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

Abstract

We use ∼ 100 cosmological galaxy formation 'zoom-in' simulations using the smoothed particle hydrodynamics code gasoline to study the effect of baryonic processes on the mass profiles of cold dark matter haloes. The haloes in our study range from dwarf (M 200 ∼ 10 10 M ⊙ ) to Milky Way (M 200 ∼ 10 12 M ⊙ ) masses. Our simulations exhibit a wide range of halo responses, primarily varying with mass, from expansion to contraction, with up to factor ∼ 10 changes in the enclosed dark matter mass at 1 per cent of the virial radius. Confirming previous studies, the halo response is correlated with the integrated efficiency of star formation:In addition, we report a new correlation with the compactness of the stellar system: ǫ R ≡ r 1/2 /R 200 . We provide an analytic formula depending on ǫ SF and ǫ R for the response of cold dark matter haloes to baryonic processes. An observationally testable prediction is that, at fixed mass, larger galaxies experience more halo expansion, while the smaller galaxies more halo contraction. This diversity of dark halo response is captured by a toy model consisting of cycles of adiabatic inflow (causing contraction) and impulsive gas outflow (causing expansion). For net outflow, or equal inflow and outflow fractions, f , the overall effect is expansion, with more expansion with larger f . For net inflow, contraction occurs for small f (large radii), while expansion occurs for large f (small radii), recovering the phenomenology seen in our simulations. These regularities in the galaxy formation process provide a step towards a fully predictive model for the structure of cold dark matter haloes.

A Testable Conspiracy: Simulating Baryonic Effects on Self-interacting Dark Matter Halos

“…Feedback could change the density profile of DM for dwarf galaxies, but it is not expected to be important for the baryondominated systems we simulate, which are Milky Way size or larger (Di Cintio et al 2014; Dutton et al 2016). It is important to note that when the equilibration timescale is short, changes due to feedback need to happen very late in order to prevent the DM from moving toward the equilibrium solution.…”

“…Fry et al (2015) find that the cores in their simulated dwarfs are not substantially different from those formed purely via feedback in their CDM simulations. It is not clear how these results generalize to larger halo masses, where feedback is expected to be less important in driving core formation in CDM halos and halo contraction effects are expected to dominate (e.g., Di Cintio et al 2014; Dutton et al 2016;Fiacconi et al 2016).…”

“…SIDM cores also may provide a natural explanation for the unexpectedly low densities of local dwarf galaxies (Vogelsberger et al 2012(Vogelsberger et al , 2014Elbert et al 2015), which is a problem known as "Too Big to Fail" (TBTF; Boylan-Kolchin et al 2011Ferrero et al 2012;Garrison-Kimmel et al 2014;Tollerud et al 2014;Klypin et al 2015;Papastergis et al 2015). There are many in the galaxy formation community who believe these issues may be resolved by baryonic processes, such as supernova feedback (Navarro et al 1996;Read & Gilmore 2005;Governato et al 2012;Di Cintio et al 2014;Maxwell et al 2015;Oñorbe et al 2015;Dutton et al 2016;Read et al 2016;Katz et al 2017), though not all authors necessarily agree (Peñarrubia et al 2012;Garrison-Kimmel et al 2013;Pace 2016). Tidal effects have been shown to solve TBTF in satellite galaxies (see e.g., Read et al 2006;Zolotov et al 2012;Arraki et al 2014;Brooks & Zolotov 2014;Del Popolo et al 2014), but the evidence for TBTF in the local field (Garrison-Kimmel et al 2014;Kirby et al 2014) necessitates another solution for these galaxies.…”

The milky way total mass profile as inferred from Gaia DR2

“…The higher than expected concentration of the MW halo could be a manifestation of the contraction of the DM halo induced by the presence of a galaxy at its centre (e.g. Schaller et al 2015;Dutton et al 2016;Lovell et al 2018). For MW and higher mass haloes, the effect of baryons on the DM halo is well described by the adiabatic contraction model (Callingham et al 2019a), in which baryons slowly accumulate at the halo centre and the DM distribution distorts in such a way that its action integrals remain approximately constant (Barnes & White 1984;Blumenthal et al 1986;Barnes 1987).…”

The orbital phase space of contracted dark matter haloes

“…The amount of DM contraction depends on many factors including the mass of the central galaxy, its assembly history and the orbital distribution of DM particles (e.g. Gnedin et al 2004;Abadi et al 2010;Duffy et al 2010;Schaller et al 2016;Dutton et al 2016;Artale et al 2019; Barnes & White 1984a;Blumenthal et al 1986).…”