NIHAO - XVIII. Origin of the MOND phenomenology of galactic rotation curves in a ΛCDM universe

The basis for Modified Newtonian Dynamics (MOND) is the radial acceleration relation (RAR) between the observed acceleration, a = V(rot)(2)rot(r)/r, and the acceleration accounted for by the observed baryons (stars and cold gas), a(bar) = V-bar(2)(r)/r. We show that the RAR arises naturally in the NIHAO sample of 89 high-resolution Lambda CDM cosmological galaxy formation simulations. The overall scatter from NIHAO is just 0.079 dex, consistent with observational constraints. However, we show that the scatter depends on stellar mass. At high masses (10(9) less than or similar to M-star 10(11) M-circle dot) the simulated scatter is just similar or equal to 0.04 dex, increasing to similar or equal to 0.11 dex at low masses (10(7) <= M-star <= 10(9) M-circle dot). Observations show a similar dependence for the intrinsic scatter. At high masses the intrinsic scatter is consistent with the zero scatter assumed by MOND, but at low masses the intrinsic scatter is non-zero, strongly disfavouring MOND. Applying MOND to our simulations yields remarkably good fits to most of the circular velocity profiles. In cases of mild disagreement the stellar mass-to-light ratio and/or 'distance' can be tuned to yield acceptable fits, as is often done in observational mass models. In dwarf galaxies with M-star similar to 10(6)M(circle dot) MOND breaks down, predicting lower accelerations than observed and in our Lambda CDM simulations. The assumptions that MOND is based on (e.g. asymptotically flat rotation curves, zero intrinsic scatter in the RAR) are only approximately true in Lambda CDM. Thus if one wishes to go beyond Newtonian dynamics there is more freedom in the observed RAR than assumed by MOND.