On the nature of infrared-faint radio sources in the Subaru X-ray Deep and Very Large Array-VIMOS VLT Deep Survey fields

Infrared-faint radio sources (IFRSs) are an unusual class of objects that are relatively bright at radio wavelengths but have faint or undetected infrared counterparts, even in deep surveys. We identify and investigate the nature of IFRSs using deep radio (S_{1.4 GHz} ∼ 100 μJy beam^-1 at 5σ), optical (m_r ∼ 26-27.7 at 5σ) and near-infrared (S_3.6 μm ∼ 1.3-2.0 μJy beam^-1 at 5σ) data that are available in two deep fields: the Subaru X-ray Deep Field (SXDF) and the Very Large Array-VIMOS VLT Deep Survey (VLA-VVDS) field. In 1.8 deg² of the two fields, we identify a total of nine confirmed and ten candidate IFRSs. We find that our IFRSs are high-redshift radio-loud active galactic nuclei, with 12/19 sources having redshift estimates in the range of z ∼ 1.7-4.3, while a limit of z ≥ 2.0 is placed on the remaining seven sources. Notably, for the first time, our study finds IFRSs with measured redshift >3.0, and also redshift estimates for IFRSs in the faintest 3.6-μm flux regime (i.e. S_{3.6 μm} < 1.3 μJy). Radio observations show that our IFRSs exhibit both compact unresolved and extended double-lobe morphologies, and have predominantly steep radio spectra between 1.4 GHz and 325 MHz. The non-detection of all but one IFRSs in the X-ray band and the optical-to-mid-infrared colour (m_r-m_{24 μm}) suggest that a significant fraction of IFRSs are likely to be hosted in dusty obscured galaxies.