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 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 vLSR=-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 vLSR=-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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- December 2001
- DOI:
- 10.1086/338050
- arXiv:
- arXiv:astro-ph/0110279
- Bibcode:
- 2001ApJ...562L.181R
- Keywords:
-
- Galaxy: Halo;
- ISM: Abundances;
- ISM: Clouds;
- Galaxies: Quasars: Absorption Lines;
- Galaxies: Quasars: Individual: Name: Fairall 9;
- Galaxies: Quasars: Individual: Alphanumeric: PG 1259+593;
- Astrophysics
- E-Print:
- 12 pages, 3 figures