The thickness of HI in galactic discs under MOND: theory and application to the Galaxy

TL;DR

This paper develops a framework to test MOND gravity using HI gas layer flaring in the Milky Way, finding good agreement beyond 17 kpc but some discrepancies closer in, suggesting MOND as a plausible alternative to dark matter.

Contribution

It introduces a simple method to derive HI disc scaleheight in MOND and applies it to the Milky Way, providing new insights into galactic gravity at low accelerations.

Findings

MOND fits HI flaring beyond 17 kpc well

Discrepancies exist between 10-16 kpc

MOND remains a plausible alternative to dark matter

Abstract

The outskirts of galaxies are a very good laboratory for testing the nature of the gravitational field at low accelerations. By assuming that the neutral hydrogen gas is in hydrostatic equilibrium in the gravitational potential of the host galaxy, the observed flaring of the gas layer can be used to test modified gravities. For the first time we construct a simple framework to derive the scaleheight of the neutral hydrogen gas disc in the MOND scenario and apply this to the Milky Way. It is shown that using a constant gas velocity dispersion of ~9 km/s, MOND is able to give a very good fit to the observed HI flaring beyond a galactocentric distance of 17 kpc up to the last measured point (~40 kpc). Between 10 and 16 kpc, however, the observed scaleheight is about 40% more than what MOND predicts for the standard interpolating function and 70% for the form suggested by Famaey & Binney. Given the uncertainties in the non-thermal pressure support by cosmic rays and magnetic fields, MOND seems to be a plausible alternative to dark matter in explaining the Milky Way flaring. Studying the flaring of extended HI discs in external edge-on galaxies may be a promising approach to assess the viability of MOND.