Rings and wiggles in Hercules A

Abstract

Hercules A (3C348) is the fourth brightest extragalactic radio source in the sky at low frequencies. It is optically identified with a 18.5 mag galaxy with redshift z = 0.154 located in a faint cluster^1,2. Although discovered as early as 1948³, and extensively studied during the 1960s, it was not mapped with high-resolution aperture-synthesis radio telescopes because of its low declination. We report here the first results from extensive observations of Her A made with the Very Large Array (VLA)⁴ of the National Radio Astronomy Observatory. We find that Her A possesses an unusual jet-dominated morphology instead of the Cygnus-A-like morphology expected for such a powerful radio source. Its two jets are quite different in appearance; one appears to be a continuous twisting jet, while the other suggests repeated ejection of individual plasmons by the central object. The presence of such different morphologies in the same source raises interesting questions concerning the generation and evol ution of extended extragalactic radio sources. Variations in the central engine provide the simplest interpretation for some of the features of this object; the role of environmental effects is less clear.