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

Abstract

Wisconsin H-Alpha Mapper (WHAM) observations reveal high-velocity and [NII]$\lambda6584$ 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 $I_{\textrm{H}\alpha}=0.84^{+0.10}_{-0.09}$ Rayleigh for emission with line center $v_\textrm{LSR}=-221\pm3~\textrm{km}~\textrm{s}^{-1}$, corresponding to an emission measure of $EM = 2.00^{+0.64}_{-0.63}~\textrm{cm}^{-6}~\textrm{pc}$. 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 $N(H^{+}) = \left(3.28 \pm 0.33\right) \times 10^{18}~\textrm{cm}^{-2}$. The comparison of ionized gas emission and absorption yields an estimate for the characteristic density of $n_{e,c} = 1.8 \pm 0.6~\textrm{cm}^{-3}$ and a characteristic length of $L_{c} =0.56 \pm 0.21~\textrm{pc}$ assuming $30\%$ solar metallicity. For a temperature of $T_{e}=8500^{+2700}_{-2600}$ K---consistent with the measured line widths and [NII]/H$\alpha$ line ratio---the gas has a thermal pressure of $p/k = 32,000^{+15,000}_{-14,000}~\textrm{cm}^{-3}~\textrm{K}$. 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.