Glitters of warm H2 in cold diffuse molecular gas

Cold molecular hydrogen, a possibly dominant gas fraction in galaxies, does not radiate due to the symmetry and small moment of inertia of the molecule. The only tracers of cold H2, the rotational lines of CO and dust thermal emission operate only in metal-rich environments. By detecting the lowest rotational lines of H2 at unexpected levels in cold diffuse medium, ISO-SWS has challenged the traditional view of the interstellar medium (ISM) by possibly revealing the transient existence of tiny fractions of gas, disseminated within the cold ISM, and warm enough to excite the H2 lines. The heating source of H2 there is the intermittent dissipation of supersonic turbulence, pervading the entire ISM. These glitters of H2 line emission may become the unique tracers of cold H2 in low metallicity environments. Given the fundamental importance of probing large hidden masses of gas in galaxies, for their implication on galaxy dynamics, star formation thresholds in metal-poor environments, and the hypothesis of H2 as baryonic dark matter in galaxies, the present SST/IRS proposal is dedicated to further search and characterization of this still elusive emission. The proposed observations consist of 27 small maps (55 by 45 arcsec) of the S(0), S(1) and S(2) H2 lines at high spectral resolution in the Milky Way and in external galaxies with massive HI disks extending far beyond their optical radius, for which the HI rotation curve cannot be accounted for by the stellar and visible gas components. The goals of the proposal are to strenghten the existence of the warm H2 pockets disseminated in the cold diffuse medium, further characterize the warm H2 emission as new tracer of hitherto undetected amounts of cold H2 by observations of nearby low metallicity environments, investigate the spatial distribution of unseen cold H2 in the external parts of galaxies, and, as a more exploratory facet, probe the presence of large amounts of baryonic dark matter in galaxies in the form of cold molecular hydrogen