Physical conditions in giant extragalactic H II regions
Giant Extragalactic H II Regions (GEHRs) in nearby galaxies have been observed at both radio and optical wavelengths. Narrow band optical imaging, large and small aperture optical spectrophotometry, radio continuum imaging at (lamda)6 cm and (lamda)21 cm, and (lamda)21 cm H I spectral line mapping have been employed to investigate the point to point variations of the physical conditions within and surrounding the GEHRs.Measurement of reddening and extinction were found to be strongly dependent on resolution. Variations in extinction as measured by the optical method were not found to correlate with those measured with the optical-radio method. In at least one case this was due to the presence of a nonthermal radio continuum source.Significant differences in optical line strength ratios exist between different points in a single GEHR. These differences point to variations of the physical conditions within the GEHR. The results of standard abundance analysis techniques are therefore dependent on the size and position of the observing aperture. Derived oxygen abundances are quite consistent throughout a single GEHR, but nitrogen abundances show significantly large variations.A supernova remnant (SNR) has been discovered in NGC 5471. Nonthermal radio continuum emission detected in other GEHRs makes it likely that there are SNRs in these regions also. It has been shown that, although SNRs may be difficult to detect in GEHRs, their presence can significantly alter the results of abundance analyses.Wolf-Rayet stars are found to be an important component of the stellar populations in only a small fraction of GEHRs.All GEHRs are found adjacent to large H I concentrations. Observations of the neutral hydrogen gas surrounding GEHRs reveals a correlation between H I surface density and velocity dispersion. The velocity dispersion in the adjacent neutral gas is comparable to the velocity dispersion in the GEHR. This strongly favors a gravitational explanation for the anomalously broad emission line widths observed in GEHRs.
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