2020ApJ...899L..11K

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_Hα=0.84_–0.09^+0.10Rayleigh for emission with line center v_LSR = -221 ± 3 km s^–1, corresponding to an emission measure of EM=2.00_–0.63^+0.64cm^–6pc. 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(H⁺)=(3.28±0.33)×10¹⁸cm^–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⁺²⁷⁰⁰K-consistent with the measured line widths and [N II]/Hα line ratio-the gas has a thermal pressure of p/k=32,000_–14,000^+15,000cm^–3K. 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.