Discovery of High-velocity Hα Emission in the Direction of the Fermi Bubble

Wisconsin H-Alpha Mapper observations reveal high-velocity Hα and [N II]λ6584 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 ${I}_{{\rm{H}}\alpha }={0.84}_{-0.09}^{+0.10}\,\mathrm{Rayleigh}$ for emission with line center v_LSR = -221 ± 3 km s^-1, corresponding to an emission measure of $\mathrm{EM}={2.00}_{-0.63}^{+0.64}\,{\mathrm{cm}}^{-6}\,\mathrm{pc}$ . 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°4, b = +11°2. We estimate the total column density of ionized gas in this velocity component to be $N({{\rm{H}}}^{+})=\left(3.28\pm 0.33\right)\times {10}^{18}\,{\mathrm{cm}}^{-2}$ . The comparison of ionized gas emission and absorption yields an estimate for the characteristic density of n_e,c = 1.8 ± 0.6 cm^-3 and a characteristic length of L_c = 0.56 ± 0.21 pc assuming 30% solar metallicity. For a temperature of ${T}_{e}={8500}_{-2600}^{+2700}\,{\rm{K}}$ —consistent with the measured line widths and [N II]/Hα line ratio—the gas has a thermal pressure of $p/k={{\rm{32,000}}}_{-14,000}^{+15,000}\,{\mathrm{cm}}^{-3}\,{\rm{K}}$ . Assuming the gas is ∼6.5 kpc distant, the derived density and pressure appear to be anomalously high for gas ∼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 Hα arises in an overpressurized zone.