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

Abstract

The shape and diversity of dwarf galaxy rotation curves is at apparent odds with dark matter halos in a $\Lambda$ Cold Dark Matter ($\Lambda$CDM) 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, large 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. This can lead to a very shallow rotation curve that falsely implies a large dark matter core. 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 $M_{200}$ and concentration parameter $c$ within our quoted uncertainties. We fit our coreNFW model to real data for four isolated dwarf irregulars, chosen to span a wide range of rotation curve shapes. We obtain an excellent fit for NGC 6822 and WLM, with tight constraints on $M_{200}$, and $c$ consistent with $\Lambda$CDM. However, IC 1613 and DDO 101 give a poor fit. For IC 1613, we show that this owes to disequilibria and its uncertain inclination $i$; for DDO 101, it owes to its uncertain distance $D$. If we assume $i_{\rm IC1613} \sim 15^\circ$ and $D_{\rm DDO101} \sim 12$ Mpc, consistent with current uncertainties, we are able to fit both galaxies very well. We conclude that $\Lambda$CDM appears to give an excellent match to dwarf galaxy rotation curves.