2001ApJ...562L.181R

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 v_LSR=+190 km.s^–1, yielding a total H₂ column density of logN(H₂)=16.40^+0.26_–0.53. In contrast, no H₂ absorption is seen in the HVC Complex C (abundance ∼0.1 solar) toward the quasar PG 1259+593 [logN(H₂)≤13.96 at v_LSR=-130 km.s^–1], although both HVCs have similar H I column densities on the order of logN(H I)~20. Weak H₂ absorption is detected in the Intermediate-Velocity Arch (IV Arch; abundance ∼1.0 solar) toward PG 1259+593 [logN(H₂)=14.10^+0.21_–0.44 at v_LSR=-55 km.s^–1 and logN(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.