Bulgeless dwarf galaxies and dark matter cores from supernova-driven outflows

Fighting the galactic bulge Observations show most dwarf galaxies to be almost 'bulgeless', consisting of a rotating stellar disc embedded in a massive near-constant-density core halo of cold dark matter. This sits uncomfortably with the predictions of models based on the dominance of cold dark matter, which invariably generate galaxies with dense stellar spheroidal bulges and steep central dark-matter profiles, as low-angular-momentum baryons and dark matter sink to the centre of galaxies through accretion and repeated mergers. Governato et al . report hydrodynamical simulations that resolve this paradox. Strong outflows from supernovae remove low-angular-momentum gas, thereby inhibiting the formation of bulges and decreasing dark-matter density around the centre of the galaxy. The properties of 'dwarf' galaxies have long challenged the cold dark matter (CDM) model of galaxy formation, as the properties of most observed dwarf galaxies contrast with models based on the dominance of CDM. Here, hydrodynamical simulations (assuming the presence of CDM) are reported in which the analogues of dwarf galaxies — bulgeless and with shallow central dark-matter profiles — arise naturally. For almost two decades the properties of ‘dwarf’ galaxies have challenged the cold dark matter (CDM) model of galaxy formation 1 . Most observed dwarf galaxies consist of a rotating stellar disk 2 embedded in a massive dark-matter halo with a near-constant-density core 3 . Models based on the dominance of CDM, however, invariably form galaxies with dense spheroidal stellar bulges and steep central dark-matter profiles 4 , 5 , 6 , because low-angular-momentum baryons and dark matter sink to the centres of galaxies through accretion and repeated mergers 7 . Processes that decrease the central density of CDM halos 8 have been identified, but have not yet reconciled theory with observations of present-day dwarfs. This failure is potentially catastrophic for the CDM model, possibly requiring a different dark-matter particle candidate 9 . Here we report hydrodynamical simulations (in a framework 10 assuming the presence of CDM and a cosmological constant) in which the inhomogeneous interstellar medium is resolved. Strong outflows from supernovae remove low-angular-momentum gas, which inhibits the formation of bulges and decreases the dark-matter density to less than half of what it would otherwise be within the central kiloparsec. The analogues of dwarf galaxies—bulgeless and with shallow