H₂ Emission as a Diagnostic of Physical Processes in Starforming Galaxies

Summary

Observations and interpretation of extragalactic rotational and rovibrational H₂ emission are reviewed. Direct observations of H₂ lines do not trace bulk H₂ mass, but excitation rate. As such, the H₂ lines are unique diagnostics, if the excitation mechanism can be determined, which generally requires high-quality spectroscopy and suitable additional data. The diagnostic power of the H₂ lines is illustrated by two cases studies: H₂ purely rotational line emission from the disk of the nearby spiral galaxy NGC891 and high resolution imaging and spectroscopy of H₂ vibrational line emission from the luminous merger NGC6240.

Introduction

Direct observations of H₂ emission from external galaxies have become standard practice in the past decade through the revolution in ground-based near-infrared instrumentation. As a result, the near-infrared H₂ rovibrational lines are now readily detectable throughout the local universe (e.g., Moorwood & Oliva 1988, 1990; Puxley et al. 1988, 1990; Goldader et al. 1995, 1997; Vanzi et al. 1998). More recently, the Short Wavelength Spectrograph (SWS) on the Infrared Space Observatory (ISO) has for the first time allowed detection of the purely rotational H₂ lines in the mid-infrared spectral regime. For instance, the first detection (outside the solar system) of the H₂S(0) line at 28.21 µm was reported by Valentijn et al. (1996) from the star forming nucleus of the nearby spiral galaxy NGC 6946.