Molecular Hydrogen in High-Velocity Clouds

We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations ofinterstellar molecular hydrogen (H_2) 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 threerotational states (J=0-2) at v(LSR)=+190 km/s, yielding a total H_2 columndensity of log N(H_2)=16.40(+0.26)(-0.53). In contrast, no H_2 absorption isseen in the high-velocity cloud Complex C (abundance ~0.1 solar) toward thequasar PG 1259+593 (log N(H_2)<13.96 at v(LSR)=-130 km/s) although both HVCshave similar HI column densities on the order of log N(HI)~20. Weak H_2absorption is detected in the Intermediate-Velocity Arch (IV Arch; abundance\~1.0 solar) toward PG 1259+593 (log N(H_2)=14.10(+0.21)(-0.44) at v(LSR)=-55km/s and log N(HI)=19.5). It thus appears that metal- and dust-poor halo cloudslike Complex C are not able to form and maintain widely distributed H_2,whereas metal and dust-rich halo clouds like the IV Arch can maintain H_2 evenat low HI column densities.Comment: 12 pages, 3 figures; Accepted for publication in ApJ Letter