Open AccessA relativistic model of the radio jets in 3C 296
Author(s) -
Laing R. A.,
Canvin J. R.,
Bridle A. H.,
Hardcastle M. J.
Publication year - 2006
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1111/j.1365-2966.2006.10903.x
Subject(s) - physics , astrophysics , line of sight , relativistic beaming , astrophysical jet , polarization (electrochemistry) , spectral index , radio galaxy , linear polarization , magnetic field , faraday effect , galaxy , active galactic nucleus , astronomy , spectral line , optics , laser , chemistry , quantum mechanics
We present new, deep 8.5‐GHz VLA observations of the nearby, low‐luminosity radio galaxy 3C 296 at resolutions from 0.25 to 5.5 arcsec. These show the intensity and polarization structures of the twin radio jets in detail. We derive the spectral‐index distribution using lower‐frequency VLA observations and show that the flatter‐spectrum jets are surrounded by a sheath of steeper‐spectrum diffuse emission. We also show images of Faraday rotation measure and depolarization and derive the apparent magnetic field structure. We apply our intrinsically symmetrical, decelerating relativistic jet model to the new observations. An optimized model accurately fits the data in both total intensity and linear polarization. We infer that the jets are inclined by 58° to the line of sight. Their outer isophotes flare to a half‐opening angle of 26° and then recollimate to form a conical flow beyond 16 kpc from the nucleus. On‐axis, they decelerate from a (poorly constrained) initial velocity β= v / c ≈ 0.8 to ≈ 0.4 around 5 kpc from the nucleus, the velocity thereafter remaining constant. The speed at the edge of the jet is low everywhere. The longitudinal profile of proper emissivity has three principal power‐law sections: an inner region (0–1.8 kpc), where the jets are faint, a bright region (1.8–8.9 kpc) and an outer region with a flatter slope. The emission is centre brightened. Our observations rule out a globally ordered, helical magnetic field configuration. Instead, we model the field as random on small scales but anisotropic, with toroidal and longitudinal components only. The ratio of longitudinal to toroidal field falls with distance along the jet, qualitatively but not quantitatively as expected from flux freezing, so that the field is predominantly toroidal far from the nucleus. The toroidal component is relatively stronger at the edges of the jet. A simple adiabatic model fits the emissivity evolution only in the outer region after the jets have decelerated and recollimated; closer to the nucleus, it predicts far too steep an emissivity decline with distance. We also interpret the morphological differences between brightness enhancements (‘arcs’) in the main and counter‐jets as an effect of relativistic aberration.