  When a black hole accretes close to the Eddington limit, the astrophysical
jet is often accompanied by radiatively driven, wide-aperture and mildly
relativistic winds. Powerful winds can produce significant non-thermal radio
emission via shocks. Among the nearby critical accretion quasars, PDS 456 has a
very massive black hole (about one billion solar masses), shows a significant
star-forming activity (about seventy solar masses per year) and hosts
exceptionally energetic X-ray winds (power up to twenty per cent of the
Eddington luminosity). To probe the radio activity in this extreme accretion
and feedback system, we performed very-long-baseline interferometric (VLBI)
observations of PDS 456 at 1.66 GHz with the European VLBI Network (EVN) and
the enhanced Multi-Element Remotely Linked Interferometry Network (e-MERLIN).
We find a rarely-seen complex radio-emitting nucleus consisting of a collimated
jet and an extended non-thermal radio emission region. The diffuse emission
region has a size of about 360 pc and a radio luminosity about three times
higher than the nearby extreme starburst galaxy Arp 220. The powerful nuclear
radio activity could result from either a relic jet with a peculiar geometry
(nearly along the line of sight) or more likely from diffuse shocks formed
naturally by the existing high-speed winds impacting on high-density
star-forming regions.
