Modelling the faint radio sky: the pathway to SKA

In the last decade our understanding of the faint radio sky has undergone a significant change. Our perception of weak radio sources has been deeply changed due to the discovery that the so-called radio-quiet active galactic nuclei are not, in fact, radio silent, but simply faint as they finally started to be detected. Moreover, a new, unexpected class of radio-loud active galactic nuclei has been recently discovered, the so-called infrared-faint radio sources. This class of rare objects discovered in deep radio surveys, is emerging as a potentially promising class of sources to pre-select high-redshift powerful radio-loud active galactic nuclei. In this thesis, I present my contribution to the ongoing efforts to understand the nature of both these types of active galactic nuclei. A more in depth analysis of their nature and properties is fundamental for a proper modelling of the faint radio sky, and will be of paramount importance for a proper interpretation of the future sky survey outcomes, on the pathway to the Square Kilometre Array (SKA). On the topic of radio-quiet active galactic nuclei, I searched for compact radio cores within a number of these sources, selected from deep radio samples. The search for these cores is particularly useful to shed light on the nature of the radio emission in high-redshift radio-quiet active galactic nuclei, which origin is currently hotly debated (star formation in the host galaxy or active nucleus). In particular, I searched for compact radio emission through long baseline radio interferometric observations of a selected sample of these sources. I also conducted a literature review, searching for such compact emissions in deep radio samples with high resolution (parsec-scale) observations already available, where radio-quiet active galactic nuclei were not recognised as such, by applying new classification criteria. In my experiment, I detected compact and bright radio cores in two active galactic nuclei classified as radio quiet, and I helped to classify other three detected objects in another experiment. Reviewing the literature, I found other five of these sources provided of a compact radio core. Therefore, I here report on a first direct evidence of radio cores in radio-quiet active galactic nuclei at cosmological redshifts. My preliminary analysis also indicates that star formation and active-galactic-nuclei-related radio emission likely coexist in these objects. On the topic of infrared-faint radio sources, I searched for the infrared counterparts of the radio faintest tail (sub-mJy/mJy) of the distribution of these objects. Moreover, I compared the radio/IR properties of the IR-detected infrared-faint radio sources with those expected for a number of known classes of objects. I found that these sources are mostly consistent with a mixture of powerful, high-redshift (z ≥ 3) radio-loud active galactic nuclei, as their radio brighter (>> 1 mJy) counterparts. Their faintness is likely to be ascribed to higher redshifts (z > 3-4). Some contamination by lower redshift (z ~ 2) heavily dust-enshrouded star forming galaxies can be present at the lowest end of their radio flux distribution (S 1:4 GHz ≤ 0.1 mJy). This research was not only useful to better understand the nature of these sources,but provided further evidence that they may potentially be a useful resource to pre-select very high redshift (z > 4) active galactic nuclei candidates and, if confirmed, to explore the onset of the radio-loud active galactic nuclei phenomenon in the distant Universe. The future, planned sub-mJy radio surveys (the ones expected to be delivered by the new facilities like ASKAP and SKA) will sample active galactic nuclei in the million, and the above mentioned classes will constitute a relevant fraction of the future radio catalogues. Developing methods to recognise them, and to correctly address their nature, is fundamental to fully exploiting these surveys.