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Since the violent relaxation in hierarchical merging is incomplete,elliptical galaxies retain a wealth of information about their formationpathways in their present-day orbital structure. A variety of evidenceindicates that gas-rich major mergers play an important role in the formationof elliptical galaxies. We simulate 1:1 disk mergers at seven different initialgas fractions ranging from 0 to 40%, using the TreeSPH code Gadget-2. Weclassify the stellar orbits in each remnant and construct radial profiles ofthe orbital content, intrinsic shape, and orientation. The dissipationlessremnants are typically prolate-triaxial, dominated by box orbits within r_c ~1.5Reff, and by tube orbits in their outer parts. As the gas fractionincreases, the box orbits within r_c are increasingly replaced by a populationof short axis tubes (z-tubes) with near zero net rotation, and the remnantsbecome progressively more oblate and round. The long axis tube (x-tube) orbitsare highly streaming and relatively insensitive to the gas fraction, implyingthat their angular momentum is retained from the dynamically cold initialconditions. Outside r_c, the orbital structure is essentially unchanged by thegas. The 15-20% gas remnants often display disk-like kinematically distinctcores (KDCs). These remnants show an interesting resemblance, in both theirvelocity maps and intrinsic orbital structure, to the KDC galaxy NGC4365 (vanden Bosch et al. 2008). At 30-40% gas, the remnants are rapidly rotating, withsharp embedded disks on ~ 1Reff scales. We predict a characteristic, physicallyintuitive orbital structure for 1:1 disk merger remnants, with a distincttransition between 1 and 3Reff that will be readily observable with combineddata from the 2D kinematics surveys SAURON and SMEAGOL.

Orbital structure of merger remnants: Trends with gas fraction in 1:1 mergers