2006ApJ...636..891G

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>20°). Most (39 out of 45) of the sight lines show detectable Galactic H₂absorption from Lyman and Werner bands between 1000 and 1126 Å, with column densities ranging from N_H2=10^14.17 to 10^19.82/cm2. In the northern Galactic hemisphere, we identify many regions of low N_H2(≤10¹⁵/cm2) between l=60° and 180° and at b>54°. These ``H<sub>2</sub>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_HI_). A comparison of high-latitude H₂with 139 OB star sight lines surveyed in the Galactic disk suggests that high-latitude and disk H₂clouds 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^hl₀₁≥124±8 K, is somewhat higher than that in the Galactic disk, <T^disk₀₁≥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_H2≥10¹⁸/cm2. The distribution of molecular fractions at high latitude shows a transition at lower total hydrogen column density (logN^hl_H~20.38±0.13) than in the Galactic disk (logN^disk_H~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.