Discovery of High-Velocity H$\alpha$ Emission in the Direction of the Fermi Bubble

Wisconsin H-Alpha Mapper (WHAM) observations reveal high-velocity and [NII]$\lambda 6584$ emission lines in the same direction and velocity as ultraviolet absorption-line features that have been previously associated with the biconical gamma-ray lobes known as the Fermi Bubbles. We measure an extinction-corrected intensity of $I_{\text{H}\alpha }=0.84_{-0.09}^{+0.10}$ Rayleigh for emission with line center $v_{\text{LSR}}=-221\pm 3\text{ km }{\text{s}}^{-1}$, corresponding to an emission measure of $EM=2.00_{-0.63}^{+0.64}{\text{cm}}^{-6}\text{ pc}$. This emission arises at the same velocity as Hubble Space Telescope/Cosmic Origins Spectrograph observations of ultraviolet absorption features detected in the PDS 456 quasar sight line that passes through the northern Bubble near $l={10}^{\circ }.4,b=+{11}^{\circ }.2$. We estimate the total column density of ionized gas in this velocity component to be $N(H^{+}) = \left(3.28 \pm 0.33\right) \times 10^{18}~$\text{cm}$^{-2}$. The comparison of ionized gas emission and absorption yields an estimate for the characteristic density of $n_{e,c}=1.8\pm 0.6{\text{cm}}^{-3}$ and a characteristic length of $L_{c} =0.56 \pm 0.21~$\text{pc}$$assuming$30%$ solar metallicity. For a temperature of $T_e=8500_{-2600}^{+2700}$ K—consistent with the measured line widths and [NII]/H$\alpha$ line ratio—the gas has a thermal pressure of $p\mathrm{/}k=32,000_{-14,000}^{+15,000}{\text{cm}}^{-3}\text{ K}$. Assuming the gas is $\sim 6.5$ kpc distant, the derived density and pressure appear to be anomalously high for gas $\sim 1.3$ kpc above the Galactic midplane. The large thermal pressure is comparable to both a hot halo or Fermi Bubble model, but suggest that the H$\alpha$ arises in an overpressurized zone.