The radial acceleration relation at the EDGE of galaxy formation: testing its universality in low-mass dwarf galaxies

TL;DR

This study tests the universality of the Radial Acceleration Relation (RAR) at the low-mass end by deriving radially resolved $g_{ m obs}$ vs $g_{ m bar}$ for 12 dwarf galaxies using GravSphere-based Jeans modelling and by comparing with EDGE ΛCDM simulations. The results show that, unlike higher-mass galaxies, low-mass dwarfs do not lie on the SPARC RAR extrapolation; many sit above it and exhibit multiple observed accelerations at a given baryonic acceleration, with substantial galaxy-to-galaxy scatter. EDGE dwarfs reproduce a similar trend and, while they show less scatter than Local Group dwarfs, environment and assembly histories likely contribute to the observed dispersion. The findings challenge the view of a universal RAR at all masses and environments and support the necessity of dark matter in the smallest dwarfs, while disfavouring simple MOND-like external-field explanations for these systems.

Abstract

A tight correlation between the baryonic and observed acceleration of galaxies has been reported over a wide range of mass ($10^8 < M_{\rm bar}/{\rm M}_\odot < 10^{11}$) - the Radial Acceleration Relation (RAR). This has been interpreted as evidence that dark matter is actually a manifestation of some modified weak-field gravity theory. In this paper, we study the radially resolved RAR of 12 nearby dwarf galaxies, with baryonic masses in the range $10^4 < M_{\rm bar}/{\rm M}_\odot < 10^{7.5}$, using a combination of literature data and data from the MUSE-Faint survey. We use stellar line-of-sight velocities and the Jeans modelling code GravSphere to infer the mass distributions of these galaxies, allowing us to compute the RAR. We compare the results with the EDGE simulations of isolated dwarf galaxies with similar stellar masses in a $Λ$CDM cosmology. We find that most of the observed dwarf galaxies lie systematically above the low-mass extrapolation of the RAR. Each galaxy traces a locus in the RAR space that can have a multi-valued observed acceleration for a given baryonic acceleration, while there is significant scatter from galaxy to galaxy. Our results indicate that the RAR does not apply to low-mass dwarf galaxies and that the inferred baryonic acceleration of these dwarfs does not contain enough information, on its own, to derive the observed acceleration. The simulated EDGE dwarfs behave similarly to the real data, lying systematically above the extrapolated RAR. We show that, in the context of modified weak-field gravity theories, these results cannot be explained by differential tidal forces from the Milky Way, nor by the galaxies being far from dynamical equilibrium, since none of the galaxies in our sample seems to experience strong tides. As such, our results provide further evidence for the need for invisible dark matter in the smallest dwarf galaxies.

Figures (12)

• Figure 1: Stellar mass $M_\star$ against the half-light radius $r_{1/2}$ for our galaxy sample. The circles indicate the classical dSphs, and the triangles represent the dwarfs from MUSE-Faint. Objects are colour-coded by the number of stars with velocities available for each object. The red stars represent the EDGE simulated dwarfs.
• Figure 2: Testing our recovery of the RAR by applying GravSphere to mock data drawn from a simulated EDGE dwarfs. For this test, we use all of the simulated star particles to fit the photometric light profile and a random 1000 stars to measure the line-of-sight velocity dispersion profile (similar to our more poorly sampled observed dwarfs; see Table \ref{['tab:properties']}). The shaded band marks the 68% confidence intervals. The true answer (as determined directly from the simulations, using equations \ref{['eq:gobs']} and \ref{['eq:gbar']}, assuming spherical symmetry) is marked by the solid lines. The black solid line marks the RAR derived by mcgaugh_radial_2016 for SPARC local spiral galaxies. The blacked dashed line marks $g_\mathrm{obs} = g_\mathrm{bar}$.
• Figure 3: The radial acceleration relation (RAR) for our sample of dwarf galaxies. Each colour corresponds to a different galaxy, where the shaded area corresponds to the $68\%$ confidence interval and the solid line marks the median value. The dotted lines correspond to extrapolations beyond the kinematic data range. The black solid line marks the RAR derived by mcgaugh_radial_2016 for SPARC local spiral galaxies. The blacked dashed line marks $g_\mathrm{obs} = g_\mathrm{bar}$.
• Figure 4: The radial acceleration relation for the EDGE simulated dwarf galaxies. The colours show each simulation as marked in the legend. The black solid line corresponds to the RAR derived by mcgaugh_radial_2016 for SPARC local spiral galaxies. The black dashed line marks $g_\mathrm{obs} = g_\mathrm{bar}$. Left: RAR of the lower resolution simulations. Right: RAR of the higher resolution simulations.
• Figure 5: Quantitative comparison of the EDGE simulated RAR with our observed dwarf galaxy sample. The black solid line corresponds to the RAR derived by mcgaugh_radial_2016 for SPARC local spiral galaxies. The median and 68% scatter in the observed $g_\mathrm{obs}-g_\mathrm{bar}$ relations are marked by the grey solid lines and shaded regions, respectively. The $g_\mathrm{obs}-g_\mathrm{bar}$ relations estimated directly from the simulations are represented by solid red lines. The blacked dashed line marks $g_\mathrm{obs} = g_\mathrm{bar}$. The small blue points mark the positions of the dSphs on this relation lelli_one_2017, while the blue circles indicate the mean and $1\sigma$ scatter of the binned data. At the bottom, the residuals relative to the mcgaugh_radial_2016 relation (black solid line) are represented for the observed, the simulated, and the lelli_one_2017 data.