Compact galaxies and the size-mass galaxy distribution from a colour-selected sample at 0.04 < z < 0.15 supplemented by ugrizYJHK photometric redshifts

The size-mass galaxy distribution is a key diagnostic for galaxy evolution. Massive compact galaxies are of particular interest as potential surviving relics of a high-redshift phase of star formation. Some compact galaxies at low redshift could be nearly unresolved in SDSS imaging and thus not included in galaxy samples. To overcome this, a sample was selected from the 9-band combination of SDSS and UKIDSS photometry to r 0.06). A sample of 163186 galaxies was obtained with 0.04 10, log S_1.5 > 10.5). The spectroscopic completeness was 76% for compact galaxies compared to 92% for normal-size galaxies. This difference is primarily attributed to SDSS fibre collisions and not the completeness of the main galaxy sample selection. Using environmental overdensities, this confirms that compact quiescent galaxies are significantly more likely to be found in high-density environments compared to normal-size galaxies. By comparison with a high-redshift sample from 3D-HST, log S_1.5 distribution functions show significant evolution, with this being a compelling way to compare with simulations such as EAGLE. The number density of compact quiescent galaxies drops by a factor of about 30 from z ~ 2 to log (n / Mpc^-3) = -5.3 +- 0.4 in the SDSS-UKIDSS sample. The uncertainty is dominated by the steep cut off in log S_1.5, which is demonstrated conclusively using this complete sample