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

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1. Introduction

From the seminal work of Toomre & Toomre (1972) we know how sensitive are galaxy disks to tidal interactions, from the formation of tidal tails and bridges up to the complete disruption of initial disks into a merged system, looking as an elliptical. Even non-merging interactions or minor mergers can thicken and destroy a stellar disk, and this has been advanced as an argument against frequent interactions in a galaxy life, or formation of the bulge through minor mergers in spiral galaxies (e.g. Gunn 1987). Although many vestiges of interactions are observed in present-day galaxies (ripples, shells, plumes, Schweizer 1990), it was assumed that these interactions must have occurred early in the galaxy life, before the formation of the disk.

Toth & Ostriker (1992) have used the argument of the fragility of disks to constrain the frequency of merging and the amount of accretion, and draw the implications on cosmological parameters. They claim that the thickness of the Milky Way disk implies that no more than 4% of its mass can have accreted within the last [FORMULA] yrs; moreover they question the currently fashionable theory of structure growth by hierarchical merging, which would not be supported by the presence of thin galactic disks, cold enough for spiral waves to develop.

Numerical simulations have been performed to check how fragile galactic disks are with respect to interaction and merging (Quinn et al 1993, Walker et al 1995). But the results depend strongly on the gas hydrodynamics and star-formation processes, since the thin disk can be re-formed continuously through gas infall. The gas accreted can, by its dissipative character, settle in a thin plane, and make the stellar disk thinner by its direct gravitational effect, and by the subsequent star formation. Other unknown parameters could influence the results, as the actual efficiency of dynamical friction, the true flattening of the dark matter, etc... Since the question is not completely settled, it is interesting to check observationally the influence of tidal interactions on the thickening of galaxy disks.

In a preliminary study, Reshetnikov et al (1993) have shown that the disks of strongly interacting spirals are 2-3 times thicker as compared with the disks of normal spiral galaxies. But the sample was only made of 6 galaxies and was too small to derive statistically significant results. Here we investigate the efficiency of tidal disk thickening by observing the thickness of planes in a large enough sample of edge-on interacting galaxies, in comparison to a control sample of isolated galaxies. In Paper I, we have presented the observations and general photometric results. In this report, after describing the reduction methods (Sect. 2), we analyse the thickness of the planes, at various radii, show that it remains almost constant in a given galaxy, i.e. stellar disks are not strongly affected by warping and flaring (Sect. 3). Then we study the statistical variations of the ratio [FORMULA] of the radial scalelength h to the scaleheight [FORMULA], and discuss the results in Sect. 4.

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© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998