Astronomy and Astrophysics – Astronomy
Scientific paper
Nov 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993aj....106.1797h&link_type=abstract
Astronomical Journal (ISSN 0004-6256), vol. 106, no. 5, p. 1797-1811
Astronomy and Astrophysics
Astronomy
158
Gas Composition, H Alpha Line, H Ii Regions, Ionized Gases, Irregular Galaxies, Star Formation, Charge Coupled Devices, Galactic Structure, Image Analysis, Massive Stars, Shell Galaxies
Scientific paper
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 greater than or equal 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.
Gallagher John S. III
Hawley Walter N.
Hunter Deidre Ann
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