A wide and collimated radio jet in 3C84 on the scale of a few hundred gravitational radii

Abstract

Understanding the formation of relativistic jets in active galactic nuclei remains an elusive problem<SUP>1</SUP>. 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 sources<SUP>2,3</SUP>. 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 RadioAstron<SUP>4</SUP> 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 observations<SUP>5</SUP> and allows us to measure the jet collimation profile from 10<SUP>2</SUP> to 10<SUP>4</SUP> gravitational radii (r<SUB>g</SUB>) from the black hole. The previously found<SUP>5</SUP>, almost cylindrical jet profile on scales larger than a few thousand r<SUB>g</SUB> is seen to continue at least down to a few hundred r<SUB>g</SUB> from the black hole, and we find a broad jet with a transverse radius of ≳250 r<SUB>g</SUB> at only 350 r<SUB>g</SUB> from the core. This implies that either the bright outer jet layer goes through a very rapid lateral expansion on scales ≲10<SUP>2</SUP> r<SUB>g</SUB> or it is launched from the accretion disk.