From the broadest perspective, the hydrogen molecule is found virtually everywhere in the Universe. Some issues concerning H2 in space clearly deserve more attention. For example, can traces of the formation process of H2 in the interstellar medium be observed? Is it possible for large quantities of very cold H2 to escape detection? How can H2 be used to probe gas at high redshift and in the centers of active galaxies?

Introduction

The hydrogen molecule plays myriad roles on the cosmic stage. During this conference, we have been reminded how the study of H2 ranges widely in space, time, and energy: from the microcosm of molecular processes to the giant molecular clouds, from the origin of structure in the early universe to places where a star will form tomorrow. We marvel at speculations about clumpuscules of H2 that might be as cold as 3 K yet contribute measurably to gamma radiation from the Galactic halo. In trying to offer a forward-looking perspective on H2 in space, it seems best to concentrate on a few topics where rapid progress in observation is taking place and where the interpretation of existing results is inadequate.

On the interpretation of astronomical spectra of H2

Dilute matter in space generally exists in a chemical and physical state far out of thermodynamic equilibrium. The state of such dilute matter reflects a competition among microscopic processes, which often operate in contact with several thermal reservoirs at different effective temperatures.