Astronomy and Astrophysics – Astrophysics
Scientific paper
2001-04-17
Astronomy and Astrophysics
Astrophysics
5 pages, A&A in press (scheduled for June/July issue). Preprint and related papers also available at http://www.xray.mpe.mpg.d
Scientific paper
10.1051/0004-6361:20010404
NGC 4151 is one of the brightest and best-studied Seyfert galaxies. Here, we present a two-phase model of the narrow-line region (NLR) of NGC 4151. This study is motivated by (i) the fact that the X-ray spectrum of NGC 4151 is among the flattest known for Seyferts, and (ii) the recent Chandra detection of an X-ray narrow-line region in this galaxy. X-ray spectra as flat as that of NGC 4151 (Gamma_x = -1.5) are expected to favor the presence of two gas phases in pressure equilibrium (Krolik, McKee & Tarter 1981). In the present study, we show that a pronounced two-phase equilibrium develops in the extended emission-line region if we use the observed multi-wavelength spectrum of NGC 4151 to ionize the clouds. The material is stable to isobaric perturbations over a wide range of temperatures. We therefore propose that such a condition has arisen in the NLR of NGC 4151, and that it explains the detection of hot, extended X-ray gas which we identify as the NLR-cloud confining medium.
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