Space telescope imaging spectrograph echelle observations of the Seyfert galaxy NGC 4151: Physical conditions in the ultraviolet absorbers

We have examined the physical conditions in intrinsic UV-absorbing gas in the Seyfert galaxy NGC 4151, using echelle spectra obtained with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope on 1999 July 19. We confirm the presence of the kinematic components detected in earlier Goddard High Resolution Spectrograph (GHRS) observations, all of which appear to be out-flowing from the nucleus, as well as a new broad absorption feature at a radial velocity of -1680 km s(-1). The UV continuum of NGC 4151 was a factor of about 4 lower than in observations taken over the previous 2 yr, and we argue that the changes in the column density of the low-ionization absorption lines associated with the broad component at -490 km s(-1) reflect the decrease in the ionizing flux. Most of the strong absorption lines (e.g., N V, C IV, Si IV, etc.) from this component are saturated but show substantial residual flux in their cores, indicating that the absorber does not fully cover the source of emission. Our interpretation is that the unocculted light is due to scattering by free electrons from an extended region, which reflects continuum, emission lines, and absorption lines. For the first time in such a study, we have been able to constrain the densities for this kinematic component and several others based on the strength of absorption lines from metastable states of C III and Fe II and/or the ratios of ground and fine structure lines of O I, CII, and Si II. We have generated a set of photoionization models that successfully match not only the ionic column densities for each component during the present low-flux state but also those seen in previous high-flux states with the GHRS and STIS, confirming that the absorbers are photoionized and respond to the changes in the continuum flux. Based on the model parameters (ionization parameter and density), we have been able to map the relative radial positions of the absorbers. We find that the absorbing gas decreases in density with distance. Finally, none of the UV absorbers is of sufficiently large column density or high enough ionization state to account for the observed X-ray absorption, while the scatterer is too highly ionized. Hence, the X-ray absorption must arise in a separate component of circumnuclear gas.