SHORES II: Multi-frequency Characterisation of the Sub-mJy Radio Population in FIR-selected Fields

TL;DR

This study conducts a deep, multi-frequency radio survey of SHORES deep fields with ATCA, delivering a 2.1 GHz catalog of 483 reliable sources (with 101 and 33 counterparts at 5.5 and 9 GHz, respectively) and robust spectral characterization across 0.1–10 GHz via RadioSED. By cross-matching to H-ATLAS FIR data, the authors quantify the FIR–radio correlation, finding a median q$_{FIR}$ ≈ 2.36 and a mild redshift evolution q$_{FIR}$(z) ∝ (1+z)^{−0.22}, while identifying a population of radio-only sources that may be high-redshift star-forming galaxies or radio-quiet AGN. The 2.1 GHz number counts align with previous deep surveys and SMB/AGN evolutionary models, underscoring a transition in the faint radio population where SFGs and radio-quiet AGN become significant. Overall, SHORES demonstrates the power of combining deep, high-frequency radio data with FIR measurements to disentangle SFG and AGN contributions and informs SKA-era strategies for the faint radio sky.

Abstract

We present a new deep multi-frequency radio survey of two extragalactic fields observed with the Australia Telescope Compact Array (ATCA) 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 deg$^2$ of two Herschel Astrophysical Terahertz Large Area Survey (H-ATLAS) fields down to rms sensitivities of 9-17 $μ$Jy$/$beam at 2.1 GHz, 28-39 $μ$Jy/beam at 5.5 GHz and 38-61 $μ$Jy/beam at 9 GHz. This setup allows us to investigate the spectral energy distributions (SEDs) 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 $α\sim -0.7$, 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 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 (FIRRC). The sources with $q_{FIR} \geq 1.69$ exhibit radio spectral indices typical of star-forming galaxies. 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.

Figures (16)

• Figure 1: 2.1 GHz maps of the DEEP 1 (top) and DEEP 2 (bottom) fields. A single colour scale is shown below.
• Figure 2: Maps of the DEEP-1 (top) and DEEP-2 (bottom) at 5.5 (left), 7.25 (center), and 9.0 (right) GHz. Notice that the 7.25 GHz is the composed by multi-frequency synthesis of the other two frequencies, on which each 2 GHz baseband was centered.
• Figure 3: Upper: Effective area curves at 2.1 GHz (DEEP-1/DEEP-2), with primary-beam correction. Lower: Effective area at 5.5-7.25-9 GHz for DEEP-1 and DEEP-2.
• Figure 4: Ratio between total flux and peak flux as a function of the SNR for the SHORES sources.
• Figure 5: Reliability of the SHORES deep field (DEEP-1 on the top and DEEP-2 below) at 2.1GHz. The green solid line indicates $\rm{SNR}=5.08$ (DEEP-1) and $\rm{SNR}=5.00$ (DEEP-2), for which the reliability is 95%.