Authors: Isabel M.E. Santos-Santos, Julio F. Navarro, et al.
First Author’s Institution: Department of Physics and Astronomy, University of Victoria, Victoria, BC, Canada
Status: Submitted to MNRAS, open access on arXiv
The known saying goes, “where there’s smoke, there’s fire.” Just like we infer the existence of a fire when we see smoke, we infer the existence of mass when we see objects moving in circles. Physics dictates that objects will revolve around a large mass — the larger the mass, the faster the rotation. When the observable matter (stars, dust, gas, etc. — collectively referred to as baryonic matter) is not sufficient to explain the rotational velocities, most astronomers conclude that there must be some form of invisible matter, or dark matter.
A successful theory needs to account for the rotational velocities as a function of radius (aka the galaxy’s “rotation curve”), which requires a precise knowledge of the density profile of baryons and dark matter. For most galaxies, ranging in size from galaxies like our own Milky Way to huge galaxy clusters with hundreds of Milky Way-sized galaxies, a simple model for dark matter is sufficient, and it is enough to know one parameter (like the total mass of the system, or the maximum rotational velocity), in order to accurately predict the full shape of a galaxy’s rotation curve.
However, the situation gets much more complicated with dwarf galaxies. Figure 1 shows the rotation curves of four dwarf galaxies that should, in theory, look similar. In some cases, the rotational velocities climb much faster than expected, meaning the central density is higher than our theories predict. In other cases, the rotational velocities climb surprisingly slowly, meaning the center of these galaxies must be less dense than we might expect.
So what is going on with these dwarf galaxies? Theorists have come up with a variety of explanations for why the rotation curves of dwarf galaxies differ from expectations. Today’s authors group these theories into four categories:
- Baryonic physics: Baryonic effects like feedback can create strong outflows of baryons from the inner regions of a galaxy, leading to a reduction in the dark matter content in those regions (causing rotation curves like the bottom right panel in Figure 1). At the same time, inflows of cold gas can have the opposite effect, increasing the density at the center of the galaxy and deepening the potential well (causing a rotation curve like the top left panel in Figure 1).
- Dark Matter physics: several theories predict that dark matter may be influenced by forces other than gravity, leading to deviations from the predicted rotation curves in Figure 1. One such example is Self-Interacting Dark Matter (SIDM), which is expected to form shallower potential wells (leading to rotation curves similar to the bottom right panel in Figure 1).
- Baryonic acceleration laws: some theorists have argued that the rotation curves of galaxies can be explained using only the spatial distribution of the baryonic components of galaxies, and that the contribution of the dark matter to the rotation curve is fully specified by that of the baryons. Such acceleration laws can (but don’t necessarily have to) arise from theories of modified gravity that do away with dark matter altogether.
- Observational uncertainties: uncertainties in the circular velocities used to measure rotation curves can arise if a large fraction of the orbits in the central region of the galaxy are non-circular. This is especially likely if the shape of the potential is triaxial instead of spherical. Such uncertainties can cause both under- and over-estimation of the central densities of dwarf galaxies, leading to the diversity of rotation curves shown in Figure 1.
The main goal of today’s paper is to evaluate each of these theories against the existing observational data. The authors make use of simulations (APOSTLE and NIHAO) to test whether any of the theories are able to reproduce the observed diversity of rotation curves. In each case, they compare the expectations from theory/simulations with the observed data. Figure 2 shows one such example to test the theory of a baryonic acceleration law (category #3 above).
So which of the four theories can justify the observed rotation curves? The authors argue that the answer is ‘none of the above.’ While each theory was able to explain certain galaxies, none of the theories reproduced the full diversity of the observed rotation curves without requiring additional assumptions (as shown in Figure 2 for the baryonic acceleration law explanation) .
It is very common for papers (and Astrobites) to end with some variation of the statement, “we need more data!” Today’s paper ends on a very different note — the data we already have highlights a clear issue with our current understanding of how dwarf galaxies behave. What we need is not more data, but rather a more satisfying theory to explain the dizzying diversity of dwarf galaxy rotation curves.
Featured image: rotation curve of a spiral galaxy. Credit: Queens University