Complex particle acceleration processes in the hotspots of 3C105 and
  3C445
  We investigate the nature of the broad-band emission associated with the
low-power radio hotspots 3C105 South and 3C445 South. Both hotspot regions are
resolved in multiple radio/optical components. High-sensitivity radio VLA,
NIR/optical VLT and HST, and X-ray Chandra data have been used to construct the
multi-band spectra of individual hotspot components. The radio-to-optical
spectra of both hotspot regions are well fitted by a synchrotron model with
steep spectral indices ~0.8 and break frequencies 10^12-10^14 Hz. 3C105 South
is resolved in two optical components: a primary one, aligned with the jet
direction and possibly marking the first jet impact with the surrounding
medium, and a secondary, further out from the jet and extended in a direction
perpendicular to it. This secondary region is interpreted as a splatter-spot
formed by the deflection of relativistic plasma from the primary hotspot. Radio
and optical images of 3C445 South show a spectacular 10-kpc arc-shape structure
characterized by two main components, and perpendicular to the jet direction.
HST images in I and B bands further resolve the brightest components into thin
elongated features. In both 3C105 South and 3C445 South the main hotspot
components are enshrouded by diffuse optical emission on scale of several kpcs,
indicating that very high energy particles, possibly injected at strong shocks,
are continuously re-accelerated in situ by additional acceleration mechanisms.
We suggest that stochastic processes, linked to turbulence and instabilities,
could provide the required additional re-acceleration.
