We show that the mass fraction of GMC gas ( n \gtrsim 100 { cm ^ { -3 } } ) in dense ( n \gg 10 ^ { 4 } { cm ^ { -3 } } ) star-forming clumps , observable in dense molecular tracers ( L _ { HCN } / L _ { CO ( 1 - 0 ) } ) , is a sensitive probe of the strength and mechanism ( s ) of stellar feedback , as well as the star formation efficiencies in the most dense gas .
Using high-resolution galaxy-scale simulations with pc-scale resolution and explicit models for feedback from radiation pressure , photoionization heating , stellar winds , and supernovae ( SNe ) , we make predictions for the dense molecular gas tracers as a function of GMC and galaxy properties and the efficiency of stellar feedback/star formation .
In models with weak/no feedback , much of the mass in GMCs collapses into dense sub-units , predicting L _ { HCN } / L _ { CO ( 1 - 0 ) } ratios order-of-magnitude larger than observed .
By contrast , models with feedback properties taken directly from stellar evolution calculations predict dense gas tracers in good agreement with observations .
Changing the strength or timing of SNe tends to move systems along , rather than off , the L _ { HCN } - L _ { CO } relation ( because SNe heat lower-density material , not the high-density gas ) .
Changing the strength of radiation pressure ( which acts efficiently in the highest density gas ) , however , has a much stronger effect on L _ { HCN } than on L _ { CO } .
We show that degeneracies between the strength of feedback , and efficiency of star formation on small scales , can be broken by the combination of dense gas , intermediate-density gas , and total SFR tracers , and favor models where the galaxy-integrated star formation efficiency in dense gas is low .
We also predict that the fraction of dense gas ( L _ { HCN } / L _ { CO ( 1 - 0 ) } ) increases with increasing GMC surface density ; this drives a trend in L _ { HCN } / L _ { CO ( 1 - 0 ) } with SFR and luminosity which has tentatively been observed .
Our results make specific predictions for enhancements in the dense gas tracers in unusually dense environments such as ULIRGs and galactic nuclei ( including the galactic center ) .