NEBULAR PROPERTIES AND THE IONIZING-RADIATION FIELD IN THE GALACTIC-CENTER

Nebulosity in the central parsec of the Milky Way exhibits a low-ioniz ation spectrum that has led previous analyses of this region to conclu de that this material is photoionized by a relatively soft continuum. We have reanalyzed the infrared emission-line spectrum of the Galactic center in order to consider whether the data could actually be explai ned with photoionization by a relatively hard, yet dilute continuum, a nd additionally to constrain the properties of the radiating plasma. W e conclude that the composite infrared spectrum does not place strong restrictions on the nature of the ionizing continuum and that much of the ionized gas in the Galactic center is probably quite dense (greate r-than-or-similar-to 10(5) cm-3). If the ionizing spectral energy dist ribution is, in fact, relatively hard, this material additionally must be highly clumped. Indications that the ionizing continuum is probabl y fairly soft are provided by radio recombination-line studies, howeve r. Shocks are unlikely to be important for generating the observed neb ulosity in light of the large far-infrared continuum luminosity of the central parsec. The infrared fine-structure spectrum provides mixed e vidence for enhanced heavy-element abundances, and enrichment is proba bly limited to less-than-or-similar-to 2 times solar levels. We predic t the optical spectrum of the Galactic center and conclude that it pro bably appears as an H II nucleus to external observers. Comparisons be tween the nucleus of the Milky Way and nuclei of external galaxies rem ain uncertain, however, since the parsec-scale metric apertures typica lly employed for studies of Galactic-center nebular emission are 1-2 o rders of magnitude smaller than those used in extragalactic measuremen ts.