SHORES. II. Multi-frequency Characterization of the Sub-mJy Radio Population in FIR-selected Fields

We present a new deep multi-frequency radio survey of two extragalactic fields observed with the Australia Telescope Compact Array as part of the SHORES project (Serendipitous H-ATLAS-fields Observations of Radio Extragalactic Sources). The observations, centred at 2.1, 5.5, and 9 GHz, cover the central 0.5 deg2 of two Herschel Astrophysical Terahertz Large Area Survey (H-ATLAS) fields down to rms sensitivities of 9-17 μJy beam-1 at 2.1 GHz, 28-39 μJy beam-1 at 5.5 GHz and 38-61 μJy beam-1 at 9 GHz. This setup allows us to investigate the spectral energy distributions of faint radio sources and probe the nature of the sub-mJy population. We extract and validate a robust catalogue of 489 sources at 2.1 GHz, 101 of which are also detected at 5.5 GHz. We perform a multi-frequency analysis of the radio number counts and derive the spectral indices of sources in the deep fields. The spectral index distribution of our sources peaks around α ∼ −0.7, consistent with synchrotron emission from the faint radio population. The number counts at 2.1 GHz are consistent with previous deep surveys and theoretical models, and provide a lower limit on the star-forming galaxy (SFG) population, which is expected to dominate the faint end. The 5.5 GHz data offer new, direct constraints on the sub-mJy radio sky at higher frequencies. By cross-matching with the H-ATLAS catalogue, we identify a sample of sources with far-infrared (FIR) counterparts and explore the far-infrared-radio correlation. The sources with qFIR ≥ 1.69 exhibit radio spectral indices typical of SFGs. Furthermore, we identify a population of radio-only sources with similar indices that may correspond to high-redshift SFGs, lacking counterparts in the FIR survey due to its limited resolution and sensitivity.