Title:

Dense Gas and Star Formation Rates in Galaxies



Phil Solomon
Astronomy Program
SUNY Stony Brook




Abstract:

Results are presented on a large survey of HCN emission from 65
spiral galaxies. including normal spirals, luminous and
ultraluminous galaxies.  HCN emission is a tracer of dense
molecular gas associated with star forming GMC cores, the sites
of star formation. I analyze the relationship between the total
far IR luminosity a tracer of the star formation rate, the HCN
line luminosity a tracer of the mass of dense gas, and the CO
luminosity a tracer of the total molecular content. There is a
tight linear correlation between the HCN luminosity and the far
IR luminosity over 3 orders of magnitude ( unlike the weaker and
nonlinear IR-Co relation) The direct consequence of the linear
relation is that the star formation law in terms of dense
molecular gas has a power law index of 1.0. The global star
formation rate in these galaxies is linearly proportional to the
amount of dense gas in normal spirals luminous and ultraluminous
galaxies, the most luminous objects in the local universe. This
is strong evidence for star formation as the power source in
ultraluminous galaxies. HCN emission is a molecular tracer
directly related to star formation.