  In order to investigate the importance of different proposed quenching
mechanisms, we use an indirect method to estimate gas masses for ~62,000 SDSS
DR7 galaxies. We infer gas surface densities from dust column densities as
traced by extinction within the fibre, applying a metallicity correction to
account for varying dust-to-gas ratios. We find that both gas fraction and star
formation efficiency (SFE) decrease moving away from the star forming main
sequence (MS) towards quiescence for all galaxy masses. We further show that
both quantities correlate similarly strongly with the departure from the MS,
implying the need for any physical model of quenching to invoke a change in
$\textit{both}$ gas fraction and SFE. Our results call for a better
understanding of the physical processes driving the decrease in star formation
efficiency, which has received relatively little attention in the theory of
quenching until now.
