2021ApJ...923L..11C

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 v_LSR = -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 v_LSR = -79 km s^–1 cloud we measure total log N(H₂) >= 16.75 cm^–2, molecular fraction f_H2_ >= 0.8%, and T₀₁ >= 97 and T₂₅ <= 439 K for the ground- and excited-state rotational excitation temperatures. At v_LSR = -108 km s^–1, we measure log N(H₂) = 16.13 ± 0.10 cm^–2, f_H2_ >= 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.