The SKA PAthfinder Radio Continuum Surveys (SPARCS) are providing deep-field imaging of the faint (sub-mJy) extragalactic 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 active galactic nucleus (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, it was hypothesized that there exists a brightness temperature cut-off point separating pure AGN from SF.
It is presented a multiresolution (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 milliarcsec-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 per cent.
At spatial scales of ∼9pc, these sources show little to no jet structure whilst at ∼0.3kpc 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 SF systems.
Read the full paper in the MNRAS here.
Image: © Ann Njeri, et al.; Monthly Notices of the Royal Astronomical Society, Volume 519, Issue 2, February 2023, Pages 1732–1744, https://doi.org/10.1093/mnras/stac3569; The SPARCS field as seen by the VLASS and e-MERLIN surveys. The central plot comprises of the VLASS survey view of this field. Overlaid are the observed phase centres for this survey in the context of the Effelsberg (assuming an effective diameter of 78 m) and the 25 m primary beams of the EVN array. The extent of each phase centre corresponds to 10 per cent half-power beam width (HPBW) smearing, that is, assuming averaging to 2 s integrations and 32kHz channel width after the initial internal wide-field correlation and a maximum baseline of 10 000 km. Surrounding the VLASS image are postage stamp cut-outs of selected sources from the e-MERLIN pilot survey (rms ∼10μJybeam−1), revealing the exquisite detail provided by the 0′′.2 high-resolution imaging.
