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

We perform a large parameter survey of 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. A set of 112 merger simulations is used to investigate the fundamental statistical properties of merger remnants as a function of the initial orientation and mass ratio of the progenitor disks. The photometric and kinematical properties of the simulated merger remnants are analyzed. The methods used to determine the characteristic properties are equivalent to the methods used for observations of giant elliptical galaxies. We take projection effects into account and analyze the remnant properties in a statistical way for comparison with observations. 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 merger remnants having * < 0.4. Observers would interpret those objects as being supported by anisotropic velocity dispersions. All 1 : 1 remnants show significant minor-axis rotation. Half of all projected 1 : 1 remnants show boxy (a4 < 0) isophotes, and the other half show disky (a4 > 0) isophotes. A distinct subclass of 4 out of 12 initial orientations leads to purely boxy remnants independent of orientation. The 1 : 1 mergers with other initial orientations show disky or boxy isophotes, depending on the viewing angle. Remnants with mass ratios of 3 : 1 and 4 : 1 have more homogeneous properties. They all rotate rapidly (maximum value of v/σ = 1.2) and show a small amount of minor-axis rotation, consistent with models of isotropic or slightly anisotropic oblate rotators. If observed in projection, they would be interpreted as being supported by rotation. About 90% of the projected 3 : 1 and 4 : 1 remnants show disky isophotes. The 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- and unequal-mass merger remnants. The correlation between isophotal twist and apparent ellipticity of every single merger remnant is in good agreement with observations. The amount of twisting strongly depends on the orientation of the remnant but is only weakly dependent on the mass ratio of the merger. The results of this study weaken the disk merger scenario as the possible formation mechanism of massive boxy giant ellipticals, as only equal-mass mergers with special initial orientations can produce purely boxy anisotropic merger remnants. Some orientations of 1 : 1 mergers can even lead to disky and anisotropic remnants that are either not observed or would be classified as S0 galaxies based on their morphology. In general, the properties of equal-mass (and 2 : 1) merger remnants are consistent with those of the observed population of giant ellipticals in the intermediate-mass regime between low-mass, fast-rotating, disky and bright, massive, boxy giant ellipticals. The 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.