2011MNRAS.417.2789L

We present deep, high-resolution imaging of the nearby Fanaroff–Riley Class I (FR I) radio galaxies NGC 193, B2 0206+35, B2 0755+37 and M84 at frequencies of 4.9 and 1.4 GHz using new and archival multiconfiguration observations from the Very Large Array. In addition, we describe lower resolution observations of B2 0326+39 and a re-analysis of our published images of 3C 296. All of these radio galaxies show twin jets and well-defined lobes or bridges of emission, and we examine the common properties of this class of source. We show detailed images of total intensity, brightness gradient, spectral index, degree of polarization and projected magnetic-field direction. The jet bases are very similar to those in tailed twin-jet sources and show the characteristics of decelerating, relativistic flows. Except on one side of M84, we find that the jets can be traced as least as far as the ends of the lobes, where they often form structures which we call `caps' with sharp outer brightness gradients. Continuing, but less well collimated, flows back into the lobes from the caps can often be identified by their relatively flat spectral indices. The lobes in these radio galaxies are similar in morphology, spectral-index distribution and magnetic-field structure to those in more powerful (FR II) sources, but they lack hotspots or other evidence for strong shocks at the ends of the jets. M84 may be an intermediate case between lobed and tailed sources, in which one jet does not reach the end of its lobe, but disrupts to form a `bubble'.