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

We present Infrared Space Observatory (ISO) Short-Wavelength Spectrometer (SWS) and ISOPHOT-S spectroscopy⁴ of the Seyfert galaxy NGC 4151. We detect a total of 17 fine-structure emission lines emitted by a wide range of low- and high-excitation ions, two rotational lines of molecular hydrogen, and the Brβ H I line. We find that the mid-IR fine-structure line profiles display blue asymmetries that are very similar to those observed in the optical lines produced in the narrow-line region. Because the mid-infrared lines are much less sensitive to extinction than the optical lines, this similarity places strong constraints on scenarios that have been invoked to explain the optical line asymmetries. For example, we are able to rule out the simplest radial motion plus dust scenarios for the production of the line asymmetries. Our preferred model is of a central, geometrically thin but optically thick, obscuring screen of subarcsecond extension, enclosing a total hydrogen gas mass of≳ 5 x 10⁶ M⊙. This mass may be molecular. The weakness of polycyclic aromatic hydrocarbon (PAH) emission features in the lowresolution spectrum is evidence that star formation plays a minor role in the circumnuclear region of NGC 4151. In a companion paper, we use the rich set of mid-infrared lines to determine the obscured photoionizing continuum produced by the active galactic nucleus.