Statistical Properties of Collisionless Equal- and Unequal-Mass Merger Remnants of Disk Galaxies

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Abstract
We perform a large parameter survey of 112 collisionless N-body simulations of binary mergers of disk-galaxies with mass ratios of 1:1, 2:1, 3:1, and 4:1 using the special purpose hardware GRAPE. The basic properties of the remnants correlate with the mass ratio of the progenitor disks. We find that about 80% of the equal-mass merger simulations lead to slowly rotating remnants with significant minor-axis rotation. One half of all projected 1:1 remnants shows boxy and the other half shows disky isophotes. Remnants with mass ratios of 3:1 and 4:1 all rotate fast and show a small amount of minor-axis rotation. About 90% of the projected 3:1 and 4:1 remnants show disky isophotes. 2:1 remnants show intermediate properties. Projection effects lead to a large spread in the data in good agreement with observations. They do not change the fundamental kinematical differences between equal-mass and unequal-mass merger remnants. The results of this study weaken the disk merger scenario as the possible formation mechanism of massive boxy giant ellipticals. In general, the properties of equal-mass (and 2:1) merger remnants are consistent with the observed population of giant ellipticals in the intermediate mass regime between low mass fast rotating disky and bright massive boxy giant ellipticals. 3:1 and 4:1 merger remnants, however, are in very good agreement with the class of low luminosity, fast rotating giant elliptical galaxies. Binary mergers of disk galaxies are therefore still very good candidates for being the main formation mechanism for intermediate and low mass giant ellipticals. The homogeneous class of massive boxy ellipticals most likely formed by a different process.

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