High Dispersion IUE Observations of NGC 3918

Extract

Our results are based on SWP 3215, a 120 min large aperture exposure. The electron density derived from N IV), Si III) and C III) ratios are log N_e < 4.3, 5.0 and 3.5. The measured resonance line ratio 1239/1243 of N V is 1.2 ± 0.1 and 1549/1551 of C IV is 1.9 ± 0.2. Both ratios are expected to be 2.0. Furthermore, it had already been reported that the C IV and N V resonance lines are fainter than predicted from models by factors from ~ 4 to ~ 6 (Torres-Peimbert et al., 1980, The First Two Years of IUE, NASA CP-2171, 641). Internal dust cannot account for any significant deviation from 2 for the resonance doublet ratio of N V (Hummer and Kunasz, 1980, Ap. J., 236, 609). However, the anomalous ratio of N V could be explained by a generally distributed hot ionized medium compatible with the C IV absorption found in approximately the same direction by Cowie et al. (1981, Ap. J., 248, 528).