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 potential surviving relics of a high-redshift phase of star formation. Some of these could be nearly unresolved in Sloan Digital Sky Survey (SDSS) imaging and thus not included in galaxy samples. To overcome this, a sample was selected from the combination of SDSS and UKIRT Infrared Deep Sky Survey (UKIDSS) photometry to r < 17.8. This was done using colour-colour selection, and then by obtaining accurate photometric redshifts (photo-z) using scaled flux matching (SFM). Compared to spectroscopic redshifts (spec-z), SFM obtained a 1 sigma scatter of 0.0125 with only 0.3 per cent outliers (vertical bar Delta ln (1 + z)vertical bar > 0.06). A sample of 163 186 galaxies was obtained with 0.04 < z < 0.15 over 2300 deg(2) using a combination of spec-z and photo-z. Following Barro et al. log Sigma(1.5) = logM(*) - 1.5log r(50, maj) was used to define compactness. The spectroscopic completeness was 76 per cent for compact galaxies (log Sigma(1.5) > 10.5) compared to 92 per cent for normal-sized 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-sized galaxies. By comparison with a high-redshift sample from 3D-HST, log Sigma(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 similar to 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 Sigma(1.)5, which is demonstrated conclusively using this complete sample.

Automated summary

The study selected a sample of galaxies using a combination of SDSS and UKIDSS photometry and accurately determined photometric redshifts. The research confirmed that compact quiescent galaxies are more likely to be found in high-density environments. Comparing with high-redshift samples, it was found that the number density of compact quiescent galaxies shows significant evolution over time.