Building the largest spectroscopic sample of ultra-compact massive
  galaxies with the Kilo Degree Survey
  Ultra-compact massive galaxies UCMGs, i.e. galaxies with stellar masses
$M_{*} > 8 \times 10^{10} M_{\odot}$ and effective radii $R_{e} < 1.5$ kpc, are
very rare systems, in particular at low and intermediate redshifts. Their
origin as well as their number density across cosmic time are still under
scrutiny, especially because of the paucity of spectroscopically confirmed
samples. We have started a systematic census of UCMG candidates within the ESO
Kilo Degree Survey, together with a large spectroscopic follow-up campaign to
build the largest possible sample of confirmed UCMGs. This is the third paper
of the series and the second based on the spectroscopic follow-up program.
Here, we present photometrical and structural parameters of 33 new candidates
at redshifts $0.15 \lesssim z \lesssim 0.5$ and confirm 19 of them as UCMGs,
based on their nominal spectroscopically inferred $M_{*}$ and $R_{e}$. This
corresponds to a success rate of $\sim 58\%$, nicely consistent with our
previous findings. The addition of these 19 newly confirmed objects, allows us
to fully assess the systematics on the system selection, and finally reduce the
number density uncertainties. Moreover, putting together the results from our
current and past observational campaigns and some literature data, we build the
largest sample of UCMGs ever collected, comprising 92 spectroscopically
confirmed objects at $0.1 \lesssim z \lesssim 0.5$. This number raises to 116,
allowing for a $3\sigma$ tolerance on the $M_{*}$ and $R_{e}$ thresholds for
the UCMG definition. For all these galaxies we have estimated the velocity
dispersion values at the effective radii which have been used to derive a
preliminary mass-velocity dispersion correlation.
