Towards a census of super-compact massive galaxies in the Kilo Degree Survey

The abundance of compact, massive, early-type galaxies (ETGs) provides important constraints to galaxy formation scenarios. Thanks to the area covered, depth, excellent spatial resolution and seeing, the ESO Public optical Kilo Degree Survey (KiDS), carried out with the VLT Survey Telescope (VST), offers a unique opportunity to conduct a complete census of the most compact galaxies in the Universe. This paper presents a first census of such systems from the first 156 square degrees of KiDS. Our analysis relies on g-, r-, and i-band effective radii ($R_{\rm e}$), derived by fitting galaxy images with PSF-convolved S��rsic models, high-quality photometric redshifts, $z_{\rm phot}$, estimated from machine learning techniques, and stellar masses, $M_{\rm \star}$, calculated from KiDS aperture photometry. After massiveness ($M_{\rm \star} > 8 \times 10^{10}\, \rm M_{\odot}$) and compactness ($R_{\rm e} < 1.5 \, \rm kpc$ in g-, r- and i-bands) criteria are applied, a visual inspection of the candidates plus near-infrared photometry from VIKING-DR1 are used to refine our sample. The final catalog, to be spectroscopically confirmed, consists of 92 systems in the redshift range $z \sim 0.2-0.7$. This sample, which we expect to increase by a factor of ten over the total survey area, represents the first attempt to select massive super-compact ETGs (MSCGs) in KiDS. We investigate the impact of redshift systematics in the selection, finding that this seems to be a major source of contamination in our sample. A preliminary analysis shows that MSCGs exhibit negative internal colour gradients, consistent with a passive evolution of these systems. We find that the number density of MSCGs is only mildly consistent with predictions from simulations at $z>0.2$, while no such system is found at $z < 0.2$.
11 pages, 6 figures, MNRAS in press, revised and improved version, figures and text have been updated