same factor. Thus, even ULIRG-like conditions could not generate the low observed HCN/CO luminosity ratios of post-starburst galaxies. The low HCN/CO luminosity ratios in the post-starburst targets are thus likely due to low dense gas mass ratios.

What prevents the CO-traced gas from further collapse? {#sec:lowdensegas}
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The low observed upper limits on the HCN-traced gas present a puzzle when coupled with the observations of significant CO-traced gas: what prevents the CO-traced gas from further collapse to denser gas? In order to explain the low dense gas mass ratios, some mechanism must prevent collapse of the CO-traced gas, affecting the ability of dense gas to form. Here, we explore possible mechanisms to prevent this collapse.

One possibility for inhibiting collapse is if kinetic energy is injected into the gas, rendering it stable from gravitational collapse. An example of this process is “morphological quenching" [@Martig2009; @Martig2013], where the increased shear in early type galaxies stabilizes the gas from gravitational collapse, decreasing the dense gas fraction and inhibiting star formation. We note that while the post-starbursts have a low dense gas fraction compared to the full set of comparison galaxies we consider, they may not have an especially low dense gas fraction 