Systematic biases on galaxy haloes parameters from Yukawa-like gravitational potentials

TL;DR

This study investigates how assuming Newtonian gravity when fitting galaxy rotation curves under Yukawa-like modified gravity leads to systematic biases in halo parameter estimates, potentially explaining the cusp/core controversy.

Contribution

It demonstrates that neglecting Yukawa-like modifications causes overestimation of halo scale length and virial mass, and underestimation of dark matter fraction, affecting galaxy modeling.

Findings

Halo scale length and virial mass are overestimated.

Dark matter fraction within optical radius is underestimated.

Cored models fit well if disc mass is biased high.

Abstract

A viable alternative to the dark energy as a solution of the cosmic speed up problem is represented by Extended Theories of Gravity. Should this be indeed the case, there will be an impact not only on cosmological scales, but also at any scale, from the Solar System to extragalactic ones. In particular, the gravitational potential can be different from the Newtonian one commonly adopted when computing the circular velocity fitted to spiral galaxies rotation curves. Phenomenologically modelling the modified point mass potential as the sum of a Newtonian and a Yukawa like correction, we simulate observed rotation curves for a spiral galaxy described as the sum of an exponential disc and a NFW dark matter halo. We then fit these curves assuming parameterized halo models (either with an inner cusp or a core) and using the Newtonian potential to estimate the theoretical rotation curve. Such a study allows us to investigate the bias on the disc and halo model parameters induced by the systematic error induced by forcing the gravity theory to be Newtonian when it is not. As a general result, we find that both the halo scale length and virial mass are significantly overestimated, while the dark matter mass fraction within the disc optical radius is typically underestimated. Moreover, should the Yukawa scale length be smaller than the disc half mass radius, then the logarithmic slope of the halo density profile would turn out to be shallower than the NFW one. Finally, cored models are able to fit quite well the simulated rotation curves, provided the disc mass is biased high in agreement with the results in literature, favoring cored haloes and maximal discs. Such results make us argue that the cusp/core controversy could actually be the outcome of an incorrect assumption about which theory of gravity must actually be used in computing the theoretical circular velocity.