A far Ultraviolet Spectroscopic Explorer survey of molecular hydrogen in intermediate-velocity clouds in the Milky Way halo

Far Ultraviolet Spectroscopic Explorer (FUSE) data are used to investigate the molecular hydrogen (H-2) content of intermediate-velocity clouds (IVCs) in the lower halo of the Milky Way. We analyze interstellar absorption toward 56 (mostly extragalactic) background sources to study H-2 absorption in the Lyman and Werner bands in 61 IVC components at H I column densities greater than or equal to10(19) cm(-2). For data with good signal-to-noise ratio (S/N) (similar to9 per resolution element and higher), H-2 in IVC gas is convincingly detected in 14 cases at column densities varying between similar to10(14) and similar to10(17) cm(-2). We find an additional 17 possible H-2 detections in IVCs in FUSE spectra with lower S/N. The molecular hydrogen fractions, f, vary between 10(-6) and 10(-3), implying a dense, mostly neutral gas phase that is probably related to the cold neutral medium (CNM) in these clouds. If the H-2 stays in formation-dissociation equilibrium, the CNM in these clouds can be characterized by compact (Dsimilar to0.1 pc) laments with volume densities on the order of n(H)similar to30 cm(-3). The relatively high detection rate of H-2 in IVC gas implies that the CNM in these clouds is ubiquitous. More dense regions with much higher molecular fractions may exist, but it would be difficult to detect them in absorption because of their small size.