Abstract
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. A variety of 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 ranging from 0 to 40%, using the TreeSPH code Gadget-2. 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 ~ 1.5Reff, and by tube orbits in their outer parts. As the gas fraction 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 the gas fraction, 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. The 15-20% gas remnants often display disk-like kinematically distinct cores (KDCs). These remnants show an interesting resemblance, in both their velocity maps and intrinsic orbital structure, to the KDC galaxy NGC4365 (van den Bosch et al. 2008). At 30-40% gas, the remnants are rapidly rotating, with sharp embedded disks on ~ 1Reff scales. We predict a characteristic, physically intuitive orbital structure for 1:1 disk merger remnants, with a distinct transition between 1 and 3Reff that will be readily observable with combined data from the 2D kinematics surveys SAURON and SMEAGOL.
Highlights
The results of our simple classification scheme were consistent with spectral classification (Carpintero & Aguilar 1998; Binney & Spergel 1982, 1984; Hoffman 2007), and we determined that the simpler algorithm was better suited for the noisy potentials and gross analysis desired in this work
The remnants are dominated by box orbits in their inner parts (r 1.5 Re), and tube orbits in their outskirts
The x-tube orbits tend to stream far more than the z-tube orbits. This trend reverses at the highest gas fractions owing to the rapid rotation of the embedded disks, and the direct modification of the gravitational potential by the large central mass concentration (CMC), which may blur the lines between x-tube and z-tube orbits as the potential becomes very round
Summary
In the standard ΛCDM concordance cosmology (Ostriker & Steinhardt 1995; Dodelson et al 1996; Spergel et al 2007), structure in the universe grows hierarchically, through a progression of smaller bodies accreting material and merging to form larger systems (e.g., White & Rees 1978). Simulated DM halos appear to share a universal internal morphology, with density and velocity anisotropy profiles similar to the generic outcome of violent relaxation following dissipationless collapse or strong tidal shocking (Dubinski & Carlberg 1991; Navarro et al 1997; Bullock et al 2001a, 2001b; Navarro et al 2010; Miller & Smith 1979; van Albada 1982; McGlynn 1984, 1990; Spergel & Hernquist 1992; Huss et al 1999; MacMillan et al 2006; Bellovary et al 2008) There is no such simple theory of hierarchical galaxy formation, because the luminous components of galaxies are formed through complex baryonic physics. Combined with simulations aimed at establishing the characteristic orbital structure arising from various formation pathways, these observational programs will provide unprecedented insight into the physics of galaxy formation
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