Cosmological puzzle resolved by stellar feedback in high redshift galaxies
Abstract
The standard cosmological model, now strongly constrained by direct
observation at early epochs, is very successful in describing the structure of
the evolved universe on large and intermediate scales. Unfortunately, serious
contradictions remain on smaller, galactic scales. Among the major small-scale
problems is a significant and persistent discrepancy between observations of
nearby galaxies, which imply that galactic dark matter (DM) haloes have a
density profile with a flat core, and the cosmological model, which predicts
that the haloes should have divergent density (a cusp) at the centre. Here we
use numerical N-body simulations to show that random bulk motions of gas in
small primordial galaxies, of the magnitude expected in these systems, result
in a flattening of the central DM cusp on short timescales (of order 10^8
years). Gas bulk motions in early galaxies are driven by supernova explosions
which result from ongoing star formation. Our mechanism is general and would
have operated in all star-forming galaxies at redshifts z>~ 10. Once removed,
the cusp cannot be reintroduced during the subsequent mergers involved in the
build-up of larger galaxies. As a consequence, in the present universe both
small and large galaxies would have flat DM core density profiles, in agreement
with observations.