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For refcode 1993AJ....106.1797H:
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1993AJ....106.1797H A SURVEY FOR EXTRA-H II REGION IONIZED GAS STRUCTURES IN IRREGULAR GALAXIES DEIDRE A. HUNTER Lowell Observatory, 1400 W. Mars Hill Rd., Flagstaff, Arizona 86001 Electronic mail: dah@lowell.edu WALTER N. HAWLEY St. Paul's School, Concord, New Hampshire 03301 and Lowell Observatory, 1400 W. Mars Hill Rd., Flagstaff, Arizona 86001 JOHN S. GALLAGHER III Department of Astronomy, University of Wisconsin, 475 N. Charter St., Madison, Wisconsin 53706 Electronic mail: gallagher%madraf.decnet@macc.wisc.edu Received 1993 April 28; revised 1993 July 14 ABSTRACT We have conducted a deep H{alpha} imaging survey of 51 irregular and amorphous galaxies with the objective of searching for large ionized gas structures outside of normal H II regions. In this sample 12% of the galaxies contain at least one ionized supershell (radius > 300 pc), 24% contain supergiant ionized filaments which are not obviously connected with a particular star forming region, and 27% contain one or more of these types of structures. For the most part, large ionized gas structures are found in galaxies that are engaging in intense star formation or that contain at least one unusually large concentration of massive stars. Thus, in most galaxies these structures are likely to have been produced by massive stars. However, there is a large range in the properties of the host galaxies, and not all galaxies that are actively forming stars or that contain luminous H II regions also contain supershells or giant filaments of ionized gas. As giant H II regions appear to be more common than large scale ionized features, we statistically find either that not all giant H II regions will produce supershells or giant filaments or that the extra-H II region structures have a shorter lifetime than the giant H II regions themselves. Two galaxies are particularly noteworthy. DDO 75 is a small galaxy with a low level of star formation activity but which nevertheless hosts several remarkably large ionized supershells. We cannot explain the origin and ionization of these structures. By contrast, DDO 50 has a neutral interstellar medium which contains numerous holes but which has no extraordinary ionized gas structures. The most likely explanations are either that the H I structures in DDO 50 are older than the lives of the massive stars that produced them or that the H I holes were formed by a process not directly associated with concentrations of massive stars. Only 2 out of 15 normal irregulars viewed at inclination angles greater than 60^deg^, plus two galaxies that are clearly in an abnormal state, show evidence for structures that are or could be extending out of the disk of the galaxy. In three of these systems the filaments which are farthest from the disks are roughly parallel to the galactic planes, and therefore have the morphologies expected of stalled supershells. Thus, galactic disk breakouts or chimneys containing optically visible ionized gas cannot be a frequent phenomenon in irregular galaxies and, even when they do occur, we cannot assume that a galactic wind is also present
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