For almost two decades the properties of 'dwarf' galaxies have challenged the cold dark matter (CDM) model of galaxy formation. Most observed dwarf galaxies consist of a rotating stellar disk embedded in a massive dark-matter halo with a near-constant-density core. Models based on the dominance of CDM, however, invariably form galaxies with dense spheroidal stellar bulges and steep central dark-matter profiles, because low-angular-momentum baryons and dark matter sink to the centres of galaxies through accretion and repeated mergers. Processes that decrease the central density of CDM halos 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. Here we report hydrodynamical simulations (in a framework 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 central dark-matter profiles-arise naturally in these simulations.

We present an analysis of star formation and feedback recipes appropriate for galactic smoothed particle hydrodynamics simulations. Using an isolated Milky Way-like galaxy, we constrain these recipes based on well-established observational results. Our star formation recipe is based on that of Katz with the additional inclusion of physically motivated supernova feedback recipes. We propose a new feedback recipe in which Type II supernovae are modelled using an analytical treatment of blastwaves. With this feedback mechanism and a tuning of other star formation parameters, the star formation in our isolated Milky Way-like galaxy follows the slope and normalization of the observed Schmidt law. In addition, we reproduce the lowdensity cut-off and filamentary structure of star formation observed in disc galaxies. Our final recipe will enable better comparison of N-body simulations with observations.

We compare the results of various cosmological gas-dynamical codes used to simulate the formation of a galaxy in the Λ cold dark matter structure formation paradigm. The various runs (13 in total) differ in their numerical hydrodynamical treatment [smoothed particle hydrodynamics (SPH), moving mesh and adaptive mesh refinement] but share the same initial conditions and adopt in each case their latest published model of gas cooling, star formation and feedback. Despite the common halo assembly history, we find large code-to-code variations in the stellar mass, size, morphology and gas content of the galaxy at z= 0, due mainly to the different implementations of star formation and feedback. Compared with observation, most codes tend to produce an overly massive galaxy, smaller and less gas rich than typical spirals, with a massive bulge and a declining rotation curve. A stellar disc is discernible in most simulations, although its prominence varies widely from code to code. There is a well-defined trend between the effects of feedback and the severity of the disagreement with observed spirals. In general, models that are more effective at limiting the baryonic mass of the galaxy come closer to matching observed galaxy scaling laws, but often to the detriment of the disc component. Although numerical convergence is not particularly good for any of the codes, our conclusions hold at two different numerical resolutions. Some differences can also be traced to the different numerical techniques; for example, more gas seems able to cool and become available for star formation in grid-based codes than in SPH. However, this effect is small compared to the variations induced by different feedback prescriptions. We conclude that state-of-the-art simulations cannot yet uniquely predict the properties of the baryonic component of a galaxy, even when the assembly history of its host halo is fully specified. Developing feedback algorithms that can effectively regulate the mass of a galaxy without hindering the formation of high angular momentum stellar discs remains a challenge

A study of metal enrichment of the intergalactic medium (IGM) using a series of smooth particle hydrodynamic (SPH) simulations is presented, employing models for metal cooling and the turbulent diffusion of metals and thermal energy. An adiabatic feedback mechanism was adopted where gas cooling was prevented on the time-scale of supernova bubble expansion to generate galactic winds without explicit wind particles. The simulations produced a cosmic star formation history (SFH) that is broadly consistent with observations until z ∼ 0.5, and a steady evolution of the universal neutral hydrogen fraction ( H I ) that compares reasonably well with observations. The evolution of the mass and metallicities in stars and various gas phases was investigated. At z = 0, about 40 per cent of the baryons are in the warm-hot intergalactic medium (WHIM), but most metals (80-90 per cent) are locked in stars. At higher redshifts the proportion of metals in the IGM is higher due to more efficient loss from galaxies. The results also indicate that IGM metals primarily reside in the WHIM throughout cosmic history, which differs from simulations with hydrodynamically decoupled explicit winds. The metallicity of the WHIM lies between 0.01 and 0.1 solar with a slight decrease at lower redshifts. The metallicity evolution of the gas inside galaxies is broadly consistent with observations, but the diffuse IGM is under enriched at z ∼ 2.5.Galactic winds most efficiently enrich the IGM for haloes in the intermediate mass range 10 10 -10 11 M . At the low-mass end gas is prevented from accreting on to haloes and has very low metallicities. At the high-mass end, the fraction of halo baryons escaped as winds declines along with the decline of stellar mass fraction of the galaxies. This is likely because of the decrease in star formation activity and decrease in wind escape efficiency. Metals enhance cooling which allows WHIM gas to cool on to galaxies and increases star formation. Metal diffusion allows winds to mix prior to escape, decreasing the IGM metal content in favour of gas within galactic haloes and star-forming gas. Diffusion significantly increases the amount of gas with low metallicities and changes the density-metallicity relation.The intergalactic medium (IGM) contains most of the baryons in the Universe, and provides the fuel for galaxies to form stars in which metals are produced. In turn, supernovae (SNe) and galactic winds enrich the IGM with metals, while stars and active galactic nuclei (AGN) emit UV photons. This interplay between the IGM and galaxies, mediated by metal cooling in the presence of UV, regulates the formation of stars in the Universe. The evolution and enrichment history of the IGM provides a record of this interplay.

We use a suite of 31 simulated galaxies drawn from the MaGICC project to investigate the effects of baryonic feedback on the density profiles of dark matter haloes. The sample covers a wide mass range: 9.4 × 10 9 < M halo /M ⊙ < 7.8 × 10 11 , hosting galaxies with stellar masses: 5.0 × 10 5 < M ⋆ /M ⊙ < 8.3 × 10 10 , i.e. from dwarf to L ⋆ . The galaxies are simulated with blastwave supernova feedback and, for some of them, an additional source of energy from massive stars is included. Within this feedback scheme we vary several parameters, such as the initial mass function, the density threshold for star formation and energy from supernovae and massive stars.The main result is a clear dependence of the inner slope of the dark matter density profile, α in ρ ∝ r α , on the ratio between stellar-to-halo mass, M ⋆ /M halo . This relation is independent of the particular choice of parameters within our stellar feedback scheme, allowing a prediction for cusp vs core formation. When M ⋆ /M halo is low, < ∼ 0.01 per cent, energy from stellar feedback is insufficient to significantly alter the inner dark matter density and the galaxy retains a cuspy profile. At higher ratios of stellar-to-halo mass feedback drives the expansion of the dark matter and generates cored profiles. The flattest profiles form where M ⋆ /M halo ∼ 0.5 per cent. Above this ratio, stars formed in the central regions deepen the gravitational potential enough to oppose the supernova-driven expansion process, resulting in cuspier profiles. Combining the dependence of α on M ⋆ /M halo with the empirical abundance matching relation between M ⋆ and M halo provides a prediction for how α varies as a function of stellar mass. Further, using the Tully-Fisher relation allows a prediction for the dependence of the dark matter inner slope on the observed rotation velocity of galaxies. The most cored galaxies are expected to have V rot ∼ 50 km s −1 , with α decreasing for more massive disc galaxies: spirals with V rot ∼ 150 km s −1 have central slopes α −0.8, approaching again the NFW profile. This novel prediction for the dependence of α on disc galaxy mass can be tested using observational data sets and can be applied to theoretical modeling of mass profiles and populations of disc galaxies.