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Y. Gao (IPAC/Caltech), P. Solomon (SUNY at Stony Brook)
We report the results of a large survey of HCN(1-0) emission from a wide range of more than ~60 galaxies (Gao & Solomon 2001a,b). HCN emission, traces dense molecular gas (n(H2 ) > 104 cm-3), typically found in the cores of giant molecular clouds with active high mass star formation while CO emission traces gas at lower densities (n(H2 ) > 200 cm-3). We find that the HCN line luminosity (L\rm HCN) is strongly correlated with the IR luminosity (L\rm IR) of galaxies over 4 orders of magnitude up to and including ultraluminous IR galaxies (ULIRGs) with L\rm IR > 1012 L\odot, which confirms an earlier result based on a much smaller sample (Solomon et al. 1992). The correlation is linear and is substantially stronger than the well known CO--IR correlation which has more scatter and breaks down above 1011 L\odot. After normalizing to CO luminosity (L\rm CO), there is still a strong correlation between L\rm IR/L\rm CO and L\rm HCN/L\rm CO. The results demonstrate that the star formation rate which produces the IR luminosity depends linearly on the mass of dense molecular gas and not on the total molecular gas or the total HI gas mass. ULIRGs are powered by star formation and have the same rate of star formation per solar mass of dense molecular gas as ordinary spiral galaxies.
The author(s) of this abstract have provided an email address for comments about the abstract: gao@ipac.caltech.edu