1998/09/08 by Tal Alexander, Eckhard Sturm, Dieter Lutz +3 · 77 citations
Physics and Astronomy · #Active galactic nucleus #Astronomy and Astrophysical Research #Astrophysical Phenomena and Observations #Emission spectrum #Galaxies: Formation, Evolution, Phenomena #Galaxy #Infrared #Line (geometry) #Photoionization #Spectral energy distribution #Spectral line #Spectroscopy #astro-ph
paper · pdf · doi:10.1086/306751
published in The Astrophysical Journal 512(1), 204-223 (IOP Publishing) · aastex, 55 pages, 18 figures. Accepted by ApJ
arxiv created 1998/09/08 · openalex publication_date 1999/02/10 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/06
The Infrared Space Observatory ( ISO ) Short-Wavelength Spectrometer (SWS) infrared spectroscopic observations of the nucleus of Seyfert galaxy NGC 4151, which are described in a companion paper, are used in conjunction with a compilation of UV to IR narrow emission line data to determine the spectral shape of the obscured extreme-UV continuum that photoionizes the narrow-line-emitting gas in the active galactic nucleus (AGN). We present a new method for determining the best-fitting photoionizing continuum and emission-line cloud model from a heterogeneous set of emission-line data. For NGC 4151, we find a best-fit model that reproduces the observed line fluxes to within a factor of 2 on average, and that is consistent with the observed geometry of the optical narrow-line region (NLR). Our model consists of a clumpy, optically thick (ionization-bounded) gas distribution, with a hydrogen gas density of ~1000 cm -3 and a volume-filling factor of 6.510 -4 . Our best-fitting spectral energy distribution (SED) falls sharply beyond the Lyman limit and then rises sharply again toward 100 eV. In particular, it does not display the "big blue bump" signature of a hot accretion disk. We find that this SED, which best reproduces the NLR line emission on the 100-500 pc scale, does not have enough UV photons to produce the observed broad-line region (BLR) recombination emission from the inner 1 pc. This suggests that the BLR is photoionized by the intrinsic continuum source, which does have a strong UV component (perhaps a big blue bump), but that this UV component is absorbed by material located between the NLR and BLR. Our analysis suggests that the absorber consists of ~510 19 cm -2 of neutral hydrogen. Such an absorber has been independently detected by UV absorption lines. Using our new method, we confirm our previous conclusion that a big blue bump is present in the SED of the Seyfert 2 galaxy Circinus.