Testing modified gravity with dwarf spheroidal galaxies

TL;DR

This study tests modified gravity (MOG) as an alternative to dark matter in dwarf spheroidal galaxies by analyzing their velocity dispersions, finding that MOG parameters need to vary per galaxy and questioning the universality of certain parameters.

Contribution

It applies MOG theory to dwarf spheroidal galaxies and demonstrates the need for galaxy-specific parameter adjustments, challenging the universality of MOG parameters.

Findings

MOG can fit velocity dispersions but requires varying parameters per galaxy.

The parameter μ correlates with galaxy luminosity, questioning its universality.

Standard stellar population models cannot explain the high mass-to-light ratios without dark matter.

Abstract

The observed velocity dispersion of the classical dwarf spheroidal (dSph) galaxies of the Milky Way (MW) requires the Newtonian stellar mass-to-light ($M_*/L$) ratios in the range of about 10 to more than 100 solar units that are well outside the acceptable limit predicted by stellar population synthesis models. Using Jeans analysis, we calculate the line-of-sight velocity dispersion ($\sigma_{\emph{los}}$) of stars in eight MW dSphs in the context of the modified gravity (MOG) theory of Moffat, assuming a constant $M_*/L$ ratio without invoking the exotic cold dark matter. First, we use the weak field approximation of MOG and assume the two parameters $ \alpha $ and $ \mu $ of the theory to be constant as has already been inferred from fitting to the observed rotational data of The HI Nearby Galaxy Survey catalogue of galaxies. We find that the derived $M_*/L$ ratios for almost all dSphs are too large to be explained by the stellar population values. In order to fit the line-of-sight velocity dispersions of the dSph with reasonable $M_*/L$ values, we must vary $\alpha$ and $\mu$ on a case by case basis. A common pair of values cannot be found for all dSphs. Comparing with the values found from rotation curve fitting, it appears that $\mu$ correlates strongly with galaxy luminosity, shedding doubt on it as a universal constant.