The significant diversity of rotation curve (RC) shapes in dwarf galaxies has recently emerged as a challenge to ΛCDM: in dark matter (DM) only simulations, DM halos have a universal cuspy density profile that results in self-similar RC shapes. We compare RC shapes of simulated galaxies from the NIHAO project with observed galaxies from the homogeneous SPARC dataset. The DM halos of the NIHAO galaxies can expand to form cores, with the degree of expansion depending on their stellar-to-halo mass ratio. By means of the V 2kpc − V Rlast relation (where V Rlast is the outermost measured rotation velocity), we show that both the average trend and the scatter in RC shapes of NIHAO galaxies are in reasonable agreement with SPARC: this represents a significant improvement compared to simulations that do not result in DM core formation, suggesting that halo expansion is a key process in matching the diversity of dwarf galaxy RCs. Note that NIHAO galaxies can reproduce even the extremely slowly rising RCs of IC 2574 and UGC 5750. Revealingly, the range where observed galaxies show the highest diversity corresponds to the range where core formation is most efficient in NIHAO simulations, 50

We use N-body simulations to revisit the globular cluster (GC) "timing problem" in the Fornax dwarf spheroidal (dSph). In agreement with earlier work, we find that, due to dynamical friction, GCs sink to the center of dark matter halos with a cuspy inner density profile but "stall" at roughly 1/3 of the core radius (r core ) in halos with constant-density cores. The timescales to sink or stall depend strongly on the mass of the GC and on the initial orbital radius, but are essentially the same for either cuspy (NFW) or cored halos normalized to have the same total mass within r core . Arguing against a cusp on the basis that GCs have not sunk to the center is thus no different from arguing against a core, unless all clusters are today at ∼ (1/3) r core . This would imply a core radius exceeding ∼ 3 kpc, much larger than seems plausible in any core-formation scenario. (The average projected distance of Fornax GCs is R GC,Fnx ∼ 1 kpc and its effective radius is ∼ 700 pc.) A simpler explanation is that Fornax GCs have only been modestly affected by dynamical friction, as expected if clusters started orbiting at initial radii of order ∼ 1-2 kpc, just outside Fornax's present-day half-light radius but well within the tidal radius imprinted by Galactic tides. This is not entirely unexpected. Fornax GCs are significantly older and more metal-poor than most Fornax stars, and such populations in dSphs tend to be more spatially extended than their younger and more metal-rich counterparts. Contrary to some earlier claims, our simulations further suggest that GCs do not truly "stall" at ∼ 0.3 r core , but rather continue decaying toward the center, albeit at reduced rates. We conclude that dismissing the presence of a cusp in Fornax based on the spatial distribution of its GC population is unwarranted.

We use a sample of galaxies with high-quality rotation curves to assess the role of the luminous component ("baryons") in the dwarf galaxy rotation curve diversity problem. As in earlier work, we find that the shape of the rotation curve correlates with baryonic surface density; high surface density galaxies have rapidly-rising rotation curves consistent with cuspy cold dark matter halos, slowly-rising rotation curves (characteristic of galaxies with inner mass deficits or "cores") occur only in low surface density galaxies. The correlation, however, seems too weak in the dwarf galaxy regime to be the main driver of the diversity. In particular, the observed dwarf galaxy sample includes "cuspy" systems where baryons are unimportant in the inner regions and "cored" galaxies where baryons actually dominate the inner mass budget. These features are important diagnostics of the viability of various scenarios proposed to explain the diversity, such as (i) baryonic inflows and outflows; (ii) dark matter self-interactions (SIDM); (iii) variations in the baryonic acceleration through the "mass discrepancy-acceleration relation" (MDAR); or (iv) non-circular motions in gaseous discs. A reanalysis of existing data shows that MDAR does not hold in the inner regions of dwarf galaxies and thus cannot explain the diversity. Together with analytical modeling and cosmological hydrodynamical simulations, our analysis shows that each of the remaining scenarios has promising features, but none seems to fully account for the observed diversity. The origin of the dwarf galaxy rotation curve diversity and its relation to the small structure of cold dark matter remains an open issue.

Using 22 hydrodynamical simulated galaxies in a ΛCDM cosmological context we recover not only the observed baryonic Tully-Fisher relation, but also the observed "mass discrepancy-acceleration" relation, which reflects the distribution of the main components of the galaxies throughout their disks. This implies that the simulations, which span the range 52

We compare the Baryonic Tully-Fisher relation (BTFR) of simulations and observations of galaxies ranging from dwarfs to spirals, using various measures of rotational velocity V rot . We explore the BTFR when measuring V rot at the flat part of the rotation curve, V flat , at the extent of HI gas, V last , and using 20% (W 20 ) and 50% (W 50 ) of the width of HI line profiles. We also compare with the maximum circular velocity of the parent halo, V DM max , within dark matter only simulations. The different BTFRs increasingly diverge as galaxy mass decreases. Using V last one obtains a power law over four orders of magnitude in baryonic mass, with slope similar to the observed BTFR. Measuring V flat gives similar results as V last when galaxies with rising rotation curves are excluded. However, higher rotation velocities would be found for low mass galaxies if the cold gas extended far enough for V rot to reach a maximum. W 20 gives a similar slope as V last but with slightly lower values of V rot for low mass galaxies, although this may depend on the extent of the gas in your galaxy sample. W 50 bends away from these other relations toward low velocities at low masses. By contrast, V DM max bends toward high velocities for low mass galaxies, as cold gas does not extend out to the radius at which halos reach V DM max . Our study highlights the need for careful comparisons between observations and models: one needs to be consistent about the particular method of measuring V rot , and precise about the radius at which velocities are measured.