The broadband spectra of infrared-faint radio sources

Infrared-faint radio sources (IFRSs) are objects found a decade ago that are characterised by their infrared faintness and enormously high radio-to-infrared flux density ratios. The nature of this extreme class of object was uncertain and several scenarios have previously been suggested. This thesis presents new multi-wavelength data in the optical, far-infrared, and radio regime that are used to study unexplored regimes of the spectral energy distribution (SED) of IFRSs, crucially narrowing down the nature of this class of objects. Optical spectroscopic data are used to measure spectroscopic redshifts of three IFRSs showing that these sources are at cosmologically relevant redshifts. Further, the spectra themselves and the first redshift-based SED modelling show IFRSs to be consistent with radio-loud active galactic nuclei (AGNs). The suggested presence of AGNs in IFRSs is confirmed based on very long baseline interferometry observations of a large sample of IFRSs in the radio regime. Based on these data, it is also argued that IFRSs might be AGNs in their earliest evolutionary stages. This suggestion is consistent with the results from a detailed radio SED study over a wide frequency range presented in this thesis, finding steep and a few curved radio SEDs. In the first far-infrared observations of IFRSs, all six targeted sources remained undetected. The resulting flux density limits provide crucial constraints for a comprehensive broadband SED modelling, showing that only high-redshift radio galaxies(HzRGs) can reproduce the characteristics of IFRSs. Estimated limits of the infrared luminosity, star formation and black hole accretion rates are also found to be consistent with the corresponding properties of HzRGs. The thesis adds significant support to the suggestion that IFRSs are radio-loud AGNs at high redshifts (z & 2) and similar to HzRGs, whereas other objects like heavily obscured star forming galaxies or galactic objects could explicitly be ruled out. Further, it hypothesises that IFRSs are young AGNs.