2001a, “Detection of a Large Arc of Ionized Hydrogen Far Above the Cassiopeia OB6 Association: A Superbubble Blowout into the Galactic Halo?,” Astrophys

The Wisconsin Ha Mapper Northern Sky Survey has revealed a loop of H ii reaching 1300 pc from the Galactic midplane above the Cassiopeia OB6 association in the Perseus spiral arm. This enormous feature surrounds and extends far above the “W4 chimney ” identified by Normandeau, Taylor, and Dewdney 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