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]$\lambda 6584$ 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 $p/k=32,000_{-14,000}^{+15,000}{\text{cm}}^{-3}\text{K}$. 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.

arXiv comment10 pages, 4 figures. in press at ApJ Letters

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