Combining LOFAR and Apertif Data for Understanding the Life Cycle of
  Radio Galaxies
  Active galactic nuclei (AGN) at the centres of galaxies can cycle between
periods of activity and of quiescence. Characterising the duty-cycle of AGN is
crucial for understanding their impact on the evolution of the host galaxy. For
radio AGN, their evolutionary stage can be identified from a combination of
morphological and spectral properties. We summarise the results we have
obtained in the last few years by studying radio galaxies in various crucial
phases of their lives, such as remnant and restarted sources. We used
morphological information derived from LOw Frequency ARray (LOFAR) images at
150 MHz, combined with resolved spectral indices maps, obtained using recently
released images at 1400 MHz from the APERture Tile In Focus (Apertif)
phased-array feed system installed on the Westerbork Synthesis Radio Telescope.
Our study, limited so far to the Lockman Hole region, has identified radio
galaxies in the dying and restarted phases. We found large varieties in their
properties, relevant for understanding their evolutionary stage. We started by
quantifying their occurrences, the duration of the 'on' (active) and 'off'
(dying) phase, and we compared the results with models of the evolution of
radio galaxies. In addition to these extreme phases, the resolved spectral
index images can also reveal interesting secrets about the evolution of
apparently normal radio galaxies. The spectral information can be connected
with, and used to improve, the Fanaroff--Riley classification, and we present
one example of this, illustrating what the combination of the LOFAR and Apertif
surveys now allow us to do routinely.
