Understanding the formation of relativistic jets in active galactic nuclei remains an elusive problem1. This is partly because observational tests of jet formation models suffer from the limited angular resolution of ground-based very-long-baseline interferometry that has thus far been able to probe the structure of the jet acceleration and collimation region in only two sources2,3. Here, we report observations of 3C84 (NGC 1275)—the central galaxy of the Perseus cluster—made with an interferometric array including the orbiting radio telescope of the RadioAstron4 mission. The data transversely resolve the edge-brightened jet in 3C84 only 30 μas from the core, which is ten times closer to the central engine than was possible in previous ground-based observations5 and allows us to measure the jet collimation profile from ~102 to ~104 gravitational radii (rg) from the black hole. The previously found5, almost cylindrical jet profile on scales larger than a few thousand rg is seen to continue at least down to a few hundred rg from the black hole, and we find a broad jet with a transverse radius of ≳250 rg at only 350 rg from the core. This implies that either the bright outer jet layer goes through a very rapid lateral expansion on scales ≲102 rg or it is launched from the accretion disk.
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We thank E. Ros for useful comments on the manuscript. The RadioAstron project is led by the Astro Space Center of the Lebedev Physical Institute of the Russian Academy of Sciences and the Lavochkin Scientific and Production Association under a contract with the State Space Corporation ROSCOSMOS, in collaboration with partner organizations in Russia and other countries. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities. The European VLBI Network is a joint facility of independent European, African, Asian and North American radio astronomy institutes. The KVN is a facility operated by the Korea Astronomy and Space Science Institute. The KVN operations are supported by the Korea Research Environment Open NETwork, which is managed and operated by the Korea Institute of Science and Technology Information. This work was partially supported by the National Research Council of Science and Technology, granted by the International Joint Research Program (EU-16-001). This research is based on observations correlated at the Bonn Correlator, jointly operated by the Max-Planck-Institut für Radioastronomie and the Federal Agency for Cartography and Geodesy. T.S. was funded by the Academy of Finland projects 274477 and 284495. Y.Y.K., M.M.L., K.V.S. and P.A.V. were supported by the Russian Science Foundation (project 16-12-10481). S.-S.L. was supported by a National Research Foundation of Korea grant funded by the Korean government (MSIP; number 987 NRF-2016R1C1B2006697).