H₂ and Infrared in Global Starburst Galaxies

Abstract

Far infrared measurements from the IRAS survey combined with estimates of the molecular gas content provide a fundamental basis for the analysis of the starburst phenomena in galaxies. When the ratio of far infrared luminosity to molecular gas mass significantly exceeds that in normal galaxies like the Milky Way (4 L_⊙ M_⊙⁻¹), star formation is occurring on a shorter timescale, possibly with an initial mass function-biased towards high mass stars. In the highest luminosity IRAS galaxies (L_(IR) ≥ 10¹¹ L_⊙), the luminosity to H₂ mass ratio is typically 40 L_⊙ M_⊙⁻¹, indicating star formation rates of 10- 100 M_⊙ yr⁻¹ and cycling times for the ISM much less than 10⁹ yr. In the very luminous infrared galaxies, the optical morphology almost invariably shows evidence of a strong galactic interaction and a substantial fraction of the total molecular gas content is seen at radii ~ 1 kpc. Dense molecular gas probably plays a pivotal role in the evolution of such dynamically disturbed systems: being dissipative, the gas can readily sink to the center of the interacting system where it may fuel a nuclear starburst and/or build up and fuel a central active nucleus. We show that the shape of the high luminosity end of the infrared galaxy luminosity function can be reproduced by a model in which normal spiral galaxies, represented in the Schecter function, undergo collision-induced starbursts. Statistics from the IRAS survey are consistent with the percentage of all spiral galaxies currently undergoing a global starburst being approximately 0.2% and the lifetime of the starburst being a dynamical time, approximately 2 x 10⁸ years. The present epoch rate is therefore such that 2% of all galaxies participate in a merger every 109 years and with standard cosmological evolution, nearly all galaxies would be undergoing such merger-induced starbursts at z= 1. Galactic merging and starburst activity must therefore play a central role in galactic evolution.

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