Galaxy evolution studies with the SPICA telescope

Lines in the mid- to far-infrared wavelength range, less affected by dust extinction than optical lines, are ideally suited to study the physical processes taking place in the dust-hidden regions in galaxies, where heavily obscured star formation and accretion onto supermassive black-holes at their nuclei take place. These tools are fundamental for the study of local galaxies, but become of paramount importance to probe galaxies at the “Cosmic Noon”, at redshift z1-3, when the peak of star formation activity has taken place and most of the baryonic mass in galaxies has been assembled. One of the main goals of this Thesis is to provide reliable calibrations of mid- to far-IR ionic fine structure lines, H2 molecular lines and PAH (polycyclic aromatic hydrocarbon features), that will be used in the near and mid term to derive crucial information on the physics driving galaxy evolution. Based on these calibrations I identify the best spectroscopic measurements for the star formation rate and the black hole accretion rate in galaxies. This is done by comparing the marginal correlation of the different line luminosities versus the total IR luminosity for three samples of galaxies in the local Universe: star-forming galaxies, active galactic nuclei, and low-metallicity dwarf galaxies. For the most commonly observed fine-structure lines in the far-IR, I compare the calibrations derived from local galaxies to the existing ALMA observations of high redshift galaxies, finding an excellent agreement. These calibrations will be exploited for distant galaxies by present and future ground-based facilities observing in the millimeter and sub-millimeter range, and for galaxies in the nearby Universe through mid-IR spectroscopic observations with the upcoming James Webb Space Telescope (JWST). The second goal of this Thesis is to design future spectro-photometric surveys that will be able to address the most challenging problems that limit our current understanding of the galaxy evolution: the co-evolution of star formation and black hole accretion, the build-up of heavy elements and the chemical evolution of galaxies, and the feedback from active galactic nuclei. Using my calibration of the mid- to far-IR lines as a basis, I have simulated deep spectro -photometric surveys covering large cosmological volumes over extended fields (1–15 deg2) with an imaging spectrometer covering the mid-IR spectral range (17–36m interval), following the study case of the SPace Infrared telescope for Cosmology and Astrophysics (SPICA). A SPICA-like mission would be able to provide an unobscured three-dimensional view of galaxy evolution back to an age of the universe of less than 3 Gyrs. This survey strategy would produce a full census of the Star Formation Rate in the Universe, using PAH bands and fine-structure ionic lines, reaching the characteristic knee of the galaxy luminosity function, where the bulk of the population is distributed, at any redshift up to z3.5. Deep follow-up pointed spectroscopic observations with grating spectrometers onboard the satellite, from mid- to far-IR spectral range, would also measure the Black Hole Accretion Rate from high-ionisation fine-structure lines down to the knee of their luminosity function. Moreover, other two relevant goals of IR spectroscopic observations from space are the studies of feedback from AGN/starburst galaxies and of the metallicity evolution. While at the present time the SPICA mission will not be further developed, a similar study on galaxy evolution can be partially addressed by the ALMA telescope in the mm/sub-mm range, and the James Webb Space Telescope (JWST) in the mid-IR. These facilities, while unable to perform wide-field blind surveys, will allow to study the processes that led to the Cosmic Noon,using the ALMA telescope to observe galaxies at redshift above z3, and the late evolution of galaxies after their peak activity, with JWST observing local galaxies up to redshift z1.