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Astron. Astrophys. 347, 112-118 (1999)

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Infrared line ratios revealing starburst conditions in galaxies

Sueli M. Viegas 1, Marcella Contini 1,2 and Thierry Contini 2,3

1 Instituto Astronômico e Geofsico, USP, Av. Miguel Stefano 4200, 04301-904 São Paulo, Brazil
2 School of Physics & Astronomy, Tel Aviv University, 69978 Tel Aviv, Israel
3 European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching bei München, Germany

Received 29 October 1998 / Accepted 16 April 1999


The physical conditions in typical starburst galaxies are investigated through critical infrared (IR) line ratios, as previously suggested by Lutz et al. (1998). The calculations by a composite model which consistently accounts for the coupled effect of shock and photoionization by hot stars definitely fit the observed line ratios of single objects and explain the observed relation between [O IV ]/([Ne II ]+0.44[Ne III ]) and [Ne III ]/[Ne II ]. The shock velocity and the gas density are the critical parameters.

Most of the shocks are produced in low density-velocity ([FORMULA] [FORMULA] 100 [FORMULA] and [FORMULA] [FORMULA] [FORMULA]) clouds which represent the bulk of the ionized gas in starburst galaxies. However, though they are by many orders less numerous, high-velocity ([FORMULA] [FORMULA]) shocks in dense ([FORMULA] [FORMULA]) clouds are necessary to reproduce the critical IR line ratios observed in the low-excitation Starburst Nucleus Galaxies (SBNGs: M 82, M83, NGC 253, NGC 3256, NGC 3690, and NGC 4945). These model predictions are in good agreement with the powerful starburst-driven superwinds and highly pressured ISM observed in SBNGs. On the contrary, the high-excitation HII galaxies (II Zw 40 and NGC 5253) do not show any clear signature of large scale outflows of gas. This difference between HII galaxies and SBNGs can be interpreted in terms of temporal evolution of their starbursts.

Key words: galaxies: starburst – galaxies: ISM – infrared: galaxies – line: formation – shock waves

Send offprint requests to: T. Contini (tcontini@eso.org)

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© European Southern Observatory (ESO) 1999

Online publication: June 18, 1999