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

Abstract

Abstract 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 for emission with line center v LSR = −221 ± 3 km s−1, corresponding to an emission measure of . 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 . 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 —consistent with the measured line widths and [N ii]/Hα line ratio—the gas has a thermal pressure of . 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.