Local Group galaxies emerge from the dark

Abstract

The "Lambda Cold Dark Matter" (ΛCDM) model of cosmic structure formation is eminently falsifiable: once its parameters are fixed on large scales, it becomes testable in the nearby Universe. Observations within our Local Group of galaxies, including the satellite populations of the Milky Way and Andromeda, appear to contradict ΛCDM predictions: there are far fewer satellite galaxies than dark matter halos (the "missing satellites" problem (1, 2)), galaxies seem to avoid the largest substructures (the "too big to fail" problem (3, 4)), and the brightest satellites appear to orbit their host galaxies on a thin plane (the "planes of satellites" problem ( 5)). We present results from the first hydrodynamic simulations of the Local Group that match the observed abundance of galaxies. We find that when baryonic and dark matter are followed simultaneously in the context of a realistic galaxy formation model, all three "problems" are resolved within the ΛCDM paradigm.The ability of the cold dark matter model to predict observables on different scales and at different epochs lies at the root of its remarkable success. The anisotropy of the microwave background radiation and the large scale distribution of galaxies were predicted after the model was formulated, and have since been spectacularly validated by observations. However, observations on scales currently testable only within the Local Group (LG) have yielded results that no simulation to date has been able to reproduce. This has renewed interest in alternatives to ΛCDM, such as warm (6) or self-interacting (7) dark matter.

The unexpected diversity of dwarf galaxy rotation curves

“…We emphasize that, although our simulations do not form "cores", they do produce galaxies whose abundance, structural properties, and evolution seem in good accord with observational constraints (see, e.g., Schaye et al 2015;Sawala et al 2014;Furlong et al 2015;Schaller et al 2014). The formation of dark matter "cores" thus does not appear to be a requisite ingredient of galaxy formation simulations that successfully reproduce the structural properties of the observed galaxy population, at least for galaxies with stellar masses M * > ∼ 10 9 M .…”

“…We discuss here the circular velocity curves of galaxies selected from the EAGLE (Schaye et al 2015;Crain et al 2015) and LO-CAL GROUPS (Sawala et al 2015) simulation projects. EAGLE is calibrated to reproduce, in a cosmological volume, the observed population of galaxies, including their abundance as a function of galaxy mass and their typical size.…”

“…The LOCAL GROUPS simulation suite produces realistic Local Group-like environments, reproducing the stellar mass function of Milky Way and M31 satellites, and that of Local Group dwarf galaxies (Sawala et al 2014) using the same calibration parameter choices as the EAGLE-Ref simulations.…”

Dynamical history of the Local Group in ΛCDM

“…This result has recently been challenged by Sawala et al (2014) and Sawala et al (2016) based on the eagle simulations, which include baryonic physics (Schaye et al 2015;Crain et al 2015). When comparing with the observed satellite systems of the MW and M31, these investigations did not take into account all of the available information, in particular the observed distances to the MW satellites.…”

Gas Inflow and Outflow Histories in Disk Galaxies as Revealed From Observations of Distant Star-Forming Galaxies

“…Indeed, earlier simulations with only modest feedback effects suffer from the so-called angular momentum problem that the simulated disk galaxies have too low angular momentum, thereby being more compact and denser than the observed ones (e.g., Navarro & Steinmetz 2000). Recent high-resolution hydrodynamical simulations taking into account stronger feedback effects based on more realistic physical background have partially resolved such problems in galaxy formation (e.g., 1 Astronomical Institute, Tohoku University, Aoba-ku, Sendai 980-8578, Japan Okamoto et al 2014;Vogelsberger et al 2014;Sawala et al 2014), but have not been completely successful yet (e.g., Sparre et al 2014;Oman et al 2015).…”