A FUSE Survey of Interstellar Molecular Hydrogen toward High-Latitude AGNs
Abstract
We report results from a Far Ultraviolet Spectroscopic Explorer (FUSE) survey of interstellar molecular hydrogen (H2) along 45 sight lines to AGNs at high Galactic latitudes (b>20deg). Most (39 out of 45) of the sight lines show detectable Galactic H2 absorption from Lyman and Werner bands between 1000 and 1126 Å, with column densities ranging from NH2=1014.17 to 1019.82 cm-2. In the northern Galactic hemisphere, we identify many regions of low NH2 (<=1015 cm-2) between l=60deg and 180° and at b>54deg. These ``H2 holes'' provide valuable, uncontaminated sight lines for extragalactic UV spectroscopy, and a few may be related to the ``Northern Chimney'' (low Na I absorption) and the ``Lockman Hole'' (low NHI). A comparison of high-latitude H2 with 139 OB star sight lines surveyed in the Galactic disk suggests that high-latitude and disk H2 clouds may have different rates of heating, cooling, and UV excitation. For rotational states J=0 and 1, the mean excitation temperature at high latitude, <Thl01>=124+/-8 K, is somewhat higher than that in the Galactic disk, <Tdisk01>=86+/-20 K. For J>=2, the mean <Texc>=498+/-28 K, and the column-density ratios, N(3)/N(1), N(4)/N(0), and N(4)/N(2), indicate a comparable degree of UV excitation in the disk and low halo for sight lines with NH2>=1018 cm-2. The distribution of molecular fractions at high latitude shows a transition at lower total hydrogen column density (logNhlH~20.38+/-0.13) than in the Galactic disk (logNdiskH~20.7). If the UV radiation fields are similar in disk and low halo, this suggests an enhanced H2 (dust-catalyzed) formation rate in higher density, compressed clouds, which could be detectable as high-latitude, sheetlike infrared cirrus.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- January 2006
- DOI:
- 10.1086/498053
- arXiv:
- arXiv:astro-ph/0507581
- Bibcode:
- 2006ApJ...636..891G
- Keywords:
-
- ISM: Clouds;
- ISM: Molecules;
- Ultraviolet: ISM;
- Astrophysics
- E-Print:
- 38 pages, 11 figures, submitted to ApJ