NIR/Optical Counterparts of Hotspots in Radio Galaxies

TL;DR

This study investigates low-power radio galaxy hotspots using high-resolution infrared, optical, and radio observations, revealing synchrotron emission with higher break frequencies than in high-power hotspots, implying recent electron acceleration.

Contribution

It provides new high-resolution multi-wavelength observations of low-power hotspots, showing higher break frequencies and suggesting ongoing electron re-acceleration processes.

Findings

Infrared/optical emission detected in 45% of hotspots.

Break frequencies between 10^5 and 10^6 GHz, higher than in high-power hotspots.

Electrons likely injected less than 10^3 years ago, indicating recent acceleration.

Abstract

We present new high spatial resolution VLT and VLA observations of a sample of nine low-power (P_{1.4 GHz} < 10^{25} W/Hz) radio hotspots. Infrared/optical emission is definitely detected in four of the nine observed objects, resulting in a detection rate of at least 45%. This emission is interpreted as synchrotron radiation from the electrons accelerated in the hot spots. The integrated spectra of these hotspots reveal typical break frequencies between 10^5 and 10^6 GHz, two orders of magnitude higher than typically found in high-power hotspots. This supports the idea that in low-power hotspots with their relatively low magnetic field strengths electrons emit most of their energy at higher frequencies. A simple spectral ageing analysis would imply that the emitting electrons have been injected into the hotspot volume less than ~10^3 years ago. We discuss possible scenarios to explain the lack of older electrons in the hotspot region. In particular, the extended morphology of the NIR/optical emission would suggest that efficient re-acceleration mechanisms rejuvenate the electron populations.