Molecular Hydrogen in High-Velocity Clouds

Abstract

We present Far Ultraviolet Spectroscopic Explorer observations of interstellar molecular hydrogen (H\r 2 ) in two Galactic high-velocity clouds (HVCs). Molecular hydrogen absorption is detected in the\r Magellanic Stream (abundance ~0.3 solar) toward the Seyfert galaxy Fairall 9 in the lowest three\r rotational states ( J = 0-2) at v LSR = +190 km s -1 , yielding a total H 2 column density of log N\r (H 2 ) = 16.40 ##IMG## [http://ej.iop.org/images/1538-4357/562/2/L181/img1.gif] {img1.gif} . In\r contrast, no H 2 absorption is seen in the HVC Complex C (abundance ~0.1 solar) toward the quasar PG\r 1259+593 [log N (H 2 ) ≤ 13.96 at v LSR = -130 km s -1 ], although both HVCs have similar H I column\r densities on the order of log N (H I ) ≈ 20. Weak H 2 absorption is detected in the\r Intermediate-Velocity Arch (IV Arch; abundance ~1.0 solar) toward PG 1259+593 [log N (H 2 ) = 14.10\r ##IMG## [http://ej.iop.org/images/1538-4357/562/2/L181/img2.gif] {img2.gif} at v LSR = -55 km s -1\r and log N (H I ) = 19.5]. It thus appears that metal- and dust-poor halo clouds like Complex C are\r not able to form and maintain widely distributed H 2 , whereas metal- and dust-rich halo clouds like\r the IV Arch can maintain H 2 even at low H I column densities.