Influence of baryons on the orbital structure of dark matter haloes

Mon. Not. R. Astron. Soc. 422, 1863–1879 (2012) doi:10.1111/j.1365-2966.2011.20298.x Influence of baryons on the orbital structure of dark matter haloes S. E. Bryan,1,2 S. Mao,1,3 S. T. Kay,1 J. Schaye,4 C. Dalla Vecchia4,5 and C. M. Booth4 1 Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, The University of Manchester, Manchester M13 9PL 2 Department of Physics & Astronomy, University of Leicester, Leicester LE1 7RH 3 National Astronomical Observatories of China, Chinese Academy of Sciences, 20A Datun Road, Beijing 100012, China 4 Leiden Observatory, Leiden University, Postbus 9513, 2300 RA Leiden, the Netherlands 5 Max Planck Institute for Extraterrestrial Physics, Giessenbachstraße 1, 85748 Garching, Germany Accepted 2011 November 30. Received 2011 November 7; in original form 2011 September 19 We explore the dynamical signatures imprinted by baryons on dark matter haloes during the formation process using the OverWhelmingly Large Simulations (OWLS), a set of state-ofthe-art high-resolution cosmological hydrodynamical simulations. We present a detailed study of the effects of the implemented feedback prescriptions on the orbits of dark matter particles, stellar particles and subhaloes, analysing runs with no feedback, with stellar feedback and with feedback from supermassive black holes. We focus on the central regions (0.25r200 ) of haloes with virial masses ∼6 × 1013 (∼7 × 1011 ) h−1 M at z = 0 (2). We also investigate how the orbital content (relative fractions of the different orbital types) of these haloes depends on several key parameters such as their mass, redshift and dynamical state. The results of spectral analyses of the orbital content of these simulations are compared, and the change in fraction of box, tube and irregular orbits is quantified. Box orbits are found to dominate the orbital structure of dark matter haloes in cosmological simulations. There is a strong anticorrelation between the fraction of box orbits and the central baryon fraction. While radiative cooling acts to reduce the fraction of box orbits, strong feedback implementations result in a similar orbital distribution to that of the dark matter only case. The orbital content described by the stellar particles is found to be remarkably similar to that drawn from the orbits of dark matter particles, suggesting that either they have forgotten their dynamical history, or subhaloes bringing in stars are not biased significantly with respect to the main distribution. The orbital content of the subhaloes is in broad agreement with that seen in the outer regions of the particle distributions. Key words: methods: numerical – galaxies: clusters: general – galaxies: evolution – galaxies: haloes – galaxies: kinematics and dynamics – cosmology: theory. 1 I N T RO D U C T I O N Dark matter structure formation is well understood within the standard cosmological model. Haloes are thought to form hierarchically, through the merging and accretion of smaller systems. As such, there should be observational signatures of these merging processes in the resulting remnants, providing dynamical information about their formation histories. We investigate the orbital content of dark matter haloes in order to explore what signatures may result. Dark matter haloes formed in a cold dark matter (CDM) cosmology appear to share a nearly universal internal morphology; they have density profiles that are well described by the Navarro,  E-mail:  C 2012 The Authors C 2012 RAS Monthly Notices of the Royal Astronomical Society  Frenk & White (hereafter NFW; 1996, 1997) profile and pseudophase space densities with a constant (α ∼ 1.9) power-law slope (Taylor & Navarro 2001; Dehnen & McLaughlin 2005; Barnes et al. 2006; Ludlow et al. 2011). There is a universal relation between the radial density profile slope and the velocity anisotropy within the inner region of dark matter haloes (Hansen & Moore 2006; Navarro et al. 2010) and the velocity distribution function is found to have a universal shape (Hansen et al. 2006). Dark matter haloes are thought to have spin distributions that are reasonably well characterized by a log-normal distribution (Bullock et al. 2001; Bailin & Steinmetz 2005; Bett et al. 2007; Macciò, Dutton & van den Bosch 2008) and are thought to be triaxial (Frenk et al. 1988; Dubinski & Carlberg 1991; Warren et al. 1992; Cole & Lacey 1996; Jing & Suto 2002; Bailin & Steinmetz 2005; Allgood et al. 2006; Macciò et al. 2006; Bett et al. 2007; Jeeson-Daniel et al. 2011). Here we explore the ABSTRACT 1864 S. E. Bryan et al. Jesseit et al. (2005) studied a statistical sample of disc galaxy mergers, using the automated spectral classification of Carpintero & Aguilar (1998) to quantify the orbital content of the resulting remnants. They found that the most abundant orbital classes were box and minor-axis tube orbits. While the inner regions of the simulated remnants were dominated by box orbits, tube orbits became more important at intermediate radii. They also found that the ratio of these two classes of orbits played a role in determining the basic properties of the remnant. Minor-axis-tube-dominated haloes were found to be discy, while those dominated by box orbits were boxy. Major-axis tubes were found to be dominant in prolate remnants. Again, it was noted that gas affects the fraction of box orbits, causing an increase in the population of minor-axis tubes. Debattista et al. (2008) studied the impact of growing a central disc on the orbital content of a halo. They find that while the central concentration does result in rounder, more radially anisotropic haloes, the halo’s shape is essentially returned to its original state if the disc is artificially ‘evaporated’. This indicates that the character of the orbits is not generally changed by the central mass concentration; the box orbits are not destroyed but simply become rounder in line with the potential. This is also considered in Valluri et al. (2010) who explore the orbital evolution induced by baryonic condensation in triaxial haloes. They find that the evolution depends on the radial distribution of the baryonic component, and that a massive compact central mass will result in the scattering of a large fraction of both box and long-axis tube orbits even at fairly large pericentric distances. A comprehensive study of the orbital structure of 1:1 merger remnants can be found in Hoffman et al. (2010). Mergers between equal mass discs at varying initial gas fractions (ranging from 0 to 40 per cent) were simulated, taking into account both star formation and feedback. They showed that, by varying the fraction of gas in a merger, a wide range of kinematic structures can be produced. The remnants formed in these simulations are typically prolate triaxial. The central regions are dominated by box orbits, while tube orbits dominate further out. The inclusion of gas acts to decrease the fraction of stellar particles on b (...truncated)