Near‐Infrared Molecular Hydrogen Emission from the Central Regions of Galaxies: Regulated Physical Conditions in the Interstellar Medium

The central regions of many interacting and early-type spiral galaxies areactively forming stars. This process affects the physical and chemicalproperties of the local interstellar medium as well as the evolution of thegalaxies. We observed near-infrared H2 emission lines: v=1-0 S(1), 3-2 S(3),1-0 S(0), and 2-1 S(1) from the central ~1 kpc regions of the archetypicalstarburst galaxies, M82 and NGC 253, and the less dramatic but still vigorouslystar-forming galaxies, NGC 6946 and IC 342. Like the far-infrared continuumluminosity, the near-infrared H2 emission luminosity can directly trace theamount of star formation activity because the H2 emission lines arise from theinteraction between hot and young stars and nearby neutral clouds. The observedH2 line ratios show that both thermal and non-thermal excitation areresponsible for the emission lines, but that the great majority of thenear-infrared H2 line emission in these galaxies arises from energy statesexcited by ultraviolet fluorescence. The derived physical conditions, e.g.,far-ultraviolet radiation field and gas density, from [C II] and [O I] linesand far-infrared continuum observations when used as inputs tophotodissociation models, also explain the luminosity of the observed H2 v=1-0S(1) line. The ratio of the H2 v=1-0 S(1) line to far-IR continuum luminosityis remarkably constant over a broad range of galaxy luminosities; L_H2/L_FIR =about 10^{-5}, in normal late-type galaxies (including the Galactic center), innearby starburst galaxies, and in luminous IR galaxies (LIRGs: L_FIR > 10^{11}L_sun). Examining this constant ratio in the context of photodissociationregion models, we conclude that it implies that the strength of the incident UVfield on typical molecular clouds follows the gas density at the cloud surface.