Infrared spectroscopy of NGC 4151: Infrared Space Observatory observationsand narrow emission line region line profiles

We present Infrared Space Observatory (ISO) Short-Wavelength Spectrometer ( SWS) and ISOPHOT-S spectroscopy(4) of the Seyfert galaxy NGC 4151. We detec t a total of 17 fine-structure emission lines emitted by a wide range of lo w- and high-excitation ions, two rotational lines of molecular hydrogen, an d the Br beta H I line. We find that the mid-IR fine-structure line profile s display blue asymmetries that are very similar to those observed in the o ptical lines produced in the narrow-line region. Because the mid-infrared l ines are much less sensitive to extinction than the optical lines, this sim ilarity places strong constraints on scenarios that have been invoked to ex plain the optical line asymmetries. For example, we are able to rule out th e simplest radial motion plus dust scenarios for the production of the line asymmetries. Our preferred model is of a central, geometrically thin but o ptically thick, obscuring screen of subarcsecond extension, enclosing a tot al hydrogen gas mass of greater than or similar to 5 x 10(6) M.. This mass may be molecular. The weakness of polycyclic aromatic hydrocarbon (PAH) emi ssion features in the low-resolution spectrum is evidence that star formati on plays a minor role in the circumnuclear region of NGC 4151. In a compani on paper, we use the rich set of mid-infrared lines to determine the obscur ed photoionizing continuum produced by the active galactic nucleus.