E. Sturm 1, T. Alexander 2, D. Lutz 1, A. Sternberg 3, H. Netzer 3, & R. Genzel 1
1 Max-Planck-Institut für Extraterrestrische Physik (MPE), Postfach 1603, 85740 Garching, Germany
2 Institute for Advanced Study, Olden Lane, Princeton, NJ 08540, USA
3 Tel Aviv University, Ramat Aviv, Tel Aviv 69978, Israel
We present ISO-SWS and ISOPHOT-S spectroscopy of the Seyfert Galaxy NGC 4151, demonstrating the great value of ISO spectra as a powerful new tool for the analysis of active galactic nuclei (AGNs). We discuss two major topics: the physical conditions in the narrow line emitting region (NLR), and the spectral energy distribution (SED) of the ionizing source.
Gas density and temperature in the NLR of NGC 4151 can be estimated from line ratios of [Ne III], [S III] and other species in the ISO range. An optical- to-infrared comparison of emission line profiles puts strong constraints on models explaining the line profile asymmetries found in the NLRs of many Seyfert galaxies. For NGC 4151 it rules out simple radial-motion-plus-dust scenarios, and lets us predict a central, geometrically thin but optically thick, obscuring screen of tens of pc extension.
The ionizing SED is of particular interest in the ultraviolet, where many models predict the so-called 'Big Blue Bump', i.e. thermal radiation from an accreting disk around a central black hole. The SED is also important for modeling the effects of the AGN on its environment, since it is a critical input for photoionization models. Such modeling is a major tool in the study of AGN emission lines and in attempts to disentangle the contribution of the nuclear emission from that of starburst regions in ultra-luminous IR galaxies.
Unfortunately, the ulraviolet SED - and thus the blue bump - of AGN continua cannot be directly observed due to galactic and intrinsic absorption. However, many emission lines in the ISO range have ionization energies which cover exactly this gap in the observable SED. Hence, by fitting a photoionization model to the observed lines, it is possible to constrain the SED. We present a new method to determine the best fitting photoionizing continuum and emission line cloud model. For NGC 4151 we find a best-fit model - consisting of a clumpy, optically thick (ionization bounded) gas distribution - that reproduces the observed line fluxes, and which is consistent with the observed geometry of the optical NLR. The best fitting SED does not display a 'Big Blue Bump', in contrast to our detection of such a bump in the SED of the Circinus Galaxy. Our analysis suggests that in NGC 4151 such a component may be absorbed by material located between the NLR and the central ionizing source.