Building the Largest Spectroscopic Sample of Ultracompact Massive Galaxies with the Kilo Degree Survey

Ultracompact massive galaxies (UCMGs), I.e., galaxies with stellar masses ${M}_{\star }\gt 8\times {10}^{10}{M}_{\odot }$ and effective radii ${R}_{{\rm{e}}}\lt 1.5\,\mathrm{kpc}$ , 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 ${M}_{\star }$ and ${R}_{{\rm{e}}}$ . 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 to 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 3σ tolerance on the ${M}_{\star }$ and ${R}_{{\rm{e}}}$ 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.