GHRS Monitoring of the Outflowing Material in NGC 4151

We present the results of a program to monitor the absorption lines in the spectrum of the Seyfert 1 galaxy NGC 4151 caused by outflowing gas from the nucleus. Spectral observations were taken in the UV with the GHRS and FOS of HST. GHRS G160M and G270M exposures were taken of the wavelength regions centered on C IV and Mg II, respectively, extending over a nearly 4 yr period. Fits to the observations at all epochs but one require at least eight distinct components spanning a range of outflow velocities from 0 to ~1600 km s-1 with respect to the nucleus. (NGC 4151 has a redshift of ~1000 km s-1). Although we see subtle changes in GHRS spectra of the broader of the absorption features, the wavelength constancy of all the features is remarkable. The limits on the secular acceleration suggest that either (1) The absorbing clouds are well beyond the broad emission line region; (2) The clouds are experiencing significant drag from an intercloud medium; or (3) We are observing an extremely complex flow pattern rather than the motions of individual clouds. The exception to this constancy occurred during one of the epochs of our monitoring when a broad shallow C IV trough appeared at an outflow velocity of 3750 km s-1 and then subsequently disappeared. An archival G160M exposure taken by Ulrich et al. was studied in order to measure the population of fine structure levels of Si II via the lines at 1526 and 1533 Å. The ratio of the upper to lower state appears to be less than the ratio of statistical weights, though additional observations are needed to secure this. This ratio is used to set upper limits on the electron density and lower limits on the distance of one of the absorbing clouds. Simultaneous FOS observations, covering a wider range of lines, but at a lower resolution, show significant changes in the Si IV absorption profile. We discuss the NGC 4151 absorption phenomenon in light of other observations of “intrinsic” absorption systems in both Seyfert nuclei and quasars.