A FUSE Survey of High‐Latitude Galactic Molecular Hydrogen

Abstract

Measurements of molecular hydrogen (H2) column densities are presented for the first six rotational levels (J = 0-5) for 73 extragalactic targets observed with the Far Ultraviolet Spectroscopic Explorer (FUSE). All of these have a final signal-to-noise ratio larger than 10 and are located at Galactic latitude |b| > 20°. The individual observations were calibrated with the FUSE calibration pipeline CalFUSE version 2.1 or higher and then carefully aligned in velocity. The final velocity shifts for all the FUSE segments are listed. H2 column densities or limits are determined for the six lowest rotational (J) levels for each H I component in the line of sight, using a curve-of-growth approach at low column densities (<16.5) and Voigt-profile fitting at higher column densities. Detections include 65 measurements of low-velocity H2 in the Galactic disk and lower halo. Eight sight lines yield nondetections for Galactic H2. The measured column densities range from log N(H2) = 14 to 20. Strong correlations are found between log N(H2) and T01, the excitation temperature of the H2, as well as between log N(H2) and the level population ratios (log[N(J')/N(J)]). The average fraction of nuclei in molecular hydrogen [f(H2)] in each sight line is calculated; however, because there are many H I clouds in each sight line, the physics of the transition from H I to H2 cannot be studied. Detections also include H2 in 16 intermediate-velocity clouds in the Galactic halo (out of 35 IVCs). Molecular hydrogen is seen in one high-velocity cloud (the Leading Arm of the Magellanic Stream), although 19 high-velocity clouds are intersected; this strongly suggests that dust is rare or absent in these objects. Finally, there are five detections of H2 in external galaxies.