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The removal of cusps from galaxy centres by stellar feedback in the early Universe


The standard cosmological model, now strongly constrained by direct observations of the Universe at early epochs, is very successful in describing the evolution of structure on large and intermediate scales1. Unfortunately, serious contradictions remain on smaller, galactic scales1,2. Among the main small-scale problems is a significant and persistent discrepancy between observations of nearby galaxies, which imply that galactic dark matter haloes have a density profile with a flat core3,4,5,6, and the cosmological model, which predicts that the haloes should have divergent density (a cusp) at the centre7,8. Here we report numerical simulations that show that random bulk motions of gas in small primordial galaxies, of the magnitude expected in these systems, will result in a flattening of the central dark matter cusp on relatively short timescales (108 years). Gas bulk motions in early galaxies are driven by supernova explosions that 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 galaxies9,10. As a consequence, in the present Universe both small and large galaxies would have flat dark matter core density profiles, in agreement with observations.

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Figure 1: Evolution of the dark matter density profile in the fiducial model.
Figure 2: Degree of flattening of the central dark matter cusp for different models.


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The simulations reported in this Letter were carried out on the McKenzie cluster at the Canadian Institute for Theoretical Astrophysics. H.M.P.C. is grateful for support from the Canadian Institute for Advanced Research, NSERC and SHARCNET. J.W. acknowledges support from NSERC.

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Mashchenko, S., Couchman, H. & Wadsley, J. The removal of cusps from galaxy centres by stellar feedback in the early Universe. Nature 442, 539–542 (2006).

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