Diversity of dwarf galaxy rotation curves and ‘cusp-core’ controversy
The rotation curves (RCs) of dwarf galaxies are observed to show a wide diversity of shapes, from slowly to steeply rising (Fig.1), implying a range of inner mass distributions in apparent disagreement with the universal, ‘cuspy’ density profile of cold dark matter halos. This discrepancy, also known as the “cusp-core” problem, is one of the most prominent so-called “small-scale” tensions with the standard cosmological paradigm. I have studied this topic extensively from multiple perspectives.
Fig.1 (Fig.1 in Santos-Santos et al. 2020c)
Fig.2 (Fig.4 in Santos-Santos et al. 2020c)
Santos-Santos et al. (2016) was the first paper to study the so-called ‘mass discrepancy-acceleration’ relation (also known as “radial-acceleration relation”) using galaxies from cosmological simulations. This observational empirical relation shows a roughly universal correlation between the local radial acceleration due to the total gravitational potential of a galaxy and the acceleration due to only its baryonic component, and has been used to argue in favor of models such as Modified Newtonian dynamics (MOND) where there is no dark matter. I showed, however, that this relation naturally arises within the LCDM framework, with no need to invoke alternative models, and predicted that its scatter is driven by galaxies with slowly-rising RCs (corresponding to shallow, ‘cored’ inner dark matter density profiles).
In Santos-Santos et al. (2018) I showed that state-of-the-art hydrodynamical simulations including strong effective feedback from star formation (from the NIHAO/Gasoline collaboration) can successfully reproduce the observed galaxies with the most slowly-rising RC shapes (e.g. IC 2574). However, these models still fail to account for the full observed diversity of shapes, as they miss dwarfs with steeply-rising RCs (i.e. with ‘cuspy’ density profiles).
In Santos-Santos et al. (2020c), we set out to extract additional information from existing observational rotation curve data. We performed a detailed analysis of baryonic rotation curve profiles and uncovered a previously overlooked, puzzling trend. Specifically, we found that in dwarf galaxies with cuspy rotation curves, baryons contribute very little to the inner regions, whereas in galaxies with cores, baryons dominate the central mass budget (Fig. 2).
I demonstrated that no existing dark matter model can account for this trend directly. However, the effect reappears when simulated galaxies are mock-observed, owing to uncertainties introduced during the modelling process. Simulations indicate that dwarf galaxies are strongly influenced by gas non-circular motions near their centres. As a result, rotation curves derived from HI data do not trace the true circular velocity and therefore cannot reliably reveal the underlying dark matter density profile. These non-circular motions are often underestimated in modelling, creating a bias in which cuspy rotation curves are misinterpreted as cored. The bias is further amplified in the presence of HI thick disks (Roper et al., including Santos-Santos 2023). Subsequent studies are now building upon these findings.