A deep 1.4 GHz survey of the J1030 equatorial field: A new window on radio source populations across cosmic time

We present deep L-Band observations of the equatorial field centered on the z = 6.3 Sloan Digital Sky Survey (SDSS) quasar (QSO). This field is rich of multiwavelength photometry and spectroscopy data, making it an ideal laboratory for galaxy evolution studies. Our observations reach a 1σ sensitivity of ~2.5 μJy at the center of the field. We extracted a catalog of 1489 radio sources down to a flux density of ~12.5 μJy (5σ) over a field of view of ~ 30′ diameter. We derived the source counts accounting for catalog reliability and completeness, and compared them with others available in the literature. Our source counts are among the deepest available so far, and, overall, are consistent with recent counts'determinations and models. They show a slight excess at flux densities ~50 μJy, possibly associated with the presence of known overdensities in the field. We detected for the first time in the radio band the SDSS J1030+0524 QSO (26 ± 5 μJy, 8σ significance level). For this object, we derived an optical radio loudness RO = 0.62±0.12, which makes it the most radio quiet among active galactic nuclei (AGN) discovered so far at z ≳ 6 and detected at radio wavelengths. We unveiled extended diffuse radio emission associated with the lobes of a bright Fanaroff-Riley type II (FRII) radio galaxy located close to the center of the J1030 field, which is likely to become the future brightest cluster galaxy of a protocluster at z = 1.7. The lobes'complex morphology, coupled with the presence of X-ray diffuse emission detected around the FRII galaxy lobes, may point toward an interaction between the radio jets and the external medium. We also investigated the relation between radio and X-ray luminosity for a sample of 243 X-ray-selected objects obtained from 500 ks Chandra observations of the same field, and spanning a wide redshift range (0 ≲ z ≲ 3). Focused on sources with a spectroscopic redshift and classification, we found that sources hosted by early-type galaxies and AGN follow log(LR)/log(LX) linear correlations with slopes of ~0.6 and ~0.8, respectively. This is interpreted as a likely signature of different efficiency in the accretion process. Finally, we found that most of these sources (≳87%) show a radio-to-X-ray radio loudness RX ≲ -3.5, classifying these objects as radio quiet.