Understanding the shape and diversity of dwarf galaxy rotation curves in ΛCDM

The shape and diversity of dwarf galaxy rotation curves is at apparent odds with dark matter halos in a Λ Cold Dark Matter (ΛCDM) cosmology. We use mock data from isolated dwarf galaxy simulations to show that this owes to three main effects. Firstly, stellar feedback heats dark matter, leading to a ‘coreNFW’ dark matter density profile with a slowly rising rotation curve. Secondly, if close to a recent starburst, HI bubbles push the rotation curve out of equilibrium, deforming the rotation curve shape. Thirdly, when galaxies are viewed near face-on, their best fit inclination is biased high. All three problems can be avoided, however, by a combination of improved mass models and a careful selection of target galaxies. Fitting our coreNFW model to mock rotation curve data, we show that we can recover the rotation curve shape, dark matter halo mass M200 and concentration parameter c within our quoted uncertainties. We fit our coreNFW model to four isolated dwarf irregulars, chosen to span a range of rotation curve shapes. We obtain an excellent fit for NGC 6822 and WLM, with tight constraints on M200, and c consistent with ΛCDM. However, IC 1613 and DDO 101 give a poor fit. For IC 1613, this owes to disequilibria and its uncertain inclination i; for DDO 101, to its uncertain distance D. Using iIC1613 ∼ 15◦ and DDDO101 ∼ 12 Mpc, consistent with current uncertainties, we obtain good fits. We conclude that ΛCDM appears to give an excellent match to dwarf galaxy rotation curves.