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\section{Introduction}  Molecular gas is the hosts of all known star formation in the local and distant Universe. The average properties of the star formation process point to roughly constant star formation rate per unit free fall time \citep[e.g.,][]{Krumholz_2007}. However, there is emerging evidence, particularly dense environments of variations in the molecular gas depletion timescale ($\tau_{\mathrm{dep}} \equiv \Sigma_{\mathrm{H2}}\Sigma_{\mathrm{H2}}$, where $\Sigma_{\mathrm{H2}}$ is the molecular gas surface density and $\Sigma_{\mathrm{H2}}$ is the star formation rate surface density). Recent work in the local \cite{Usero_2015,Bigiel_2015,Pereira_Santaella_2016,Bigiel_2016} and high-redshift \citep{Genzel_2015,Aravena_2016,Scoville_2016}  universe point to significant variation in depletion times in times, converging to  the sense that higher density environments have shorter depeletion times times. Whether the changes in depletion time reflect a different mode of star formation or a continuous variation along a continuum \citep[e.g.,][]{Krumholz_2012} remains unsettled.  \begin{itemize}