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 ${}_{-0.8}^{+1.4}$ 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 ${f}_{{{\rm{H}}}_{2}}$ ≥ 0.8%, and T ₀₁ ≥ 97 and T ₂₅ ≤ 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, ${f}_{{{\rm{H}}}_{2}}$ ≥ 0.5%, and T ₀₁ = 77 ${}_{-18}^{+34}$ and T ₂₅ = 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.