A counterjet in the elliptical galaxy M87

Abstract

JETS in radio galaxies have a variety of observed morphologies, in particular appearing in both one-sided and two-sided forms. It is not known whether jets can be intrinsically one-sided, or if the one-sided appearance is the result of an asymmetry in a two-sided jet. An asymmetry could be real, or apparent because of geometrical and possibly relativistic effects creating a large difference in the apparent visibility of the two sides. The elliptical galaxy M87 hosts a weak but well known one-sided radio jet, which is very visible at optical and even X-ray wavelengths. Using the New Technology Telescope of the European Southern Observatory in Chile, we have identified an optical counterpart to the radio hotspot in the southeast lobe of M87, at about the same distance as the radio jet but oppositely located. Polarization data identify the optical emission as synchrotron radiation, and the severe energetic requirements of maintaining this emission suggest con-tinuous replenishment at the site of the hotspot. We propose that this is provided by an invisible 'counterjet'. We argue that the counterjet cannot be identical to the visible known jet, but must be intrinsically somewhat fainter.