Extremely deep 150 MHz source counts from the LoTSS Deep Fields
  With the advent of new generation low-frequency telescopes, such as the LOw
Frequency ARray (LOFAR), and improved calibration techniques, we have now
started to unveil the sub GHz radio sky with unprecedented depth and
sensitivity. The LOFAR Two Meter Sky Survey (LoTSS) is an ongoing project in
which the whole northern radio sky will be observed at 150 MHz with a
sensitivity better than 100 $\mu$Jy beam$^{-1}$ at a resolution of \asec{6}.
Additionally, deeper observations are planned to cover smaller areas with
higher sensitivity. The Lockman Hole, the Bo\"otes and the Elais-N1 regions are
among the most well known northern extra-galactic fields, and the deepest of
the LoTSS Deep Fields so far. We exploit these deep observations to derive the
deepest radio source counts at 150~MHz to date. Our counts are in broad
agreement with those from the literature, and show the well known upturn at
$\leq$ few mJy, mainly associated with the emergence of the star-forming galaxy
population. More interestingly, our counts show for the first time a very
pronounced drop around S$\sim$2 mJy, which results in a prominent `bump' at
sub-mJy flux densities. Such a feature was not observed in previous counts'
determinations (neither at 150 MHz nor at higher frequency). While sample
variance can play a role in explaining the observed discrepancies, we believe
this is mostly the result of a careful analysis aimed at deblending confused
sources and removing spurious sources and artifacts from the radio catalogues.
This `drop and bump' feature cannot be reproduced by any of the existing
state-of-the-art evolutionary models, and appears to be associated with a
deficiency of AGN at intermediate redshift ($1<z<2$) and an excess of
low-redshift ($z<1$) galaxies and/or AGN.
