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.