2006MNRAS.368...48L

We present images of the jets in the nearby radio galaxy NGC315 made with the Very Large Array at five frequencies between 1.365 and 5GHz with resolutions between 1.5 and 45arcsec. Within 15arcsec of the nucleus, the spectral index of the jets is α= 0.61. Further from the nucleus, the spectrum is flatter, with significant transverse structure. Between 15 and 70arcsec from the nucleus, the spectral index varies from ~0.55 on-axis to ~0.44 at the edge. This spectral structure suggests a change of dominant particle acceleration mechanism with distance from the nucleus and the transverse gradient may be associated with shear in the jet velocity field. Further from the nucleus, the spectral index has a constant value of 0.47. We derive the distribution of Faraday rotation over the inner ±400arcsec of the radio source and show that it has three components: a constant term, a linear gradient (both probably due to our Galaxy) and residual fluctuations at the level of 1-2rad/m2. These residual fluctuations are smaller in the brighter (approaching) jet, consistent with the idea that they are produced by magnetic fields in a halo of hot plasma that surrounds the radio source. We model this halo, deriving a core radius of ~225arcsec and constraining its central density and magnetic field strength. We also image the apparent magnetic field structure over the first ±200arcsec from the nucleus.