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Context. Supermassive black holes in the centres of radio-loud activegalactic nuclei (AGN) can produce collimated relativistic outflows (jets).Magnetic fields are thought to play a key role in the formation and collimationof these jets, but the details are much debated. Aims. We study the innermostjet morphology and magnetic field strength in the AGN 3C 345 with anunprecedented resolution using images obtained within the framework of the keyscience programme on AGN polarisation of the Space VLBI mission RadioAstron.Methods. We observed the flat spectrum radio quasar 3C 345 at 1.6 GHz on 2016March 30 with RadioAstron and 18 ground-based radio telescopes in fullpolarisation mode. Results. Our images, in both total intensity and linearpolarisation, reveal a complex jet structure at 300 $\mu$as angular resolution,corresponding to a projected linear scale of about 2 pc or a few thousandgravitational radii. We identify the synchrotron self-absorbed core at the jetbase and find the brightest feature in the jet 1.5 mas downstream of the core.Several polarised components appear in the Space VLBI images that cannot beseen from ground array-only images. Except for the core, the electric vectorposition angles follow the local jet direction, suggesting a magnetic fieldperpendicular to the jet. This indicates the presence of plane perpendicularshocks in these regions. Additionally, we infer a minimum brightnesstemperature at the largest $(u,v)$-distances of $1.1\times 10^{12}$ K in thesource frame, which is above the inverse Compton limit and an order ofmagnitude larger than the equipartition value. This indicates locally efficientinjection or re-acceleration of particles in the jet to counter the inverseCompton cooling or the geometry of the jet creates significant changes in theDoppler factor, which has to be $>11$ to explain the high brightnesstemperatures.

astronomy and astrophysics

Probing the innermost regions of AGN jets and their magnetic fields with  RadioAstron IV. The quasar 3C 345 at 18 cm: Magnetic field structure and  brightness temperature