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Astron. Astrophys. 337, 9-16 (1998)

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5. Discussion and conclusion

We have simulated spiral projection effects cumulated to dust-absorption effects on the appearance of warps in inclined galaxies. We find that indeed inclined [FORMULA] spirals can mimick warps, but the effect is not so serious for very flat galaxies, at high inclination on the sky. We estimated that no more than 15% of S-shaped warps could be due to this phenomenon. We also emphasize that these projection effects could also subtract some genuine warps from the resulting apparent warp galaxies, and therefore, the effect can somewhat be compensated.

From the simulations of real warped planes, we find that the fraction of missed warps because of projection effects, i.e. those with line of nodes perpendicular to the line of sight, is less than half, contrary to what could be expected. This is due to the fact that a warp along the line of sight thickens the apparent disk, and the resulting galaxy is then not included in the sample of "flat" galaxies, as we have defined here. We estimate that in our sample, we do not miss more than 20% of the true warps.

Our observed statistics confirm the suspected influence of tidal interactions in warping distortions. The fact that large galaxies appear less warped in average might then be only due to the fact that they require more massive companions, which are less frequent. The fact that most galaxies are warped, even in isolation, confirms the persistence of the perturbation, even if triggered by a companion.

The percentage of warps is also to be compared to the percentage of asymmetric galaxies. Richter & Sancisi (1994) on a sample of 1700 galaxies observed in HI deduced that at least 50% of them show asymmetries, non-circularities, lopsidedness. Zaritsky & Rix (1997) on an optical sample find that 30% of field spirals galaxies exhibit significant lopsidedness at large radii (Zaritsky & Rix 1997); this is attributed to companions. If warps could be attributed to companions, this will be compatible with the fraction of warps that we find, provided that the warp is more long-lived than lopsidedness.

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© European Southern Observatory (ESO) 1998

Online publication: August 6, 1998