Trends observed in galaxies , such as the Gao & Solomon relation , suggest a linear relation between the star formation rate and the mass of dense gas available for star formation .
Validation of such relations requires the establishment of reliable methods to trace the dense gas in galaxies .
One frequent assumption is that the HCN ( J = 1 –0 ) transition is unambiguously associated with gas at { } H _ { 2 } densities \gg { } 10 ^ { 4 } ~ { } { } cm ^ { -3 } .
If so , the mass of gas at densities \gg { } 10 ^ { 4 } ~ { } { } cm ^ { -3 } could be inferred from the luminosity of this emission line , L _ { { } HCN ( 1 \text { - - } 0 ) } .
Here we use observations of the Orion A molecular cloud to show that the HCN ( J = 1 –0 ) line traces much lower densities \sim { } 10 ^ { 3 } ~ { } { } cm ^ { -3 } in cold sections of this molecular cloud , corresponding to visual extinctions A _ { V } \approx { } 6 ~ { } { } mag .
We also find that cold and dense gas in a cloud like Orion produces too little HCN emission to explain L _ { { } HCN ( 1 \text { - - } 0 ) } in star–forming galaxies , suggesting that galaxies might contain a hitherto unknown source of HCN emission .
In our sample of molecules observed at frequencies near 100 GHz ( also including { } ^ { 12 } CO , { } ^ { 13 } CO , { } C ^ { 18 } O , CN , and CCH ) , { } N _ { 2 } H ^ { + } is the only species clearly associated with rather dense gas .