  Radio galaxies dominate the radio sky and are essential to the galaxy
evolution puzzle. High-resolution studies of statistical samples of radio
galaxies are expected to shed light on the triggering mechanisms of the AGN,
alternating between the phases of activity and quiescence. In this work, we
focus on the sub-arcsec radio structures in the central regions of the 35 radio
galaxies over 6.6 $deg^2$ of the Lockman Hole region. These sources were
previously classified as active, remnant, and candidate restarted radio
galaxies using 150 MHz LOFAR observations. We examine the morphologies and
study the spectral properties of their central regions to explore their
evolutionary stages and revise the criteria used to select the initial sample.
We use the newly available LOFAR 150 MHz image obtained using international
baselines, achieving 0.38'' x 0.30'' resolution, making this the first
systematic study of the nuclear regions at high resolution and low frequency.
We use publicly available images from the FIRST survey at 1.4 GHz and the Karl
G. Jansky VLA Sky Survey at 3 GHz to achieve our goals. In addition, for one
restarted candidate we present new dedicated observations with the VLA at 3
GHz. We have found various morphologies of the central regions of the radio
galaxies in our sample, some resembling miniature double-double radio galaxies.
We also see the beginnings of active jets or distinct detections unrelated to
the large-scale structure. Furthermore, we have found diverse radio spectra in
our sample - flat, steep, or peaked between 150 MHz and 3 GHz, indicative of
the different life-cycle phases. Based on these analyses, we confirm five of
six previously considered restarted candidates and identify three more from the
active sample, supporting previous results suggesting that the restarted phase
can occur after a relatively short remnant phase (i.e. a few tens of millions
of years).
