HOPKINS ULTRAVIOLET TELESCOPE OBSERVATIONS OF THE FAR-ULTRAVIOLET SPECTRUM OF NGC-4151

We observed the far-ultraviolet spectrum of the Seyfert galaxy NGC 4151 from 912 to 1860 angstrom with the Hopkins Ultraviolet Telescope during the flight of Astro-1 aboard the space shuttle Columbia in 1990 December. Broad emission lines with full-width at half-maximum of approximately 8500 km s-1 dominate the spectrum. Numerous absorption features modify the continuum shape, particularly at wavelengths shortward of Ly-alpha. The continuum turns over sharply below 1000 angstrom and disappears by 924 angstrom, well above the redshifted Lyman edge of NGC 4151 at 915 angstrom. The continuum flux and the intensity of broad C IV lambda-1549 are slightly above the historical mean values observed with the International Ultraviolet Explorer. Many of the spectral features in the 912-1150 angstrom range have never before been seen in a low-redshift activity galaxy, including emission from O VI lambda-lambda-1032, 1037 and He II lambda-1085, and absorption by C III lambda-977, N III lambda-991, 0 VI lambda-lambda-1032, 1037, and the hydrogen Lyman series. The broad emission lines all have similar profiles, and their relative intensities are consistent with previously calculated photoionization models. The absorption lines have intrinsic widths of approximately 1000 km s-1 and are blueshifted relative to the systemic velocity of NGC 4151 by 200-1300 km s-1. The observed equivalent widths of the C III absorption lines lambda-977 and lambda-1176 require absorbing material with a density in excess of 10(9.5) cm-3. The equivalent widths of these same lines, the Si IV doublet lambda-lambda-1393, 1402, and the hydrogen Lyman lines all favor a Doppler parameter b congruent-to 200 km s-1 for the absorbing material. Absorption of the continuum by the converging higher order Lyrman lines explains the sharp turnover of the continuum below 1000 angstrom. The blueshifts of the absorption lines, their large intrinsic widths, and the. inferred high densities are all consistent with outflowing material originating in the broad-line region.