Molecular Gas within the Milky Way's Nuclear Wind
Abstract
We report the first direct detection of molecular hydrogen associated with the Galactic nuclear wind. The Far-Ultraviolet Spectroscopic Explorer spectrum of LS 4825, a B1 Ib-II star at l,b = 1.67,-6.63 lying d = 9.9+1.4-0.8 kpc from the Sun, 1 kpc below the Galactic plane near the Galactic Center, shows two high-velocity H2 components at vLSR = -79 and -108 km s-1. In contrast, the FUSE spectrum of the nearby (0.6 away) foreground star HD 167402 at d=4.9+0.8-0.7 kpc reveals no H2 absorption at these velocities. Over 60 lines of H2 from rotational levels J = 0 to 5 are identified in the high-velocity clouds. For the vLSR = -79 km s-1 cloud we measure total log N(H2) ≥ 16.75 cm-2, molecular fraction fH2 ≥ 0.8%, and T01 ≥ 97 and T25 ≤ 439 K for the ground- and excited-state rotational excitation temperatures. At vLSR = -108 km s-1, we measure log N(H2) = 16.13 0.10 cm-2, fH2 ≥ 0.5%, and T01 = 77+34-18 and T25 = 1092+149-117 K, for which the excited state ortho- to para-H2 is 1.0+0.3-0.1, much less than the equilibrium value of 3 expected for gas at this temperature. This non-equilibrium ratio suggests that the -108 km s-1 cloud has been recently excited and has not yet had time to equilibrate. As the LS 4825 sight line passes close by a tilted section of the Galactic disk, we propose that we are probing a boundary region where the nuclear wind is removing gas from the disk.
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