The life cycle of radio galaxies in the LOFAR Lockman Hole field
  Radio galaxies are known to go through cycles of activity, where phases of
apparent quiescence can be followed by repeated activity of the central
supermassive black hole. A better understanding of this cycle is crucial for
ascertaining the energetic impact that the jets have on the host galaxy, but
little is known about it. We used deep LOFAR images at 150 MHz of the Lockman
Hole extragalactic field to select a sample of 158 radio sources with sizes $>
60^{\prime\prime}$ in different phases of their jet life cycle. Using a variety
of criteria (e.g. core prominence combined with low-surface brightness of the
extended emission and steep spectrum of the central region) we selected a
subsample of candidate restarted radio galaxies representing between 13% and
15% of the 158 sources of the main sample. We compare their properties to the
rest of the sample, which consists of remnant candidates and active radio
galaxies. Optical identifications and characterisations of the host galaxies
indicate similar properties for candidate restarted, remnant, and active radio
galaxies, suggesting that they all come from the same parent population. The
fraction of restarted radio galaxies is slightly higher with respect to
remnants, suggesting that the restarted phase can often follow after a
relatively short remnant phase (the duration of the remnant phase being a few
times 10$^{7}$ years). This confirms that the remnant and restarted phases are
integral parts of the life cycle of massive elliptical galaxies. A preliminary
investigation does not suggest a strong dependence of this cycle on the
environment surrounding any given galaxy.
