Molecular Hydrogen in High-Velocity Clouds

Abstract

We present Far Ultraviolet Spectroscopic Explorer 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 km s-1, yielding a total H2 column density of log N(H2) = 16.40. In contrast, no H2 absorption is seen in the HVC Complex C (abundance ~0.1 solar) toward the quasar PG 1259+593 [log N(H2) ≤ 13.96 at vLSR = -130 km s-1], 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 PG 1259+593 [log N(H2) = 14.10 at vLSR = -55 km s-1 and log N(H I) = 19.5]. It thus appears that metal- and 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.