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${}^{\circ }$,$-$6.63${}^{\circ }$ lying $d$ = 9.9${}_{-0.8}^{+1.4}$ kpc from the Sun, $\sim$1 kpc below the Galactic plane near the Galactic Center, shows two high-velocity H${}_2$ components at $v_{\mathrm{LSR}}$ = $-79$ and $-108$ km s${}^{-1}$. In contrast, the FUSE spectrum of the nearby ($\sim$0.6${}^{\circ }$ away) foreground star HD 167402 at $d$=4.9${}_{-0.7}^{+0.8}$ kpc reveals no H${}_2$ absorption at these velocities. Over 60 lines of H${}_2$ from rotational levels $J$ = 0 to 5 are identified in the high-velocity clouds. For the $v_{\mathrm{LSR}}$ = $-79$ km s${}^{-1}$ cloud we measure total log $N$(H${}_2$) $\ge$ 16.75 cm${}^{-2}$, molecular fraction $f_{{\mathrm{H}}_2}$ $\ge$ 0.8%, and $T_{01}$ $\ge$ 97 and $T_{25}$ $\le$ 439 K for the ground- and excited-state rotational excitation temperatures. At $v_{\mathrm{LSR}}$ = $-108$ km s${}^{-1}$, we measure log $N$(H${}_2$) = 16.13 $\pm$ 0.10 cm${}^{-2}$, $f_{{\mathrm{H}}_2}$ $\ge$ 0.5%, and $T_{01}$ = 77${}_{-18}^{+34}$ and $T_{25}$ = 1092${}_{-117}^{+149}$ K, for which the excited state ortho- to para-H${}_2$ is 1.0${}_{-0.1}^{+0.3}$, 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.

arXiv commentAccepted to ApJ Letters, 11 pages, 4 figures, 1 table

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