High-redshift blazar classification though multi-wavelength data

High-redshift radio-loud active galactic nuclei (AGN) are key laboratories for studying the formation of the earliest galaxies and supermassive black holes. Among them, blazars, AGNs with relativistic jets oriented close to the line of sight, appear to dominate at redshifts above four. Blazars can be used to obtain an estimate of jetted AGN. However, distinguishing blazars from misaligned radio sources remains challenging, particularly given the limited number of known high-redshift radio quasars. In general, the jet orientation can be inferred from X-ray data and milliarcsecond (mas)-scale angular resolution radio measurements. The latter can be achieved by very long-baseline interferometry (VLBI) imaging. VLBI is suitable for distinguishing between compact, high-brightness temperature radio emission of blazars and the more extended structures of misaligned jetted AGN. Previous high-resolution VLBI studies revealed that some of the radio sources among blazar candidates in fact show unbeamed radio emission on mas scales. For this reason, it is extremely important to compare the classifications obtained with different methods and from different wavelength bands. We found that the combination of multi-band (radio, optical, and X-ray) data and the combination of different classification methods is able to properly recover all the oriented sources present in high-redshift radio quasar samples.