Nuclear Emission in Spiral Nebulae.

Abstract

Spectrograms of dispersion 37-200 A/mm have been obtained of six extragalactic nebulae with high- excitation nuclear emission lir~es superposed on a normal G-type spectrum. All the stronger emission lines from X 3727 to X 6731 found in planetaries like NGC 7027 appear in the spectra of the two brightest spirals observed, NGC 1068 and NGC 4151. Apparent relative intensities of the emission lines in the six spirals were reduced to true relative in- tensities. Color temperatures of the continua of each spiral were determined for this purpose. The observed relative intensities of the emission lines exhibit large variations from nebula to nebula. Profiles of the emission lines show that all the lines are broadened, presumably by Doppler motion, by - amounts varying up to 8500 km/sec for the total width of the hydrogen lines in NGC 3516 and NGC 7469. The hydrogen lines in NGC 4151 have relatively narrow cores with wide wings, 7500 km/sec in total breadth. Similar wings are found for the Balmer lines in NGC 7469. The lines of the other ions show no evidence of wide wings. Some of the lines exhibit strong asymmetries, usually in the sense that the violet side of the line is stronger than the red. In NGC 7469 the absorption K line of Ca ii is shallow and 50 A wide, at least twice as wide as in normal spirals. Absorption minima are found in six of the stronger emission lines in NGC 1068, in one line in NGC 4151, and one in NGC 7469. Evidence from measures of wave length and equivalent widths suggests that these absorption minima arise from the G-type spectra on which the emissions are superposed. The maximum width of the Balmer emission lines seems to increase with the absolute magnitude of the nucleus and with the ratio of the light in the nucleus to the total light of the nebula. The emission lines in the brightest diffuse nebulae in other extragalactic objects do not appear to have wide emission lines similar to those found in the nuclei of emission spirals