SPARCS-North Wide-field VLBI Survey: Exploring the resolved MicroJy extra-galactic radio source population with EVN+e-MERLIN

TL;DR

This study uses high-resolution VLBI and e-MERLIN imaging to investigate the transition between active galactic nuclei and star-forming regions in faint extragalactic radio sources, providing insights into their emission mechanisms.

Contribution

It presents the first milliarcsecond-scale VLBI observations of the SPARCS Northern field, revealing the spatial scale transition from pure AGN to combined AGN and star-formation activity.

Findings

Detection of 11 sources with VLBI at 9 pc scale

Emergence of radio jets at 0.3 kpc scale in some sources

Evidence of a transition from pure AGN to combined emission

Abstract

The SKA PAthfinder Radio Continuum Surveys (SPARCS) are providing deep-field imaging of the faint (sub-mJy) extra-galactic radio source populations through a series of reference surveys. One of the key science goals for SPARCS is to characterize the relative contribution of radio emission associated with AGN from star-formation (SF) in these faint radio source populations, using a combination of high sensitivity and high angular resolution imaging over a range of spatial scales (arcsec to mas). To isolate AGN contribution from SF, we hypothesise that there exists a brightness temperature cut-off point separating pure AGN from SF. We present a multi-resolution (10-100 mas) view of the transition between compact AGN and diffuse SF through a deep wide-field EVN+e-MERLIN, multiple phase centre survey of the centre of the Northern SPARCS (SLOAN) reference field at 1.6 GHz. This is the first (and only) VLBI (+e-MERLIN) milliarcsecond angular resolution observation of this field, and of the wider SPARCS reference field programme. Using these high spatial resolution (9 pc - 0.3 kpc at z ~ 1.25) data, 11 milliarcsecond-scale sources are detected from a targeted sample of 52 known radio sources from previous observations with the e-MERLIN, giving a VLBI detection fraction of ~ 21%. At spatial scales of ~ 9 pc, these sources show little to no jet structure whilst at ~ 0.3 kpc one-sided and two-sided radio jets begin to emerge on the same sources, indicating a possible transition from pure AGN emissions to AGN and star-formation systems.