Understanding Interstellar Molecular Hydrogen: Unraveling the Message of its Spectrum

Atomic hydrogen and helium are by far the most abundant elements in the Universe and there is probably more mass in molecular hydrogen than in any other species. Therefore the simplest neutral molecule plays a significant role in a wide variety of astrophysical environments. However, the scarcity of both accurate and complete data sets for rovibrational inelastic cross sections and rate coefficients has set a serious limitation regarding the development of many reliable astrophysical models. In particular, the investigation of cooling processes, molecular emissioIl1 and non equilibrium effects in nlOlecular gaseous nebulae and other molecular environnlents requires collisional excitation data for H, He, and Hz impacting the dominant molecular species (e.g. Hz, HD, and CO). These collisional processes play important roles in starburst galaxies, the formation of the first stars, the study of high-redshift damped Lyman-a systems, and galactic molecular clouds and star-forming regions, for example. It is well known that the uncertainties in the existing collisional rate coefficient data can be significant. In addition, within the next few years, the observational investigation of such environments will be significantly improved owing to the ongoing operation of NASA's Spitzer Infrared Telescope Facility, and other upcoming infrared astrophysics missions such as SOFIA, the James Webb Space Telescope, Herschel, and Astro-F. To extract the maximum scientific return from the unprecedented observations that