Detection of a large arc of ionized hydrogen far above the cassiopeia OB6 association: A superbubble blowout into the galactic halo?

The Wisconsin H alpha Mapper Northern Sky Survey has revealed a loop of H I I reaching 1300 pc from the Galactic midplane above the Cassiopeia OB6 asso ciation in the Perseus spiral arm. This enormous feature surrounds and exte nds far above the "W4 chimney" identified by Normandeau, Taylor, and Dewdne y 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 suggest s that past episodes of massive star formation have cleared the H I from an enormous volume above the Perseus arm, allowing Lyman continuum photons fr om O stars near the Galactic midplane to reach into the halo.