Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992aas...181.5401s&link_type=abstract
American Astronomical Society, 181st AAS Meeting, #54.01D; Bulletin of the American Astronomical Society, Vol. 24, p.1210
Astronomy and Astrophysics
Astronomy
Scientific paper
An optical study has been made of the H II region populations in three galaxies, M101, M51 and NGC 4449. Emission line imagery has allowed several identification techniques to be used to spatially select, and to derive emission line properties for, 625 H II regions in M101, 465 in M51 and 163 in NGC 4449, making this the most complete study of its kind to date. Several trends have been discovered concerning the properties of the HII regions with radial position within their galaxy. It has been known for some time that M101 exhibits large gradients in excitation, and oxygen abundance; as well as a gradient in the line-of-sight reddening. We find that these gradients have a `knee' at an intermediate radius, as well as being a strong function of the surface brightness used to delimit the regions. No positional variation in the derived ionization parameter for each H II region was found. Possible local variations in the effective collapse density for neutral gas have been detected for both M101 and M51. M51 shows systematic emission variations only in the brightest cores of its largest H II regions, an effect attributed to a larger influence of the local ISM on the properties of the fainter, and more obscured, H II regions. M51 exhibits a spiral pattern that does not follow a single mathematical description, with the most pronounced departure occuring around the corotation radius. A possible variation in the evolutionary time from peak local compression to peak star formation with radius has been detected for only one of the two arms in the galaxy. NGC 4449 displays no sytematic variations in the derived emission properties of its H II region population since it is driven by a star formation mechanism that is independent of the radial ordinate, in direct contrast to the spiral density wave mechanism dominant in spiral systems. Each of the galaxies exhibits a different level of blending between the H II region structures and the warm ionized phase of the ISM, allowing the study of the differences in the local structure in each galaxy.
Dufour Reginald James
Hester Jeff J.
Scowen Paul A.
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