Dark Matter Heating and Early Core Formation in Dwarf Galaxies

TL;DR

This study uses cosmological simulations to show how supernova-driven gas outflows transform dark matter cusps into cores in dwarf galaxies, explaining observed properties and addressing the too-big-to-fail problem.

Contribution

It demonstrates that dark matter core formation in dwarf galaxies occurs early and is energetically feasible, with implications for galaxy formation theories.

Findings

Dark matter cores form in dwarfs with stellar mass > 1e6 Msun.

Core formation is energetically less demanding than previously thought.

Simulated inner dark matter profiles match observations of Fornax and Sculptor.

Abstract

We present more results from a fully cosmological LCDM simulation of a group of isolated dwarf galaxies that has been shown to reproduce the observed stellar mass and cold gas content, resolved star formation histories, and metallicities of dwarfs in the Local Volume. Here we investigate the energetics and timetable of the cusp-core transformation. As suggested by previous work, supernova-driven gas outflows remove dark matter (DM) cusps and create kpc-size cores in all systems having a stellar mass above 1e6 Msun. The "DM core mass removal efficiency" -- dark mass ejected per unit stellar mass -- ranges today from a few to a dozen, and increases with decreasing host mass. Because dwarfs form the bulk of their stars prior to redshift 1 and the amount of work required for DM heating and core formation scales approximately as Mvir^{5/3}, the unbinding of the DM cusp starts early and the formation of cored profiles is not as energetically onerous as previously claimed. DM particles in the cusp typically migrate to 2-3 core radii after absorbing a few percent of the energy released by supernovae. The present-day slopes of the inner dark matter mass profiles, Gamma=d log M/dlog R=2.5-3, of the simulated "Bashful" and "Doc" dwarfs are similar to those measured in the luminous Fornax and Sculptor dwarf spheroidals. None of the simulated galaxies has a circular velocity profile exceeding 20 km/s in the inner 1 kpc, implying that supernova feedback is key to solve the "too-big-to-fail" problem for Milky Way subhalos.