LoTSS DR1: Double-double radio galaxies in the HETDEX field
  Double-double radio galaxies (DDRGs) represent a short but unique phase in
the life-cycle of some of the most powerful radio-loud active galactic nuclei
(RLAGN). These galaxies display large-scale remnant radio plasma in the
intergalactic medium left behind by a past episode of active galactic nuclei
(AGN) activity, and meanwhile, the radio jets have restarted in a new episode.
The knowledge of what causes the jets to switch off and restart is crucial to
our understanding of galaxy evolution, while it is important to know if DDRGs
form a host galaxy dichotomy relative to RLAGN. We utilised the LOFAR Two-Metre
Sky Survey DR1, using a visual identification method to compile a sample of
morphologically selected candidate DDRGs, showing two pairs of radio lobes. To
confirm the restarted nature in each of the candidate sources, we obtained
follow-up observations with the VLA at higher resolution to observe the inner
lobes or restarted jets, the confirmation of which created a robust sample of
33 DDRGs. We created a comparison sample of 777 RLAGN from the DR1 catalogue,
and compared the optical and infrared magnitudes and colours of their host
galaxies. We find that there is no statistically significant difference in the
brightness of the host galaxies between double-doubles and single-cycle RLAGN.
The DDRG and RLAGN samples also have similar distributions in WISE mid-infrared
colours, indicating similar ages of stellar populations and dust levels in the
hosts of DDRGs. We conclude that DDRGs and 'normal' RLAGN are hosted by
galaxies of the same type, and that DDRG activity is simply a normal part of
the life cycle of RLAGN. Restarted jets, particularly for the class of
low-excitation radio galaxies, rather than being a product of a particular
event in the life of a host galaxy, must instead be caused by smaller scale
changes, such as in the accretion system surrounding the black hole.
