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| Paper: |
FUSE Survey of Interstellar Molecular Hydrogen Toward 45 High-Latitude AGN |
| Volume: |
348, Astrophysics in the Far Ultraviolet: Five Years of Discovery with FUSE |
| Page: |
439 |
| Authors: |
Gillmon, K.; Shull, J.M.; Danforth, C.; Tumlinson, J. |
| Abstract: |
We report results from a Far Ultraviolet Spectroscopic Explorer (FUSE) survey of interstellar molecular hydrogen (H2) along 45 sightlines to background AGN at high Galactic latitudes (|b| > 20°). The high-latitude sightlines are effective at probing diffuse gas in the low Galactic halo, including large-scale gaseous structures. FUSE spectra of 87% (39 of 45) of the observed AGN show detectable Galactic H2 absorption from the Lyman and Werner bands between 920 and 1126 Å, with detected column densities ranging from N(H2) = 1014.17−19.82 cm−2. Our survey is sensitive to N(H2) > 1013.8−14.6 cm−2, depending on the S/N (2—11 per pixel) and spectral resolution (R = 15,000 − 20,000). The broad range of column densities indicates large fluctuations in the spatial structure of high-latitude H2 clouds, particularly at b > 54°. In the northern hemisphere, we identify many regions of low N(H2) (≤ 1015 cm−2) between (l = 60° − 180° and b > 54°). These “H2 holes” may be related to the “Northern Chimney ” (region of low Na I absorption, Lallement et al. 2003) and the “Lockman Hole ” (region of low NHI, Lockman et al. 1986). A comparison of these high-latitude results with those from our FUSE survey of ∼ 130 OB-star sightlines in the Galactic disk suggests that that halo and disk clouds have different physical properties. The mean rotational temperature, {Thalo01} = 125±15 K, is higher than that found in the Galactic disk, {Tdisk01} = 86±20 K. Because this H2 temperature is elevated compared to gas in the Milky Way disk (Savage et al. 1977; Shull et al. 2004) or in the LMC/SMC (Tumlinson et al. 2002), it may imply enhanced heating or reduced cooling rates in Galactic halo diffuse H2 clouds. The transition to appreciable molecular fractions, fH2 ≥ 0.01, in our halo sample occurs at a lower total hydrogen column density (log N Hhalo = 20.35) than in the Galactic disk (log NHdisk = 20.7), possibly indicating reduced H2 photodissociation rates offsetting a lower formation rate on dust grains in low-metallicity gas. By looking for correlations between high-latitude H2 detections and IRAS maps of infrared cirrus clouds, we hope to further characterize the halo clouds and explain why they differ from disk clouds. |
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