Molecular Hydrogen in High-Velocity Clouds

We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations of interstellar molecular hydrogen (H2) in two Galactic high-velocity clouds (HVCs). Molecular hydrogen absorption is detected in the Magellanic Stream (abundance ∼ 0.3 solar) toward the Seyfert galaxy Fairall 9 in the lowest three rotational states (J = 0− 2) at vLSR = +190 kms , yielding a total H2 column density of log N(H2)= 16.40 +0.26 −0.53. In contrast, no H2 absorption is seen in the high-velocity cloud Complex C (abundance ∼ 0.1 solar) toward the quasar PG1259+593 (log N(H2)≤ 13.96 at vLSR = −130 km s ), although both HVCs have similar H i column densities on the order of log N(H i)≈ 20. Weak H2 absorption is detected in the Intermediate-Velocity Arch (IV Arch; abundance ∼ 1.0 solar) toward PG1259+593 (log N(H2)= 14.10 +0.21 −0.44 at vLSR = −55 km s −1 and log N(H i) = 19.5). It thus appears that metaland dust-poor halo clouds like Complex C are not able to form and maintain widely distributed H2, whereas metal and dust-rich halo clouds like the IV Arch can maintain H2 even at low H i column densities. Subject headings: ISM: clouds – ISM: abundances – quasars: absorption lines – quasars: individual (Fairall 9, PG1259+593) – Galaxy: halo Department of Astronomy, University of Wisconsin-Madison, 475 N. Charter Street, Madison, WI 53706; [email protected] Department of Physics and Astronomy, Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD21218 Current address: Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD21218