What Do Gas-Rich Galaxies Actually Tell Us about Modified Newtonian Dynamics?

It has recently been claimed that measurements of the baryonic Tully-Fisher relation (BTFR), a power-law relationship between the observed baryonic masses and outer rotation velocities of galaxies, support the predictions of modified Newtonian dynamics for the slope and scatter in the relation, while challenging the cold dark matter (CDM) paradigm. We investigate these claims, and find that (1) the scatter in the data used to determine the BTFR is in conflict with observational uncertainties on the data, (2) these data do not make strong distinctions regarding the best-fit BTFR parameters, (3) the literature contains a wide variety of measurements of the BTFR, many of which are discrepant with the recent results, and (4) the claimed CDM “prediction” for the BTFR is a gross oversimplification of the complex galaxy-scale physics involved. We conclude that the BTFR is currently untrustworthy as a test of CDM.

Figure 1

Histograms (green bars) of values of $a=V_{\mathrm{f}}^4/G{M}_{\mathrm{b}}$ (in units of $\mathrm{m}{\mathrm{s}}^{-2}$) for MG11 and FS11 data sets, along with fitted Gaussians (black, solid curves) and Gaussians with the most likely mean and variance for data sets with the same statistical properties as the measured data (blue, dashed curves). For MG11, the Monte Carlo-generated distribution is much wider than the fitted distribution, indicating a discrepancy in the level of scatter in the data as compared to the stated error bars. This discrepancy is less severe for FS11.

Figure 2

68% (light blue, inner) and 95% (dark blue, outer) confidence regions for the best-fit values of $a_0$ (in units of $\mathrm{m}{\mathrm{s}}^{-2}$) and $\alpha$ for the MG11 and FS11 data sets (plotted on different sets of axes). It is apparent that the quality of the data makes it difficult to distinguish $\alpha =4$ (the prediction of MOND) from lower values; this difficulty is reflected in the wide range of slopes measured in other studies [7, 9, 10, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32].