Toward a physical model for the evolution of galaxies and active galactic nuclei through cosmic times

A major part of this Thesis consists of a comprehensive investigation of the cosmological evolution of the luminosity function (LF) of galaxies and active galactic nuclei (AGNs) in the infrared (IR). Building on previous work by the Trieste--Padova group, we have elaborated a model that, based on the observed dichotomy in the ages of stellar populations of early-type galaxies on one side and late-type galaxies on the other, interprets the epoch-dependent LFs at $z \gtrsim 1$ using a physical approach for the evolution of proto-spheroidal galaxies and of the associated AGNs, while IR galaxies at $z \lesssim 2$ are interpreted as being mostly late-type ``cold'' (normal) and ``warm'' (starburst) galaxies. As for proto-spheroids, in addition to the epoch-dependent LFs of stellar and AGN components separately, we have worked out, for the first time, the evolving LFs of these objects as a whole (stellar plus AGN component), taking into account in a self-consistent way the variation with galactic age of the global spectral energy distribution (SED). This high-$z$ model provides a physical explanation for the observed positive evolution of both galaxies and AGNs up to $z \simeq 2.5$ and for the negative evolution at higher redshifts, for the sharp transition from Euclidean to extremely steep counts at (sub-)millimeter wavelengths, as well as for the (sub-)millimeter counts of strongly lensed galaxies that are hard to account for by alternative, physical or phenomenological approaches. The evolution of late-type galaxies and $z \lesssim 2$ AGNs is described using a phenomenological parametric approach complemented with luminosity-independent empirical SEDs, allowing us to deal simultaneously with data over a broad wavelength range. The ``cold'' population has a mild luminosity evolution and no density evolution, while the ``warm'' population evolves significantly in luminosity and negligibly in density. Type 1 AGNs have similar evolutions in luminosity and density, while the type 2 AGNs only evolve in density. This ``hybrid'' model provides a good fit to the multi-wavelength (from the mid-IR to millimeter waves) data on LFs at different redshifts and on number counts (both global and per redshift slices). The modeled AGN contributions to the counts and to the cosmic infrared background (CIB) are always sub-dominant. They are maximal at mid-IR wavelengths: the contributions to the 15 and 24 $\mu$m counts reach 20\% above 10 and 2 mJy, respectively, while the contributions to the CIB are of 8.6\% and of 8.1\% at 15 and 24 $\mu$m, respectively. A prediction of the present model, useful to test it, is a systematic variation with wavelength of the populations dominating the counts and the contributions to the CIB intensity. This implies a specific trend for cross-wavelength CIB power spectra, which is found to be in good agreement with the data. Updated predictions for the number counts and the redshift distributions of star-forming galaxies spectroscopically detectable by future mission, such as the SPace Infrared telescope for Cosmology and Astrophysics (SPICA), have been obtained exploiting this ``hybrid'' model for the evolution of the dusty star-forming galaxies. Preliminary radio counts of star-forming galaxies, resulting from the combination of the ``hybrid'' model with the well-known IR-radio correlation, are also made to explain the sub-mJy excess of radio source counts that will be determined precisely by future Square Kilometer Array (SKA) surveys. To understand the role played by star-forming galaxies at $z \gtrsim 6$ on the cosmic re-ionization, the high-$z$ physical model has been extended to very small halos to investigate the evolution of the ultraviolet (UV) LF of high-$z$ star-forming galaxies taking into account in a self-consistent way their chemical evolution and the associated evolution of dust extinction. The model yields good fits of the UV and Ly$\alpha$ LFs at all redshifts ($z \gtrsim 2$) at which they have been measured, providing a simple explanation for the weak evolution observed between $z = 2 $ and $z =6$. The observed range of UV luminosities at high-$z$ implies a minimum halo mass capable of hosting active star formation $M_{\rm crit} \lesssim 10^{9.8}\ M_\odot$, consistent with the constraints from hydrodynamical simulations. The derived relationships linking the optical depths for absorption of ionizing photons by dust and neutral hydrogen to the star formation rate and, in the case of dust absorption, to the metallicity of the galaxies, imply higher effective escape fractions for galaxies with lower intrinsic UV luminosities or lower halo/stellar masses, and also a mild increase of the escape fraction with increasing redshift at fixed luminosity or halo/stellar mass. Galaxies already represented in the UV LF ($M_{\rm UV} \lesssim -18$ or $M_{\rm crit} \gtrsim 10^{10}\,M_\odot$) can keep the universe fully ionized up to $z \simeq 6$. To get complete ionization up to $z \simeq 7$, the population of star-forming galaxies at this redshift must extend in luminosity to $M_{\rm UV} \sim -13$ ($M_{\rm crit} \sim 10^{8.5}\,M_\odot$) or fainter. Although a complete ionization of intergalactic medium up to $z \simeq 7$ is disfavored by some (uncertain) data at $z \simeq 6$--7, pointing to a rapid drop of the ionization degree above $z \simeq 6$, the electron scattering optical depth inferred from Cosmic Microwave Background experiments favor an ionization degree close to unity up to $z \simeq 9$--10. Since all these constraints on the re-ionization history are affected by substantial uncertainties, better data are needed to reach firm conclusions