A FUSE survey 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 (vertical bar b vertical bar > 20 degrees). Most ( 39 out of 45) of the sight lines show detectable Galactic H-2 absorption from Lyman and Werner bands between 1000 and 1126 Angstrom, with column densities ranging from N-H2 10(14.17) to 10(19.82) cm(-2). In the northern Galactic hemisphere, we identify many regions of low N-H2 (<= 10(15) cm(-2)) between l = 60 degrees and 180 degrees and at b > 54 degrees. 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-HI). A comparison of high-latitude H-2 with 139OB 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-01(hl)> = 124 +/- 8 K, is somewhat higher than that in the Galactic disk, < T-01(disk)> = 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 >= 10(18) cm(-2). The distribution of molecular fractions at high latitude shows a transition at lower total hydrogen column density (log N-H(hl) approximate to 20.38 +/- 0.13) than in the Galactic disk (log N-H(disk) approximate to 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.