PROPERTIES OF H-II REGIONS IN THE CENTERS OF NEARBY GALAXIES

As part of an optical spectroscopic survey of nearby bright galaxies, we have identified a sample of over 200 emission-line nuclei having op tical spectra resembling those of giant extragalactic H II regions. Su ch ''H II nuclei,'' powered by young, massive stars, are found in a su bstantial fraction of nearby galaxies, especially those of late Hubble type. This paper summarizes the observational characteristics of H II nuclei, considers the variation of their properties with Hubble type, and compares the nuclear H II regions with those found in galaxy disk s. Similarities and differences between H II nuclei and luminous starb urst nuclei are additionally noted. Nuclei in early-type disk galaxies (SO-SBc) on average have low excitation, and hence high oxygen abunda nce (from similar to 1.1 to 3.3 times the solar value), whereas those in late-type systems (Sc-IO) have excitations spanning a wide range (c orresponding to less than 0.25-3.5 times the solar oxygen abundance). The H alpha luminosities of early-type nuclei greatly exceed those of later types. The enhancement of massive star formation may be linked t o the higher efficiency with which bars can drive gaseous inflow in sy stems with prominent bulges. The early-type systems also have higher a mounts of internal extinction and higher electron densities. The physi cal properties of H II nuclei resemble those of giant H II regions in spiral disks in some ways, but they differ in several others. The two groups emit comparable H alpha luminosities and generally have similar electron densities. Because of their unique location in the galaxies, nuclear H II, regions are characterized by much higher oxygen abundan ces. H II nuclei systematically emit stronger low-ionization forbidden lines than disk H II regions, confirming a trend recognized by Kennic utt, Keel, & Blaha. We discuss several possibilities for the origin of the spectral variations.