Dark matter cores all the way down

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: <it>M</it><inf>200</inf> = 107–109 M<inf>⊙</inf>. We work at a resolution (∼4 pc minimum cell size; ∼250 M<inf>⊙</inf> 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 <it>r</it><inf>1/2</inf> always form if star formation proceeds for long enough. Cores fully form in less than 4 Gyr for the <it>M</it><inf>200</inf> = 108 M<inf>⊙</inf> and ∼14 Gyr for the 109 M<inf>⊙</inf> dwarf. We provide a convenient two parameter ‘<scp>core</scp>NFW’ 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 Λcold dark matter is correct, then ‘pristine’ dark matter cusps will be found either in systems that have truncated star formation and/or at radii <it>r</it> > <it>r</it><inf>1/2</inf>; (ii) complete core formation lowers the projected velocity dispersion at <it>r</it><inf>1/2</inf> by a factor of ∼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 sub-halo mass function inside galaxies and groups.