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

Baldry, Sullivan, Rani & Turner 2021, MNRAS, 500, 1557. (link: ADS)

Abstract

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 SDSS imaging and thus not included in galaxy samples. To overcome this, a sample was selected from the combination of SDSS and 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% outliers (Delta ln(1+z) > 0.06). A sample of 163186 galaxies was obtained with 0.04 < z < 0.15 over 2300 deg2 using a combination of spec-z and photo-z. Following Barro et al., log Sigma1.5 = log M* - 1.5 log r50,maj was used to define compactness. The spectroscopic completeness was 76% for compact galaxies (log Sigma1.5 > 10.5) 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 Sigma1.5 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 Sigma1.5, which is demonstrated conclusively using this complete sample.

Data:

  • SDSS and UKIDSS selected sample over an extended redshift range 0.002 < z < 0.35: Description File, FITS binary table.
  • Environmental measurements for the SDSS and UKIDSS selected sample over 0.04 < z < 0.15: Description File, FITS binary table.
  • Compact galaxies (including flag 0/1 for star-forming/quenched): CSV File.

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