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

TL;DR

This paper investigates the apparent discrepancies between dwarf galaxy rotation curves and $ m f ext{LCDM}$ predictions, showing that many issues can be resolved with improved models and careful galaxy selection, leading to a good overall match.

Contribution

The study introduces the coreNFW model and demonstrates its effectiveness in fitting dwarf galaxy rotation curves within $ m f ext{LCDM}$, accounting for feedback effects and observational biases.

Findings

CoreNFW model accurately fits most dwarf galaxy rotation curves.

Biases in inclination and distance can explain some poor fits.

$ m f ext{LCDM}$ remains consistent with dwarf galaxy data.

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.