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 optical spectra resembling those of giant extragalactic H II regions. Such ''H II nuclei,'' powered by young, massive stars, are found in a substantial 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 disks. Similarities and differences between H II nuclei and luminous starburst nuclei are additionally noted. Nuclei in early-type disk galaxies (SO-SBc) on average have low excitation, and hence high oxygen abundance (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 (corresponding 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 to the higher efficiency with which bars can drive gaseous inflow in systems with prominent bulges. The early-type systems also have higher amounts of internal extinction and higher electron densities. The physical 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 abundances. H II nuclei systematically emit stronger low-ionization forbidden lines than disk H II regions, confirming a trend recognized by Kennicutt, Keel, & Blaha. We discuss several possibilities for the origin of the spectral variations.