AFUSESurvey of Interstellar Molecular Hydrogen toward High‐Latitude AGNs

We report results from a Far Ultraviolet Spectroscopic Explorer ( FUSE ) survey of interstellar molecular hydrogen (H 2 ) along 45 sight lines to AGNs at high Galactic latitudes ( > 20°). Most (39 out of 45) of the sight lines show detectable Galactic H 2 absorption from Lyman and Werner bands between 1000 and 1126 Å, with column densities ranging from N = 10 14.17 to 10 19.82 cm -2 . In the northern Galactic hemisphere, we identify many regions of low N (≤10 15 cm -2 ) between l = 60° and 180° and at b > 54°. These "H 2 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 N ). A comparison of high-latitude H 2 with 139 OB star sight lines surveyed in the Galactic disk suggests that high-latitude and disk H 2 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, ⟨ T ⟩ = 124 ± 8 K, is somewhat higher than that in the Galactic disk, ⟨ T ⟩ = 86 ± 20 K. For J ≥ 2, the mean ⟨ T exc ⟩ = 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 N ≥ 10 18 cm -2 . The distribution of molecular fractions at high latitude shows a transition at lower total hydrogen column density (log N ≈ 20.38 ± 0.13) than in the Galactic disk (log N ≈ 20.7). If the UV radiation fields are similar in disk and low halo, this suggests an enhanced H 2 (dust-catalyzed) formation rate in higher density, compressed clouds, which could be detectable as high-latitude, sheetlike infrared cirrus.