Response of Dark Matter Halos to Condensation of Baryons: Cosmological Simulations and Improved Adiabatic Contraction Model

Abstract

The cooling of gas in the centers of dark matter halos is expected to lead to a more concentrated dark matter distribution. The response of dark matter to the condensation of baryons is usually calculated using the model of adiabatic contraction, which assumes spherical symmetry and circular orbits. In contrast, halos in the hierarchical structure formation scenarios grow via multiple violent mergers and accretion along filaments, and particle orbits in the halos are highly eccentric. We study the effects of the cooling of gas in the inner regions of halos using highresolution cosmological simulations that include gas dynamics, radiative cooling, and star formation. We find that the dissipation of gas indeed increases the density of dark matter and steepens its radial profile in the inner regions of halos compared to the case without cooling. For the first time, we test the adiabatic contraction model in cosmological simulations and find that the standard model systematically overpredicts the increase of dark matter density in the inner 5% of the virial radius. We show that the model can be improved by a simple modification of the assumed invariant from M (r)r to M (r)r, where r andr are the current and orbit-averaged particle positions. This modification approximately accounts for orbital eccentricities of particles and reproduces simulation profiles to within 10%-20%. We present analytical fitting functions that accurately describe the transformation of the dark matter profile in the modified model and can be used for interpretation of observations.

NIHAO IX: the role of gas inflows and outflows in driving the contraction and expansion of cold dark matter haloes

“…However, these simulations generally find that the contraction is weaker than predicted by the Blumenthal et al (1986) and Gnedin et al (2004) models. A potentially serious problem with these simulations is they over predict the stellar masses of galaxies, and thus it is not clear how applicable they are to galaxies in the real Universe.…”

“…Several authors have studied the halo response in cosmological galaxy formation simulations finding contraction in haloes of mass ∼ 10 12 M⊙ (Gnedin et al 2004;Abadi et al 2010;Pedrosa et al 2010). It should be noted that these studies suffered from over-cooling and small sample sizes (the Abadi et al 2010 simulations do not include star formation).…”

“…tribution of the baryons. In the adiabatic contraction formalism (Blumenthal et al 1986) and its variants (Gnedin et al 2004;Abadi et al 2010;Dutton et al 2015), at a fixed halo and stellar mass, more compact baryon distributions result in more contraction. For the expansion case the stars are collisionless, so they are expected to expand with the dark matter.…”

“…Comparison between halo response in NIHAO simulations (binned by star formation efficiency, solid lines), and various models in the literature. Left: long-dashed lines show the halo response predicted by the model of Gnedin et al (2004), which is different for each NIHAO stack, while the short dashed line shows the relation from Abadi et al (2010). Right: dotted lines show r f /r i = (M i /M f ) ν which is sometimes used to parametrize the halo response in galaxy mass models.…”

Formation of disc galaxies in preheated media: a preventative feedback model

“…(23) in Mo, Mao & White (1998) to compute fc for a NFW halo density profile. In our modeling, the effect of contraction of the dark matter halo due to disk formation (Blumenthal et al 1986;Gnedin et al 2004;Choi et al 2006) is ignored. We assume λ = 0.035, which is the median value of dark matter halos in cosmological simulations (Bullock et al 2001;Macciò et al 2007).…”