Abstract
We use high resolution simulations of isolated dwarf galaxies to study the
physics of dark matter cusp-core transformation at the edge of galaxy formation
(Mvir = 10^7 - 10^9 Msun). We work at a resolution (4 pc) 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 half light radius r_1/2 always form if star formation
proceeds for long enough. Cores fully form in less than 4 Gyrs for the Mvir
=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 half light radius.
Our results have several important implications: (i) we make a strong
prediction that if LambdaCDM 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' (though we stress that a full solution likely also involves
unmodelled environmental effects); 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. We will explore such environmental effects
in a forthcoming paper.
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