MOND and the dynamics of NGC 628 [GA]

Aniyan et al. (2018) have recently published direct measurements of the baryonic mass distribution and the rotation curve of the almost-face-on disc galaxy NGC 628. While its very low inclination makes this galaxy anything but ideal for rotation-curve analysis, these new results, taken at face value, have interesting ramifications for MOND. The methods employed afford a direct determination of the stellar mass in the disc, which, in turn, affords a parameter-free MOND prediction of the rotation curve, which I show. In comparison, the dark-matter fits that Aniyan et al. present have two free parameters. To boot, these results further negate an earlier claim deleterious to MOND. It is that stellar $M/L$ ratios deduced from vertical velocity dispersions in disc galaxies are rather lower than what is required by MOND fits to rotation curves. Specifically, it was claimed that even high-surface-density discs are, by and large, sub maximal; viz., that they show substantial mass discrepancies near their center. This is contrary to the prediction of MOND that in such high-acceleration regions only small discrepancies should appear, if any to speak of. Such claims of low $M/L$ values have been rebutted before, and the fallacy that may have led to them pointed out. The new results strongly buttress these rebuttals.

Read this paper on arXiv…

M. Milgrom
Thu, 15 Feb 18

Comments: 3 pages, 1 figure