Towards a census of supercompact massive galaxies in the Kilo Degree Survey

Abstract

The abundance of compact, massive, early-type galaxies (ETGs) provides important constraintsto galaxy formation scenarios. Thanks to the area covered, depth, excellent spatial resolutionand seeing, the ESO Public optical Kilo Degree Survey (KiDS), carried out with the VLTSurvey Telescope, offers a unique opportunity to conduct a complete census of the mostcompact galaxies in the Universe. This paper presents a first census of such systems from the first 156 deg2 of KiDS. Our analysis relies on g-, r- and i-band effective radii (Re), derived byfitting galaxy images with point spread function (PSF)-convolved Sérsic models, high-qualityphotometric redshifts, zphot, estimated from machine learning techniques, and stellar masses,M*, calculated from KiDS aperture photometry. After massiveness (M* ≥∼ 8 × 1010M⊙) andcompactness (Re ≤∼ 1.5 kpc in g, r and i bands) criteria are applied, a visual inspection of thecandidates plus near-infrared photometry from VIKING-DR1 are used to refine our sample.The final catalogue, to be spectroscopically confirmed, consists of 92 systems in the redshiftrange z ~ 0.2-0.7. This sample, which we expect to increase by a factor of 10 over the totalsurvey area, represents the first attempt to select massive supercompact ETGs (MSCGs) inKiDS. We investigate the impact of redshift systematics in the selection, finding that thisseems to be a major source of contamination in our sample. A preliminary analysis shows thatMSCGs exhibit negative internal colour gradients, consistent with a passive evolution of thesesystems.We find that the number density of MSCGs is only mildly consistent with predictionsfrom simulations at z ≥ 0.2, while no such system is found at z ≤ 0.2.