Molecular Hydrogen in High-Velocity Clouds
Abstract
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 v(LSR)=+190 km/s, 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 PG 1259+593 (log N(H2)<13.96 at v(LSR)=-130 km/s) although both HVCs have similar HI column densities on the order of log N(HI)~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(+0.21)(-0.44) at v(LSR)=-55 km/s and log N(HI)=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 HI column densities.
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