NIHAO project II: halo shape, phase-space density and velocity distribution of dark matter in galaxy formation simulations
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abstract
We use the NIHAO (Numerical Investigation of Hundred Astrophysical Objects)
cosmological simulations to study the effects of galaxy formation on key
properties of dark matter (DM) haloes. NIHAO consists of $\simeq 90$
high-resolution SPH simulations that include (metal-line) cooling, star
formation, and feedback from massive stars and SuperNovae, and cover a wide
stellar and halo mass range: $10^6 < M_* / M_{\odot} < 10^{11}$ ( $10^{9.5} <
M_{\rm halo} / M_{\odot} < 10^{12.5}$). When compared to DM-only simulations,
the NIHAO haloes have similar shapes at the virial radius, R_{\rm vir}, but are
substantially rounder inside $\simeq 0.1R_{\rm vir}$. In NIHAO simulations
$c/a$ increases with halo mass and integrated star formation efficiency,
reaching $\sim 0.8$ at the Milky Way mass (compared to 0.5 in DM-only),
providing a plausible solution to the long-standing conflict between
observations and DM-only simulations. The radial profile of the phase-space $Q$
parameter ($\rho/\sigma^3$) is best fit with a single power law in DM-only
simulations, but shows a flattening within $\simeq 0.1R_{\rm vir}$ for NIHAO
for total masses $M>10^{11} M_{\odot}$. Finally, the global velocity
distribution of DM is similar in both DM-only and NIHAO simulations, but in the
solar neighborhood, NIHAO galaxies deviate substantially from Maxwellian. The
distribution is more symmetric, roughly Gaussian, with a peak that shifts to
higher velocities for Milky Way mass haloes. We provide the distribution
parameters which can be used for predictions for direct DM detection
experiments. Our results underline the ability of the galaxy formation
processes to modify the properties of dark matter haloes.
