How Do the Velocity Anisotropies of Halo Stars, Dark Matter, and Satellite Galaxies Depend on Host Halo Properties?

Abstract

We investigate the mass ( M _200 ) and concentration ( c _200 ) dependencies of the velocity anisotropy ( β ) profiles for different components in the dark matter halo—including halo stars, dark matter, and subhalos—using systems from the IllustrisTNG simulations. Beyond a critical radius, β becomes more radial with the increase of M _200 , reflecting more prominent radial accretion around massive halos. The critical radius is r ∼ r _s , 0.3 r _s , and r _s for halo stars, dark matter, and subhalos, with r _s being the scale radius of the host halos. This dependence on M _200 is the strongest for subhalos and the weakest for halo stars. In central regions, the β of halo stars and dark matter particles get more isotropic with the increase of M _200 in TNG300 due to baryons. By contrast, the β of dark matter from the dark-matter-only TNG300-Dark run shows much weaker dependence on M _200 within r _s . Dark matter in TNG300 is slightly more isotropic than in TNG300-Dark at 0.2 r _s < r < 10 r _s and ${\mathrm{log}}_{10}{{M}}_{200}/{{M}}_{\odot }\lt 13.8$ . Halo stars and dark matter also become more radial with the increase in c _200 , at fixed M _200 . Halo stars are more radial than the β profile of dark matter by approximately a constant beyond r _s . Dark matter particles are more radial than subhalos. The differences can be understood, as subhalos on more radial orbits are more easily stripped, contributing more stars and dark matter to the diffuse components. We provide the fitting formula for the differences between the β of halo stars and dark matter at r _s < r < 3 r _s as ${\beta }_{\mathrm{star}}-{\beta }_{\mathrm{DM}}=(-0.034\pm 0.012){\mathrm{log}}_{10}{{M}}_{200}/{{M}}_{\odot }\,+(0.772\pm 0.163)$ for ${\mathrm{log}}_{10}{{M}}_{200}/{{M}}_{\odot }\geqslant 13$ and as β _star − β _DM = 0.328 for ${\mathrm{log}}_{10}{{M}}_{200}/{{M}}_{\odot }\lt 13$ .

Keywords

• Galaxy dark matter halos
• Galaxy stellar halos
• Milky Way stellar halo
• Hydrodynamical simulations
• N-body simulations