Dark matter cores all the way down

TL;DR

This study uses high-resolution simulations to demonstrate that dark matter cores form in dwarf galaxies with prolonged star formation, significantly impacting galaxy dynamics and the interpretation of dark matter distribution.

Contribution

It provides the first detailed simulation-based analysis showing dark matter core formation in dwarf galaxies at high resolution, with a new fitting function for core growth.

Findings

Dark matter cores form if star formation lasts long enough.

Cores form within 4-14 Gyrs depending on galaxy mass.

Core formation reduces velocity dispersion, explaining the 'too big to fail' problem.

Abstract

We use high resolution simulations of isolated dwarf galaxies to study the physics of dark matter cusp-core transformations at the edge of galaxy formation: M200 = 10^7 - 10^9 Msun. We work at a resolution (~4 pc minimum cell size; ~250 Msun per particle) at which the impact from individual supernovae explosions can be resolved, becoming insensitive to even large changes in our numerical 'sub-grid' parameters. We find that our dwarf galaxies give a remarkable match to the stellar light profile; star formation history; metallicity distribution function; and star/gas kinematics of isolated dwarf irregular galaxies. Our key result is that dark matter cores of size comparable to the stellar half mass radius (r_1/2) always form if star formation proceeds for long enough. Cores fully form in less than 4 Gyrs for the M200 = 10^8 Msun and 14 Gyrs for the 10^9 Msun dwarf. We provide a convenient two parameter 'coreNFW' fitting function that captures this dark matter core growth as a function of star formation time and the projected stellar half mass radius. Our results have several implications: (i) we make a strong prediction that if LCDM is correct, then 'pristine' dark matter cusps will be found either in systems that have truncated star formation and/or at radii r > r_1/2; (ii) complete core formation lowers the projected velocity dispersion at r_1/2 by a factor ~2, which is sufficient to fully explain the 'too big to fail problem'; and (iii) cored dwarfs will be much more susceptible to tides, leading to a dramatic scouring of the subhalo mass function inside galaxies and groups.