The unexpected diversity of dwarf galaxy rotation curves

TL;DR

This study compares simulated and observed dwarf galaxy rotation curves, revealing a significant diversity in observed profiles that current models struggle to reproduce, indicating gaps in understanding dark matter or baryonic effects.

Contribution

It highlights the discrepancy between simulated and observed dwarf galaxy rotation curves, emphasizing the need to reconsider dark matter models or baryonic physics in simulations.

Findings

Simulated galaxy rotation curves are less diverse than observed.

Many dwarf galaxies show an inner mass deficit not explained by current models.

The 'cusp vs core' problem is better seen as an 'inner mass deficit' issue.

Abstract

We examine the circular velocity profiles of galaxies in {\Lambda}CDM cosmological hydrodynamical simulations from the EAGLE and LOCAL GROUPS projects and compare them with a compilation of observed rotation curves of galaxies spanning a wide range in mass. The shape of the circular velocity profiles of simulated galaxies varies systematically as a function of galaxy mass, but shows remarkably little variation at fixed maximum circular velocity. This is especially true for low-mass dark matter-dominated systems, reflecting the expected similarity of the underlying cold dark matter haloes. This is at odds with observed dwarf galaxies, which show a large diversity of rotation curve shapes, even at fixed maximum rotation speed. Some dwarfs have rotation curves that agree well with simulations, others do not. The latter are systems where the inferred mass enclosed in the inner regions is much lower than expected for cold dark matter haloes and include many galaxies where previous work claims the presence of a constant density "core". The "cusp vs core" issue is thus better characterized as an "inner mass deficit" problem than as a density slope mismatch. For several galaxies the magnitude of this inner mass deficit is well in excess of that reported in recent simulations where cores result from baryon-induced fluctuations in the gravitational potential. We conclude that one or more of the following statements must be true: (i) the dark matter is more complex than envisaged by any current model; (ii) current simulations fail to reproduce the effects of baryons on the inner regions of dwarf galaxies; and/or (iii) the mass profiles of "inner mass deficit" galaxies inferred from kinematic data are incorrect.