A comparison of the physical conditions in nuclear, hotspot, and disk H II regions

The physical properties of H II regions situated in galactic nuclei, in regions immediately surrounding the nuclei ('hotspots'), and in the disks are compared, using optical spectrophotometry and radio-continuum data. The integrated properties (size, H-alpha luminosity) of the nuclei and hotspots are similar to those of the brightest disk H II regions, but important differences are found in the emission-line spectra, internal structure, and radio properties. Roughly half of the H II region and starburst nuclei show evidence for a secondary ionization component, either an active nucleus or large-scale shocks. This feature is seen both in the line ratios and line profiles. This supports the idea that many spiral nuclei are composite in nature, with a central LINER or Seyfert-like nucleus surrounded by star-forming regions. 76 refs