Title:

  Dense Molecular Gas and Global Star Formation in Galaxies


Abstract:

This talk explores the global relationships between the star formation
rate, indicated by the far-infrared (FIR) emission, the total molecular
gas, and the DENSE molecular gas content based on our HCN(1-0) survey 
from a wide range of 65 galaxies. HCN emission, traces dense molecular 
gas (n(H2) > 10^4 cm^-3), typically found in the cores of giant molecular 
clouds with active High Mass star formation whereas CO emission traces gas 
at lower densities (n(H2) > 200 cm^-3). We found that the HCN luminosity 
is strongly correlated with the FIR luminosity over 4 orders of magnitude 
up to and including ultraluminous IR galaxies. The correlation is linear 
and is substantially stronger than the conventional CO--IR correlation 
which has more scatters and breaks down at high luminosity end for
luminous IR galaxies. The results demonstrate that the star formation rate
may depend linearly on the mass of dense molecular gas, but not on the 
mass of total molecular gas. Finally, we revisit the global Schmidt law 
of star formation in galaxies in light of our new results.