Hotspots in radio galaxies: a comparison with hydrodynamic simulations

Abstract

Surface brightness distributions are derived for the beam cap of an axisymmetric supersonic jet ramming through its confining medium. The detailed structure and time-evolution of the working surface is computed numerically with a high-resolution gas dynamics code employing a special technique for accurately tracking and preserving the jet boundary. The synchrotron emissivity is assumed to be proportional to the square of the gas pressure inside the jet, and zero outside. The geometry of the beam cap is found to change on a dynamical time-scale, producing an enormous variety of hotspot shapes in a single evolution. A classification of well-resolved hotspots in radio galaxies is made with which the derived contour maps are compared. Several quite common features of observed hotspots are reproduced, such as: (i) a bright head with sharp outer edge having a length-to-width ratio 3–5; (ii) twin wings and single tails extending backwards in the general direction of the nucleus; (iii) complex subcomponent geometries, including two bright peaks transverse to the source axis. Hotspots viewed at an angle appear rounder, have smoother edges and have lower dynamic range than when viewed from the side. Distorted hotspots can be accommodated by a more general model. Three-dimensional simulations should be able to reproduce observed asymmetries. Relativistic beaming effects, non-circular jets, or strong magnetic fields may also contribute to distortion.