IPAC Colloquia Y. Gao The dense molecular gas and star formation laws in galaxies

We first show that the disk-averaged surface density of dense molecular gas (traced by HCN) has the tightest linear correlation with that of star formation rate (SFR), using a large sample of ~150 galaxies including (ultra)luminous IR galaxies [(U)LIRGs]. This is consistent with the tight, linear FIR and HCN correlation found globally in galaxies. The traditional Kennicutt-Schmidt law that relates the total gas and SFR appears to have no unique power-law slope as the correlation slopes change from ~1 for normal spirals to ~1.5 when more and more (U)LIRGs are included. New observations using other dense gas tracers, such as multi-J CS lines, as well as high-J CO lines from Herschel SPIRE/FTS observations, further reveal that the SFR and dense gas relationship is linearly in all galaxies observed so far. Additionally, the locally resolved SFR-HCN correlation in M51 is also roughly consistent with the globally established linear SFR-HCN correlation. All of these suggest that SFR depends linearly upon the mass of dense molecular gas.