A Map of the Ionized Component of the Intermediate Velocity Cloud Complex K

Abstract

The Wisconsin H-Alpha Mapper (WHAM) Northern Sky Survey is revealing that many intermediate-velocity (|v_LSR| <= 100 km/s) neutral clouds and complexes have an associated ionized component. We present the first map of the H-Alpha emission from an intermediate- or high-velocity structure: Complex K. This large, high-latitude feature stretches from l = 10 to 70 degrees, b = +30 to +70 degrees and peaks in velocity over v_LSR ~ -60 to -80 km/s. The neutral and ionized gas generally trace each other quite well in the complex, but the detailed structure is not identical. In particular, the H-Alpha emission peaks in brightness at slightly higher Galactic longitudes than corresponding 21 cm features. The ionized gas has a peak H-Alpha intensity of 0.5 Rayleighs, corresponding to an emission measure of 1.1 cm^-6 pc. Structures in the complex are traced by WHAM down to about 0.1 Rayleighs (0.2 cm^-6 pc). Typical line widths of the H-Alpha emission are ~ 30 km/s, limiting temperatures in the ionized gas to < 20,000 K. If radiation is the primary ionizing mechanism, the Lyman continuum flux required to sustain the most strongly emitting ionized regions is 1.2 x 10^6 photons cm^-2 s^-1. There appears to be no local, stellar source capable of maintaining the ionization of the gas; however, the required ionizing flux is consistent with current models of the escape of Lyman continuum radiation from OB stars in the disk and of ionizing radiation produced by cooling supernova remnants.