The impact of baryonic physics on the shape and radial alignment of substructures in cosmological dark matter haloes

Abstract

We use two simulations performed within the Constrained Local UniversE Simulation (CLUES) project to study both the shape and radial alignment of (the dark matter component of) subhaloes; one of the simulations is a dark matter only model while the other run includes all the relevant gas physics and star formation recipes. We find that the involvement of gas physics does not have a statistically significant effect on either property – at least not for the most massive subhaloes considered in this study. However, we observe in both simulations including and excluding gas dynamics a (pronounced) evolution of the dark matter shapes of subhaloes as well as of the radial alignment signal since infall time. Further, this evolution is different when positioned in the central and outer regions of the host halo today; while subhaloes tend to become more aspherical in the central 50 per cent of their host’s virial radius, the radial alignment weakens in the central regime while strengthening in the outer parts. We confirm that this is due to tidal torquing and the fact that subhaloes at pericentre move too fast for the alignment signal to respond.

The shapes and alignments of the satellites of the Milky Way and Andromeda

“…Pereira et al (2008) find that for subhaloes within the virial radius the alignment is mass-independent with a median alignment cos η ∼ 0.66 and σcos η = 0.55. Despite these simulations being darkmatter-only, it appears that they match our results well, which points towards dark matter being only weakly influenced by and approximately tracing the baryons on the Local Group subhalo scale as suggested by Knebe et al (2010).…”

“…If the shape of low-mass dark matter subhaloes are unaffected by any central baryonic component (e.g. Knebe et al 2010) then we conclude that the stellar populations in dSphs are intrinsically flatter than their dark matter haloes by approximately a factor of 2. However, further hydrodynamic simulations at low mass scales are required to study how the growth of a baryonic component reshapes the dark matter halo and it is difficult to draw any strong conclusions from the observed discrepancy.…”

“…Additionally, the addi- tion of baryons does not affect the distribution of dark matter. It is expected that due to the high mass-to-light ratio in dSphs the effect of the baryons on the dark matter is small (Knebe et al 2010). Finally, the simulations do not include a disc which is anticipated to have a significant effect on the lifetime and structure of subhaloes (D'Onghia et al 2010).…”

“…However, other baryonic effects are important on the cluster scale and the degree of radial alignment from full hydrodynamical simulations on these scales is an on-going area of active research (Tenneti et al 2015), particularly for interpretation of weak cosmological gravitational lensing signals (Kiessling et al 2015;Joachimi et al 2015). The effect of baryons on the Milky Way subhalo scale is still an open question with, for instance, Knebe et al (2010) suggesting baryons have little effect on the alignment of the dark matter haloes at these low mass scales.…”

Tidal disruption of dwarf spheroidal galaxies: the strange case of Crater II

“…those for which the stars and dark matter are stratified on concentric self-similar ellipsoids with axis ratios p = b/a and q = c/a. There is some evidence for such a simplification from simulations (Knebe et al 2010). When viewed at standard spherical polar angles θ and φ relative to the Cartesian coordinate system defined by the principal axes, the observed velocity dispersion and on-sky half-light radius can be computed using simple analytic expressions (Roberts 1962;.…”