The influence of gas on the structure of merger remnants

We present a large set of merger simulations of early-type disc galaxies with mass ratios of 1:1 and 3:1 and 10 per cent of the total disc mass in gas. The internal orbital structure and the kinematic and photometric properties of the remnants are analysed in detail and compared to pure stellar mergers. In contrast to the collisionless case, equal-mass mergers with gas do not result in very boxy remnants which is caused by the suppression of box orbits and the change of the projected shape of minor-axis tube orbits in the more axisymmetric remnants. The isophotal shape of 3:1 remnants and the global kinematic properties of 1:1 and 3:1 remnants are only weakly affected by the presence of gas. 1:1 remnants are slowly rotating, whereas 3:1 remnants are fast rotating and discy. The shape of the stellar line-of-sight velocity distributions (LOSVDs) is strongly influenced by gas. Within the effective radius, the LOSVDs of collisionless remnants have broad leading wings while their gaseous counterparts show steep leading wings, more consistent with observations of elliptical galaxies. We show that this change is also caused by the suppressed populating of box orbits and it is amplified by the formation of extended gas discs in the merger remnants which might eventually turn into stars. If elliptical galaxies have formed from mergers, our results indicate that massive, slowly rotating boxy elliptical galaxies cannot have formed from dissipative mergers of discs. Pure stellar (dry) mergers are the more likely candidates. On the other hand, lower mass, fast rotating and discy ellipticals can have formed from dissipative (wet) mergers of early-type discs. So far, only unequal-mass disc mergers with gas can successfully explain their observed substructure. This is consistent with the revised morphological classification scheme of increasing importance of gas dissipation when moving from boxy to discy ellipticals and then to spiral galaxies, proposed by Kormendy & Bender.