We present accurate , spatially resolved imaging of radio spectra at the bases of jets in eleven low-luminosity ( Fanaroff-Riley I ) radio galaxies , derived from Very Large Array ( VLA ) observations .
We pay careful attention to calibration and to the effects of random and systematic errors , and we base the flux-density scale on recent measurements of VLA primary amplitude calibrators by Perley & Butler ( 54 ) .
We show images and profiles of spectral index over the frequency range 1.4 – 8.5 GHz , together with values integrated over fiducial regions defined by our relativistic models of the jets .
We find that the spectral index \alpha , defined in the sense I ( \nu ) \propto \nu ^ { - \alpha } , decreases with distance from the nucleus in all of the jets .
The mean spectral indices are 0.66 \pm 0.01 where the jets first brighten abruptly and 0.59 \pm 0.01 after they recollimate .
The mean change in spectral index between these locations , which is independent of calibration and flux-density scale errors and is therefore more accurately and robustly measured , is -0.067 \pm 0.006 .
Our jet models associate the decrease in spectral index with a bulk deceleration of the flow from \approx 0.8 c to \la 0.5 c .
We suggest that the decrease is the result of a change in the characteristics of ongoing particle acceleration .
One possible acceleration mechanism is the first-order Fermi process in mildly relativistic shocks : in the Bohm limit , the index of the electron energy spectrum , p = 2 \alpha + 1 , is slightly larger than 2 and decreases with velocity upstream of the shock .
This possibility is consistent with our measurements , but requires shocks throughout the jet volume rather than at a few discrete locations .
A second possibility is that two acceleration mechanisms operate in these jets : one ( with p = 2.32 ) dominant close to the galactic nucleus and associated with high flow speeds , another ( with p = 2.18 ) taking over at larger distances and slower flow speeds .