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Astron. Astrophys. 324, 80-90 (1997)

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5. Conclusions

We have studied a sample of 24 edge-on interacting galaxies and compared them to other edge-on isolated galaxies, to investigate the effects of tidal interaction on disk thickening. We find for most of the galaxies a constant (within 20%) scaleheight with radius. Only one system (VV 490) reveals a significant flaring. The average scaleheight [FORMULA] is higher for the interacting sample, and the scalelength is also smaller, so that the [FORMULA] ratio is 1.5-2 times smaller than in isolated galaxies. This corresponds quite well to the predictions of N-body simulations (Quinn et al 1993, Walker et al 1996): the gravity torques induced by the tidal interaction produce a central mass concentration, while the outer disk spreads out radially, leading to a decrease of h. Most of the heating is expected to be vertical, since the planar heating is taken away by the stripped stars either in the primary or in the satellite. The quantitative agreement between observations and simulations is rather good, given the large dispersion expected due to the initial morphology of the interacting galaxies: a dense satellite will produce much more heating than a diffuse one, where stripped stars take the orbital energy away; a mass-condensed primary will inhibit tidally-induced spiral and bar perturbations, that are the source of heating both radially and vertically.

The fact that tidal interactions and minor mergers must have involved every galaxy in a Hubble time, and therefore also the presently isolated and undisturbed galaxies, tells us that the lower values of [FORMULA] observed for the interacting sample must be transient. Radial gas inflow induced by the interaction may have contributed to reform a thin young stellar disk, while the vertical thickening have formed the thick disk components now observed in the Milky Way and many nearby galaxies (e.g. Burstein 1979, Shaw & Gilmore 1989). This process might be occurring all along the interaction, so that the galaxy is never observed without a thin disk. One cannot therefore date back the period of the last interaction by the age of the thin disk, as has been proposed by Toth & Ostriker (1992) and Quinn et al (1993). The Milky Way, experiencing now interactions with the Magellanic Clouds and a few dwarf spheroidal companions, has still a substantial gaseous and stellar thin disk. More self-consistent simulations, including gas and star-formation, must be performed to derive more significant predictions.

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© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998