To the Editor — We performed a Bayesian analysis1 on galaxy rotation curves from the SPARC database2 and found strong evidence for a characteristic acceleration scale a0. We considered fits to individual galaxies where a0 is either held fixed or allowed to vary. Results are not meaningfully improved with variable a0, so there is no added value with such freedom. The data are consistent with a single value of a0.
A recent paper3 performs a subset of the same analysis using the same data, reaching the opposite conclusion. These authors neglect errors on galaxy inclination and assume flat priors with hard boundaries on stellar mass-to-light ratio and galaxy distance. Distance and inclination are measured quantities with associated errors, so the only sensible choice is a Gaussian prior with standard deviation given by the error1. A Gaussian prior on mass-to-light ratios is motivated by stellar population models and is superior to the flat prior in breaking parameter degeneracies1. The priors adopted in ref. 3 are prone to give unphysical results, with many mass-to-light ratios pinned to the extremes of the imposed boundaries.
Despite these important technical aspects, the difference between the conclusions of ref. 1 and ref. 3 is mostly driven by the interpretation of outliers. Figure 1 shows the baryonic Tully–Fisher relation from both analyses and a histogram of the scatter around the line of constant acceleration. The data tell a consistent story: the vast majority of the data cluster around a characteristic acceleration scale. The claim in ref. 3 that the width of this distribution cannot be ascribed to observational errors is driven by degeneracies in Bayesian fits and neglecting uncertainties in inclination. We performed error budget calculations many times4,5,6 and consistently found that they are enough to explain outliers.
The claim that modified Newtonian dynamics (MOND)7 is ruled out at high significance is not supported by the SPARC data. Rather, MOND predicts rotation curves with an average accuracy of 13% (ref. 1).
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The authors declare no competing interests.
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McGaugh, S.S., Li, P., Lelli, F. et al. Presence of a fundamental acceleration scale in galaxies. Nat Astron 2, 924 (2018). https://doi.org/10.1038/s41550-018-0615-9