The sub-mJy radio population in the Extended Chandra Deep Field South
Deep radio observations provide a dust unbiased view of both black hole (BH) and star\nformation (SF) activity and therefore represent a powerful tool to investigate their evolution\nand their possible mutual influence across cosmic time. Radio astronomy is therefore\nbecoming increasingly important for galaxy evolution studies thanks also to the many new\nradio facilities under construction or being planned. To maximise the potentiality of these\nnew instruments it is crucial to make predictions on what they will observe and to see how\nbest to complement the radio data with multi-wavelength information.\nThese are the motivations of my Thesis in which I studied a sample of 900 sources\ndetected in one of the deepest radio surveys ever made. The observations have been\nperformed at 1.4 GHz with the Very Large Array on the Extended Chandra Deep Field\nSouth. I developed a multi-wavelength method to identify the optical-infrared counterparts\nof the radio sources and to classify them as radio-loud active galactic nuclei (RL AGNs),\nradio-quiet (RQ) AGNs, and star forming galaxies (SFGs). I was able for the first time to\nquantify the relative contribution of these different classes of sources down to a radio flux\ndensity limit of \u223c30 \u03bcJy.\nI characterized the host galaxy properties (stellar masses, optical colors, and morphology)\nof the radio sources; RQ AGN hosts and SFGs have similar properties with disk\nmorphology and blue colors while radio-loud AGN hosts are more massive, redder and\nmostly ellipticals. This suggests that the RQ and RL activity occurs at two different evolutionary\nstages of the BH-host galaxy co-evolution. The RQ phase occurs at earlier times\nwhen the galaxy is still gas rich and actively forming stars while the radio activity of the\nBH appears when the galaxy has already formed the bulk of its stellar population, the gas\nsupply is lower, and the SF is considerably reduced.\nI quantified the star formation rate (SFR) of the radio sources using two independent\ntracers, the radio and far-infrared luminosities. I found evidence that the main contribution\nto the radio emission of RQ AGNs is the SF activity in their host galaxy. This result\ndemonstrates the remarkable possibility of using the radio band to estimate the SFR even in\nthe hosts of bright RQ AGNs where the optical-to-mid-infrared emission can be dominated\nby the AGN. I have shown that deep radio surveys can be used to study the cosmic star\nformation history; I estimated the contribution of the so-called \u201dstarburst\u201d mode to the\ntotal SFR density and quantified the AGN occurrence in galaxies with different levels of\nSF.