ORBITAL STRUCTURE OF MERGER REMNANTS. I. EFFECT OF GAS FRACTION IN PURE DISK MERGERS

Since the violent relaxation in hierarchical merging is incomplete, elliptical galaxies retain a wealth of information about their formation pathways in their present-day orbital structure. Recent advances in integral field spectroscopy, multi-slit infrared spectroscopy, and triaxial dynamical modeling techniques have greatly improved our ability to harvest this information. A variety of observational and theoretical evidence indicates that gas-rich major mergers play an important role in the formation of elliptical galaxies. We simulate 1: 1 disk mergers at seven different initial gas fractions (f(gas)) ranging from 0% to 40%, using a version of the TreeSPH code Gadget-2 that includes radiative heating and cooling, star formation, and feedback from supernovae and active galactic nuclei. We classify the stellar orbits in each remnant and construct radial profiles of the orbital content, intrinsic shape, and orientation. The dissipationless remnants are typically prolate-triaxial, dominated by box orbits within r(c) similar to 1.5 R-e, and by tube orbits in their outer parts. As f(gas) increases, the box orbits within r(c) are increasingly replaced by a population of short-axis tubes (z-tubes) with near zero net rotation, and the remnants become progressively more oblate and round. The long-axis tube (x-tube) orbits are highly streaming and relatively insensitive to f(gas), implying that their angular momentum is retained from the dynamically cold initial conditions. Outside r(c), the orbital structure is essentially unchanged by the gas. For f(gas) greater than or similar to 15%, gas that retains its angular momentum during the merger re-forms a disk that appears in the remnants as a highly streaming z-tube population superimposed on the hot z-tube distribution formed by the old stars. In the 15%-20% gas remnants, this population appears as a kinematically distinct core (KDC) within a system that is slowly rotating or dominated by minor-axis rotation. These remnants show an interesting resemblance, in both their velocity maps and intrinsic orbital structure, to the KDC galaxy NGC 4365. At 30%-40% gas, the remnants are rapidly rotating, with sharp embedded disks on similar to 1 R-e scales. We predict a characteristic, physically intuitive orbital structure for 1:1 disk merger remnants, with a distinct transition between 1 and 3 R-e that will be readily observable with combined data from the two-dimensional kinematics surveys SAURON and SMEAGOL. Our results illustrate the power of direct comparisons between N-body simulations and dynamical models of observed systems to constrain theories of galaxy formation.