Detection of a Large Arc of Ionized Hydrogen Far Above the Cas OB6 Association: A Superbubble Blowout into the Galactic Halo?

The Wisconsin Hα Mapper (WHAM) Northern Sky Survey has revealed a loop of H II reaching 1300 pc from the Galactic midplane above the Cas OB6 association in the Perseus sprial arm. This enormous feature surrounds and extends far above the “W4 Chimney” identified by Normandeau et al. and appears to be associated with the star formation activity near the W3/W4/W5 H II region complex. The existence of this ionized structure suggests that past episodes of massive star formation have cleared the H I from an enormous volume above the Perseus arm, allowing Lyman continuum photons from O stars near the Galactic midplane to reach into the halo. Subject headings: galaxies: ISM — Galaxy: halo — ISM:bubbles — ISM: general — ISM:HII regions — ISM:structure