AFUSESurvey 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 ( > 20°). Most (39 out of 45) of the sight lines show detectable Galactic absorption from Lyman and Werner bands between 1000 and 1126 A, with column densities ranging from N = 1014.17 to 1019.82 cm-2. In the northern Galactic hemisphere, we identify many regions of low N (≤1015 cm-2) between l = 60° and 180° and at b > 54°. 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 N). A comparison of high-latitude with 139 OB star sight lines surveyed in the Galactic disk suggests that high-latitude and disk 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 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 N ≥ 1018 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 (dust-catalyzed) formation rate in higher density, compressed clouds, which could be detectable as high-latitude, sheetlike infrared cirrus.