LOFAR discovery of a 700-kpc remnant radio galaxy at low redshift
  Remnant radio galaxies represent the final dying phase of radio galaxy
evolution, in which the jets are no longer active. Due to their rarity in flux
limited samples and the difficulty of identification, this dying phase remains
poorly understood and the luminosity evolution largely unconstrained. Here we
present the discovery and detailed analysis of a large (700 kpc) remnant radio
galaxy with a low surface brightness that has been identified in LOFAR images
at 150 MHz. By combining LOFAR data with new follow-up Westerbork observations
and archival data at higher frequencies, we investigated the source morphology
and spectral properties from 116 to 4850 MHz. By modelling the radio spectrum
we probed characteristic timescales of the radio activity. The source has a
relatively smooth, diffuse, amorphous appearance together with a very weak
central compact core which is associated with the host galaxy located at
z=0.051. From our ageing and morphological analysis it is clear that the
nuclear engine is currently switched off or, at most, active at a very low
power state. The host galaxy is currently interacting with another galaxy
located at a projected separation of 15 kpc and a radial velocity offset of 300
km/s. This interaction may have played a role in the triggering and/or shut
down of the radio jets. The spectral shape of this remnant radio galaxy differs
from the majority of the previously identified remnant sources, which show
steep or curved spectra at low to intermediate frequencies. In light of this
finding and in preparation for new-generation deep low-frequency surveys, we
discuss the selection criteria to be used to select representative samples of
these sources.
