Surface density effects in quenching: cause or effect?

Understand

There are very strong observed correlations between the specific star-formation rates (sSFR) of galaxies and their mean surface mass densities, {\Sigma}, as well as other aspects of their internal structure.

• These strong correlations have often been taken to indicate that the internal structure of a galaxy must play a major physical role, directly or indirectly, in the control of star-formation.
• In this paper we show by means of a very simple toy model that these correlations can arise naturally without any such physical role once the observed evolution of the size-mass relation for star-forming galaxies is taken into account.
• In particular, the model reproduces the sharp threshold in {\Sigma} between galaxies that are star-forming and those that are quenched, and the evolution of this threshold with redshift.