Orbital structure of collisionless merger remnants: on the origin of photometric and kinematic properties of elliptical and S0 galaxies

R. Jesseit T. Naab 0 A. Burkert Universitatssternwarte Scheinerstr. Munchen Germany Insititute of Astronomy Madingley Road Cambrige CB HA 0 Present address: Universita tssternwarte , Scheinerstr. 1, 81679 Mu nchen , Germany A B S T R A C T We present a detailed investigation of the relation between the orbital content of merger remnants and observable properties of elliptical and S0 galaxies. Our analysis is based on the statistical sample of collisionless mergers of disc galaxies with different mass ratios and orbital parameters, published by Naab & Burkert. We use the spectral method by Carpintero & Aguilar to determine the orbital content of every remnant and correlate it with its intrinsic shape, and its projected kinematic and photometric properties. We discuss the influence of the bulge component and varying pericentre distances. The classified orbit families are box orbits, minor-axis tubes, inner and outer major-axis tubes and boxlets. In general, box orbits dominate the inner parts of the remnant. Major and minor-axis tubes become dominant at intermediate radii and boxlets at large radii. The two most abundant orbit classes are the minor-axis tubes and the box orbits. Their ratio seems to determine the basic properties of a remnant. On average, the fraction of minor-axis tubes increases by a factor of 2 from a merger mass ratio of 1:1 to 4:1, whereas the fraction of box orbits decreases by 10 per cent. At a given mass the central velocity dispersion of a remnant scales with the ratio of minor-axis tubes to box orbits. Interestingly, the division line between rotational supported systems and pressure supported objects, (v maj/ 0) = 0.7, turns out to coincide with a box to minor-axis tube ratio of unity. The observed h 3 v/ anticorrelation for ellipticals cannot be reproduced by collisionless merger remnants. We propose that this can only be reconciled by an additional physical process that significantly reduces the box orbit content. Remnants that are dominated by minor-axis tube orbits have predominantly discy projections. Boxy remnants have always a box to minor-axis tube ratio larger than one. This study will enable us to identify observed ellipticals that could have formed, in the collisionless limit, by gas-poor disc mergers. In addition, it demonstrates how observable properties of spheroidal stellar systems are connected with their intrinsic orbital structure. - Elliptical galaxies show a richness of distinct substructure in their isophotal shape and velocity anisotropy. Bender (1988) and Bender, Dobereiner & Mollenhoff (1988) found that elliptical galaxies can be divided into objects with boxy and discy isophotal shape. Interestingly, ellipticals with boxy isophotes show X-ray emission in excess of discrete sources indicative of a hot gaseous halo, rotate slowly and have higher luminosity, while discy ellipticals are fast rotators, with low luminosities and no detectable X-ray haloes (Bender et al. 1989). These features are probably linked to the formation process of these galaxies. Toomre & Toomre (1972) were the first to propose that ellipticals could originate from mergers of two disc galaxies. Subsequent N-body simulations of collisions of disc galaxies showed that merger remnants mimic important features of elliptical galaxies such as global kinematic and photometric properties, kinematic misalignments or kinematically decoupled cores (Barnes 1992; Hernquist 1992, 1993; Heyl, Hernquist & Spergel 1994, 1996; Weil & Hernquist 1996; Naab, Burkert & Hernquist 1999; Bendo & Barnes 2000; Cretton et al. 2001; Naab & Burkert 2003; Burkert & Naab 2004). Bender, Saglia & Gerhard (1994) showed that the asymmetry of the line-of-sight velocity distribution (LOSVD), correlates with the ratio of local rotation velocity to velocity dispersion (v/ ). In general, the LOSVD shows a steep leading wing. These findings have been confirmed by newer observations (Halliday et al. 2001; Pinkney et al. 2003). If we consider ellipticals as pure stellar systems all kinematic and photometric properties will originate from the projected superposition of the orbits of individual stars, which build up the galaxy. Unfortunately, the trajectories of the stars themselves are not observable. Reliable constraints concerning the intrinsic shape can only be inferred statistically if a large sample of observed ellipticities is available such as for the SDSS (Alam & Ryden 2002). To deproject an individual galaxy and determine its orbital content is much harder. The only tools available are Schwarzschild models (Schwarzschild 1979), where a library of orbits is fitted to the light distribution and, if available, to the detailed kinematics of a galaxy (see Rix et al. 1997 and references therein). One limitation is that most of these models assume that the fitted galaxy is intrinsically axisymmetric, which might not always be the case in nature (Statler et al. 2004). Simulated galaxies have the advantage that intrinsic and projected properties can be studied at the same time. Several authors investigated the orbital content of simulated merger remnants. One of the basic results was that the centre of the remnants is dominated by box orbits, while minor-axis tubes dominate at larger radii (Barnes 1992). In agreement with theory they found that major-axis tubes dominate in prolate remnants. The presence of gas in mergers has a significant impact on the orbital structure, as the box orbits at the centre are destroyed and minor-axis tubes become more populated (Barnes & Hernquist 1996). Bendo & Barnes (2000) extracted the LOSVD for single orbit classes for a few merger remnants. Although these studies highlighted the importance of orbital structure and sometimes also discussed the effect of viewing angles (Heyl, Hernquist & Spergel 1994, 1995), they were, however, limited to a small number of merger remnants. Binney & Spergel (1982) originally proposed to use spectral dynamics for stellar dynamic problems. Laskar (1993) devised an algorithm to analyse dynamic spectra with high accuracy, which was also applied to analytical models of galactic systems (Papaphilippou & Laskar 1996, 1998). Carpintero & Aguilar (1998, henceforth CA98) created a fully automated code to classify orbits in arbitrary twoor three-dimensional potentials through spectral dynamics and we will use this in all our analyses presented in this paper. Whereas the orbital classification algorithms, used for simulations previously, picked up the different orbit families by checking whether they change the sign of the angular momentum or not, the code of CA98 allows one to distinguish orbit classes, which could not have been classified with simpler methods. The purpose of this paper is to identify the orbital content of the merger remnants and connect it to their observable and intrinsic properties. The main ingredients are a large sample of collisionless merger remnants, software that mimics real life observations (Naab & (...truncated)