Molecular Gas within the Milky Way's Nuclear Wind

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${}_{-0.8}^{+1.4}$ 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${}_{-0.7}^{+0.8}$ 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 ${f}_{{{\rm{H}}}_{2}}$ ≥ 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, ${f}_{{{\rm{H}}}_{2}}$ ≥ 0.5%, and T01 = 77${}_{-18}^{+34}$ and T25 = 1092${}_{-117}^{+149}$ K, for which the excited-state ortho- to para-H2 is ${1.0}_{-0.1}^{+0.3}$, much less than the equilibrium value of 3 expected for gas at this temperature. This nonequilibrium 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.