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

The Wisconsin H-Alpha Mapper (WHAM) Northern Sky Survey is revealing thatmany intermediate-velocity (|v_LSR| <= 100 km/s) neutral clouds and complexeshave an associated ionized component. We present the first map of the H-Alphaemission from an intermediate- or high-velocity structure: Complex K. Thislarge, 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 andionized gas generally trace each other quite well in the complex, but thedetailed structure is not identical. In particular, the H-Alpha emission peaksin brightness at slightly higher Galactic longitudes than corresponding 21 cmfeatures. 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 complexare traced by WHAM down to about 0.1 Rayleighs (0.2 cm^-6 pc). Typical linewidths of the H-Alpha emission are ~ 30 km/s, limiting temperatures in theionized gas to < 20,000 K. If radiation is the primary ionizing mechanism, theLyman continuum flux required to sustain the most strongly emitting ionizedregions is 1.2 x 10^6 photons cm^-2 s^-1. There appears to be no local, stellarsource capable of maintaining the ionization of the gas; however, the requiredionizing flux is consistent with current models of the escape of Lymancontinuum radiation from OB stars in the disk and of ionizing radiationproduced by cooling supernova remnants.