Dark matter response to galaxy formation

Abstract

We have resimulated the six galaxy-sized haloes of the Aquarius Project including metal-dependent cooling, star formation and supernova feedback. This allows us to study not only how dark matter haloes respond to galaxy formation, but also how this response is affected by details of halo assembly history. In agreement with previous work, we find baryon condensation to lead to increased dark matter concentration. Dark matter density profiles differ substantially in shape from halo to halo when baryons are included, but in all cases the velocity dispersion decreases monotonically with radius. Some haloes show an approximately constant dark matter velocity anisotropy with $ \beta \approx 0.1-02$, while others retain the anisotropy structure of their baryon-free versions. Most of our haloes become approximately oblate in their inner regions, although a few retain the shape of their dissipationless counterparts. Pseudo-phase-space densities are described by a power law in radius of altered slope when baryons are included. The shape and concentration of the dark matter density profiles are not well reproduced by published adiabatic contraction models. The significant spread we find in the density and kinematic structure of our haloes appears related to differences in their formation histories. Such differences already affect the final structure in baryon-free simulations, but they are reinforced by the inclusion of baryons, and new features are produced. The details of galaxy formation need to be better understood before the inner dark matter structure of galaxies can be used to constrain cosmological models or the nature of dark matter.Comment: 14 pages, 9 figures. Accepted MNRAS. Revised version includes discussion on resolution effects and minor changes

Systematic Problems With Using Dark Matter Simulations to Model Stellar Halos

“…When galaxies form, baryons cool and collapse into a centrifugally rotating disk, whose morphology can then be altered by further accreted material and interactions with other galaxies; these processes do not occur in pure DM simulations, which do not have the capability to radiatively cool. The gravitational potential in which the stars that constitute the stellar halo orbit is therefore different in the real universe than in a pure DM simulation: it is more concentrated, and is flattened in the inner regions due to the disk (Kazantzidis et al 2004;Bailin et al 2005;Debattista et al 2008;Tissera et al 2010). Different groups have taken different approaches to account for this effect: C10 and Rashkov et al (2012) use pure Nbody cosmological simulations and neglect any changes in the potential due to baryonic physics; BJ05 grow an analytic disk potential inside an analytic growing halo potential; and finally, a particularly interesting approach is that of L11, who compare a full hydrodynamic cosmological simulation to the identical DM-only simulation using the same initial conditions.…”

Constraints on the presence of a 3.5 keV dark matter emission line fromChandraobservations of the Galactic centre

“…Simulations show that α can vary quite significantly (Dutton & Macciò 2014). We adopt a default value of α = 0.17 (Dutton & Macciò 2014) as well as the larger value of 0.20 (Tissera et al 2010) representing a more cored profile and leading to ∼40% uncertainty on the density integral and a factor of two for the squared integral. Simulations indicate that the dark matter profiles may be steeper than the Einasto profile for cold dark matter e.g.…”

Fitting functions for a disk-galaxy model with differentΛCDM-halo profiles

“…They are widely used in semi-analytic cosmological models, pre-prepared numerical 1 In modern literature, the mode and amount of halo contraction are actually a matter of debate (e.g. Dutton et al 2007;Tissera et al 2010). The outcome, however, remains unchanged: the properties of disk galaxies are linked to those of their host halos.…”