Remnant Radio Galaxies Discovered in a Multi-frequency Survey
  The remnant phase of a radio galaxy begins when the jets launched from an
active galactic nucleus are switched off. To study the fraction of radio
galaxies in a remnant phase, we take advantage of a $8.31$\,deg$^2$ sub-region
of the GAMA~23~field which comprises of surveys covering the frequency range
0.1--9\,GHz. We present a sample of 104 radio galaxies compiled from
observations conducted by the Murchison Wide-field Array (216\,MHz), the
Australia Square Kilometer Array Pathfinder (887\,MHz), and the Australia
Telescope Compact Array (5.5\,GHz). We adopt an `absent radio core' criterion
to identify 10 radio galaxies showing no evidence for an active nucleus. We
classify these as new candidate remnant radio galaxies. Seven of these objects
still display compact emitting regions within the lobes at 5.5\,GHz; at this
frequency the emission is short-lived, implying a recent jet switch-off. On the
other hand, only three show evidence of aged lobe plasma by the presence of an
ultra-steep spectrum ($\alpha<-1.2$) and a diffuse, low surface-brightness
radio morphology. The predominant fraction of young remnants is consistent with
a rapid fading during the remnant phase. Within our sample of radio galaxies,
our observations constrain the remnant fraction to $4\%\lesssim
f_{\mathrm{rem}} \lesssim 10\%$; the lower limit comes from the limiting case
in which all remnant candidates with hotspots are simply active radio galaxies
with faint, undetected radio cores. Finally, we model the synchrotron spectrum
arising from a hotspot to show they can persist for 5--10\,Myr at 5.5\,GHz
after the jets switch off -- radio emission arising from such hotspots can
therefore be expected in an appreciable fraction of genuine remnants.
