2-D spectroscopy and modeling of the biconical ionized gas in NGC 4388

Free access article

Issue		A&A Volume 400, Number 3, March IV 2003


Page(s)		859 - 870
Section		Galactic structure and dynamics
DOI		http://dx.doi.org/10.1051/0004-6361:20030008

A&A 400, 859-870 (2003)
DOI: 10.1051/0004-6361:20030008

2-D spectroscopy and modeling of the biconical ionized gas in NGC 4388

S. Ciroi¹, M. Contini^{1, 2}, P. Rafanelli¹ and G. M. Richter³

¹ Dipartimento di Astronomia, Vicolo dell'Osservatorio 2, 35122 Padova, Italy
    e-mail: piraf@pd.astro.it
² School of Physics and Astronomy, Tel-Aviv University, Ramat-Aviv, Tel-Aviv, 69978 Israel
    e-mail: contini@post.tau.ac.il
³ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
    e-mail: gmrichter@aip.de

(Received 9 September 2002 / Accepted 23 December 2002)

Abstract
We present recent results from spectroscopic data and modeling of the biconical ionized gas in the Seyfert-2 galaxy NGC 4388. A field of ${\sim} 2.6\times2.4$ kpc centered on the nucleus has been observed by means of the modern technique of integral field spectroscopy. The analysis of more than two hundred spectra allowed to study the physical characteristics of the gas in the surroundings of the active nucleus. The South-West ionization cone, revealed by the [ $\ion{O}{III}$ ] $\lambda$ 5007/H $\beta$ excitation map, shows high emission line ratios not completely supported by simple photoionization. Composite models which account for the combined effects of photoionization and shock show that such high [ $\ion{O}{III}$ ]/H $\beta$ line ratios are emitted by low density ( n₀=30 cm^-3) gas inside large ( D>1 pc) shocked clouds ( ${V_{\rm s}}=100$ $\mathrm{km\, s^{-1}}$ ) reached by a relatively low flux from the active nucleus. The data of the VEELR in the North-East cone by Yoshida et al. (2002) have been modeled. The results confirm that photoionization is the prevailing mechanism, but nontheless weak shocks are under way between colliding clouds with small ( <1 pc) sizes and low densities $n_{0}\le100$ cm^-3, moving outward at relatively low velocities ( ${V_{\rm s}}=100$ $\mathrm{km\, s^{-1}}$ ).

Key words: techniques: spectroscopic -- galaxies: nuclei -- galaxies: Seyfert -- shock waves -- galaxies: individual: NGC 4388

Offprint request: S. Ciroi, ciroi@pd.astro.it

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