Remnant radio-loud AGN in the Herschel-ATLAS field
  Only a small fraction of observed Active Galactic Nuclei display large-scale
radio emission associated with jets, yet these radio-loud AGN have become
increasingly important in models of galaxy evolution. In determining the
dynamics and energetics of the radio sources over cosmic time, a key question
concerns what happens when their jets switch off. The resulting `remnant'
radio-loud AGN have been surprisingly evasive in past radio surveys, and
therefore statistical information on the population of radio-loud AGN in their
dying phase is limited. In this paper, with the recent developments of LOFAR
and the VLA, we are able to provide a systematically selected sample of remnant
radio-loud AGN in the Herschel-ATLAS field. Using a simple core-detection
method, we constrain the upper limit on the fraction of remnants in our
radio-loud AGN sample to 9 per cent, implying that the extended lobe emission
fades rapidly once the core/jets turn off. We also find that our remnant sample
has a wide range of spectral indices ($-1.5\leqslant
\alpha^{1400}_{150}\leqslant -0.5$), confirming that the lobes of some remnants
may possess flat spectra at low frequencies just as active sources do. We
suggest that, even with the unprecedented sensitivity of LOFAR, our sample may
still only contain the youngest of the remnant population.
