EDP Sciences
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Volume 417, Number 2, April II 2004
Page(s) 515 - 525
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20035714

A&A 417, 515-525 (2004)
DOI: 10.1051/0004-6361:20035714

The unusual emission line spectrum of I Zw 1

M.-P. Véron-Cetty1, M. Joly2 and P. Véron1

1  Observatoire de Haute Provence, CNRS, 04870 Saint-Michel l'Observatoire, France
    e-mail: philippe.veron@oamp.fr
2  Observatoire de Paris-Meudon, 5 place J. Janssen, 92195 Meudon, France
    e-mail: Monique.Joly@obspm.fr

(Received 20 November 2003 / Accepted 17 December 2003)

Most Seyfert 1 galaxies show strong Fe II lines in their spectrum having the velocity and width of the broad emission lines. To remove the Fe II contribution in these objects, an accurate template is necessary. We used very high signal-to-noise, medium resolution archive optical spectra of I Zw 1 to build such a template. I Zw 1 is a bright narrow-line Seyfert 1 galaxy. As such it is well suited for a detailed analysis of its emission line spectrum. Furthermore it is known to have a very peculiar spectrum with, in addition to the usual broad and narrow line regions, two emission regions emitting broad and blue shifted [O III] lines making it a peculiarly interesting object. While analysing the spectra, we found that the narrow-line region is, unlike the NLR of most Seyfert 1 galaxies, a very low excitation region dominated by both permitted and forbidden Fe II lines. It is very similar to the emission spectrum of a blob in $\eta$ Carinae which is a low temperature ( $T_{\rm e}\sim6500$ K), relatively high density ( $N_{\rm e}=10^{6}$ cm -3) cloud. The Fe II lines in this cloud are mainly due to pumping via the stellar continuum radiation field (Verner et al. 2002). We did not succeed in modelling the spectrum of the broad-line region, and we suggest that a non radiative heating mechanism increases the temperature in the excited H I region, thus providing the necessary additional excitation of the Fe II lines. For the low-excitation narrow-line region, we are able to apply boundaries to the physical conditions accounting for the forbidden and permitted Fe II lines (10 $^{6}<N_{\rm e}<10^{7}$ cm -3; 10 -6 <U<10-5).

Key words: galaxies: active -- galaxies: individual: I Zw 1

Offprint request: M.-P. Véron-Cetty, mira.veron@oamp.fr

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