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 formation (SF) activity and therefore represent
a powerful tool to investigate their evolution and their possible
mutual influence across cosmic time. Radio astronomy is therefore
becoming increasingly important for galaxy evolution studies thanks
also to the many new radio facilities under construction or being
planned. To maximise the potentiality of these new instruments it
is crucial to make predictions on what they will observe and to see
how best to complement the radio data with multi-wavelength
information. These are the motivations of my Thesis in which I
studied a sample of 900 sources detected in one of the deepest
radio surveys ever made. The observations have been performed at
1.4 GHz with the Very Large Array on the Extended Chandra Deep
Field South. I developed a multi-wavelength method to identify the
optical-infrared counterparts of the radio sources and to classify
them as radio-loud active galactic nuclei (RL AGNs), radio-quiet
(RQ) AGNs, and star forming galaxies (SFGs). I was able for the
first time to quantify the relative contribution of these different
classes of sources down to a radio flux density limit of ∼30 μJy. I
characterized the host galaxy properties (stellar masses, optical
colors, and morphology) of the radio sources; RQ AGN hosts and SFGs
have similar properties with disk morphology and blue colors while
radio-loud AGN hosts are more massive, redder and mostly
ellipticals. This suggests that the RQ and RL activity occurs at
two different evolutionary stages of the BH-host galaxy
co-evolution. The RQ phase occurs at earlier times when the galaxy
is still gas rich and actively forming stars while the radio
activity of the BH appears when the galaxy has already formed the
bulk of its stellar population, the gas supply is lower, and the SF
is considerably reduced. I quantified the star formation rate (SFR)
of the radio sources using two independent tracers, the radio and
far-infrared luminosities. I found evidence that the main
contribution to the radio emission of RQ AGNs is the SF activity in
their host galaxy. This result demonstrates the remarkable
possibility of using the radio band to estimate the SFR even in the
hosts of bright RQ AGNs where the optical-to-mid-infrared emission
can be dominated by the AGN. I have shown that deep radio surveys
can be used to study the cosmic star formation history; I estimated
the contribution of the so-called ”starburst” mode to the total SFR
density and quantified the AGN occurrence in galaxies with
different levels of SF.