A review is presented of ISO observations of molecular hydrogen, H2, toward various Galactic source types, such as shocks and photon dominated regions. In so doing I examine the similarities and differences in the H2 spectrum found under these different excitation conditions and mechanisms, and how the observations impact on some of the latest models.

Introduction

Before the launch of the Infrared Space Observatory (ISO, Kessler et al. 1996) observations of H2 were restricted to a hot component with an excitation temperature of about 2000 K in shock excited sources, and a non-thermally (i.e. fluorescently) excited component in photon dominated regions (PDRs). These components were typically probed with the 1–0 S(1) line at 2.12 µm as well as several other near-infrared ro-vibrational transitions, and in some cases pure rotational transitions from high J levels (e.g. Gredel 1994, Knacke & Young 1981). Only a few observations, principally toward the Orion star forming region, of lower energy pure rotational transitions existed, e.g. the 0–0 S(2) and 0–0 S(1) lines at 12.2786 and 17.0348 µm, but which already pointed to the existence of a lower temperature component in such sources (e.g. Beck, Lacy & Geballe 1979; Parmar, Lacy & Achtermann 1991, 1994; Richter et al. 1995; Burton & Haas 1997).