ar X iv : a st ro - p h / 96 10 04 3 v 1 7 O ct 1 99 6 Resolving the C IV and Mg II Absorption in NGC 4151

Observations of the C iv and Mg ii absorption lines in the Seyfert 1 galaxy NGC 4151 obtained with the GHRS in October 1994 are presented. The data from the STScI archive show multiple broad and narrow components in both species. In addition to Galactic absorption, four narrow and four broad systems associated with NGC 4151 are identified. Two broad systems dominate the total equivalent width, and their mean blueshift and width are comparable to the broad Lyman line and continuum absorption seen in far-UV spectra from the Hopkins Ultraviolet Telescope. Narrow-line C iv emission is present on the red side of the broadest absorption trough, and narrow absorption at the systemic velocity of NGC 4151, presumably in its own ISM, absorbs the core of the narrow emission line. Strong Mg ii absorption is present in all but two velocity systems. Ratios relative to the corresponding C iv components suggest a low ionization parameter for the absorbing gas: U ∼ 1−3×10. This makes none of the identified UV absorption systems a good candidate for association with the warm X-ray absorbing gas. 1. UV and X-ray Absorption in NGC 4151 A persistent problem in understanding the absorbing material in NGC 4151 has been reconciling the vastly different gas columns inferred for the X-ray absorption and for the UV absorption. The X-ray absorbing column varies between 10 and 10 cm. Bromage et al. (1985) estimated a total column for the UV-absorbing material of no more than ∼ 10 cm. The neutral hydrogen column is variable (Kriss et al. 1995). The bulk of the absorption is in low column density gas with NH ∼ 10 18 cm and Doppler parameter b ∼ 300 km s. Any low-b component has a neutral column no greater than 5× 10 cm. One possibility for reconciling these differences has been the recent success of warm absorber models for characterizing the X-ray absorption and the associated UV absorption lines in 3C 351 and NGC 5548 (Mathur et al. 1994; Mathur et al. 1995). In such models the absorption arises in gas photoionized by the central engine (e.g., Netzer 1993; Krolik & Kriss 1995). The X-ray absorption is dominated by highly ionized species of heavy ions (e.g., O vii and O viii). The total gas columns can be quite high (10–10 cm), with relatively low columns in the lower ionization species responsible for the UV absorption. Warm absorber models with a reflection component can fit the X-ray spectrum of NGC 4151 (Weaver et al. 1994a,b). Kriss et al. (1995) find that