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Astron. Astrophys. 330, 873-880 (1998)

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Radial brightness profiles of extragalactic H II regions

M. Rozas 1, H.O. Castañeda 1, 2 and J.E. Beckman 1

1 Instituto de Astrofísica de Canarias, E-38200 La Laguna, Tenerife, Spain
2 ISO, Satellite Tracking Station, E-28080 Madrid, Spain

Received 12 November 1996 / Accepted 11 August 1997


We present a systematic study of the radial brightness profiles in H [FORMULA] of a set of H II regions in 6 late-type spiral galaxies, complete down to stated limiting luminosities. We have sufficient linear resolution in our observations to be able to fit models in which the effects of the radial fall off in density and in mean degree of ionization can be distinguished. We find a sharp break in the plot of internal radial brightness gradient against luminosity at a luminosity in H [FORMULA] of 1038.6 erg/s in all these objects. For lower luminosities the gradients are almost invariant, showing only a slight tendency to steepen with luminosity; at this critical value there is a jump to higher gradients, and at higher luminosities the gradients steepen progressively. This is consistent with the regions showing a transition from ionization-bounded to density-bounded behaviour, in conformity with evidence from H II region luminosity functions presented in previous studies.

Key words: ISM: H ii regions – galaxies: individual (NGC 157, NGC 3631, NGC 4321, NGC 6814, NGC 6951, NGC 7479) – galaxies: spiral

Send offprint requests to: J.E. Beckman

© European Southern Observatory (ESO) 1998

Online publication: January 27, 1998