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The size-mass galaxy distribution is a key diagnostic for galaxy evolution.Massive compact galaxies are potential surviving relics of a high-redshiftphase of star formation. Some of these could be nearly unresolved in SDSSimaging and thus not included in galaxy samples. To overcome this, a sample wasselected from the combination of SDSS and UKIDSS photometry to r<17.8. This wasdone using colour-colour selection, and then by obtaining accurate photometricredshifts (photo-z) using scaled flux matching (SFM). Compared to spectroscopicredshifts (spec-z), SFM obtained a 1-sigma scatter of 0.0125 with only 0.3%outliers (Delta:ln(1+z)>0.06). A sample of 163186 galaxies was obtained with0.04<z<0.15 over 2300 sq.deg. using a combination of spec-z and photo-z.Following Barro et al., log:Sigma_1.5=log:M_*-1.5log:reff was used to definecompactness. The spectroscopic completeness was 76% for compact galaxies(log:Sigma_1.5>10.5) compared to 92% for normal-size galaxies. This differenceis primarily attributed to SDSS `fibre collisions' and not the completeness ofthe main galaxy sample selection. Using environmental overdensities, thisconfirms that compact quiescent galaxies are significantly more likely to befound in high-density environments compared to normal-size galaxies. Bycomparison with a high-redshift sample from 3D-HST, log:Sigma_1.5 distributionfunctions show significant evolution, with this being a compelling way tocompare with simulations such as EAGLE. The number density of compact quiescentgalaxies drops by a factor of about 30 from z~2 to log(n/Mpc^-3)=-5.3+-0.4 inthe SDSS-UKIDSS sample. The uncertainty is dominated by the steep cut off inlog:Sigma_1.5, which is demonstrated conclusively using this complete sample.

monthly notices of the royal astronomical society

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