Abstract

Molecular gas is the raw material for star formation and hence a crucial factor in galactic evolution. Ultraluminous infrared galaxies emit the bulk of their power in the far infrared, show disturbed morphologies indicative of recent mergers, and rival QSOs in their bolometric luminosities, but are more numerous in the local universe. Although they are as rich in molecular gas as the most gas rich normal spiral galaxies, they have elevated ratios of infrared luminosity to molecular mass that suggest they are undergoing bursts of very rapid and efficient star formation. A survey of HCN(1→0) emission from ten ultraluminous and normal galaxies shows far infrared emission correlates better with the amount of dense, n(H2)>104 cm−3, molecular gas than with the total amount of molecular gas. The star formation efficiency appears to depend on the fraction of the molecular gas reservoir at high density.

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