoted to the study of the classical and ultra-faint dwarf spheroidal galaxies, satellites of the Milky Way. The models for the dwarf spheroidals assume a very slow star formation rate and relatively high rate of galactic winds, and these characteristics are more marked in the ultra-faint dwarfs. One of the main conclusions is that it is unlikely that the actual ultra-faint dwarfs have been the building blocks of the whole Galactic halo, although more accurate data are necessary to draw firm conclusions. In my Thesis, I also explore the effect of the so-called integrated galactic initial mass function (IGIMF) and neutron star mergers on the chemical evolution of the Sagittarius dwarf spheroidal galaxy. The results of my models support the idea that the initial mass function in Sagittarius is truncated, favouring the IGIMF theory; moreover, I conclude that the observed Eu abundances can be explained only by assuming neutron star mergers for the Eu nucleosynthesis. Finally, I present the results of a novel "photochemical evolution model", which I have developed during my PhD research activity and applied to the study of the stellar populations in the Sculptor dwarf spheroidal galaxy. This new stellar population synthesis model ``lights up'' the stars with different mass, metallicity and age of chemical evolution model and represents a novel method to draw the color-magnitude diagram of galaxies. In the third part of my Thesis, I pass from the chemical evolution of the Local Group dwarf galaxies to the star forming galaxies in the Local Universe. In particular, I present detailed chemical evolution models to explain the nitrogen and oxygen abundances, which have been inferred in a sample of Sloan Digital Sky Survey galaxies in the Local Universe. This collection of data - integrated with the abundances derived in metal-poor, star forming dwarf galaxies - clearly demonstrates the existence of a plateau in the (N/O) ratio at very low metallicity, followed by an increase of this ratio which steepens as the metallicity increases. The conclusions are that primary nitrogen by massive stars is always needed to reproduce the plateau at very low metallicity, together with differential galactic winds, which are responsible for the steepening of the (N/O) vs. (O/H) relation at large metallicities. I conclude also that the secondary nitrogen component by low- and intermediate-mass stars, increasing as a function of the metallicity, is also necessary to explain the final high (N/O) ratios in the data.