28 March 2008

Molecular Star-formation Rate Indicators in Galaxies

Desika Narayanan
(CfA)


We derive a physical model for the observed relations between star
formation rate (SFR) and molecular line (CO and HCN) emission in
galaxies, and show how these observed relations are reflective of the
underlying star formation law. We do this by combining 3D non-LTE
radiative transfer calculations with hydrodynamic simulations of
isolated disk galaxies and galaxy mergers. We demonstrate that the
observed SFR-molecular line relations are not necessarily
representative of a more direct tracer of the SFR. Rather, they are
driven by the relationship between molecular line emission and gas
density, and anchored by the index of the underlying Schmidt law. Our
model for SFR-molecular line relations quantitatively reproduces the
slopes of the observed SFR-CO (J=1-0), CO (J=3-2) and HCN (J=1-0)
relations when a Schmidt law with index of ~1.5 describes the SFR.  We
use these results to make imminently testable predictions for the
SFR-molecular line relations of unobserved transitions.