Is Holmberg-II beyond MOND theory?

TL;DR

This paper compares the gas-rich dwarf galaxies KK246 and Holmberg II, finding that Holmberg II's rotation curve challenges MOND predictions due to its similar rotation velocity despite higher luminosity and gas mass.

Contribution

It provides observational evidence that Holmberg II's rotation curve may be inconsistent with MOND, questioning its universality in explaining galaxy dynamics.

Findings

Holmberg II's rotation velocity is similar to KK246 despite higher luminosity.

MOND predicts a higher asymptotic velocity for Holmberg II than observed.

The rotation curve of Holmberg II challenges MOND unless parameters are finely tuned.

Abstract

We compare the basic properties and kinematics of two gas-rich dwarf galaxies: KK246 and Holmberg II (HoII). HoII is 20 times more luminous in the blue-band than KK246 and its HI mass is a factor of 6 higher than in KK246. However, the amplitudes of the rotation curves (at the last measured point) of both galaxies are very similar, of about 40 km/s at a galactocentric radius of 7 kpc. This fact is challenging for modified theories of gravity that predict a one-to-one relation between gravity at any radius and the enclosed baryonic mass in galaxies. In particular, MOdified Newtonian Dynamics (MOND) predicts an asymptotic flat velocity of 60 km/s in HoII. Since the baryonic mass of HoII is dominated by the gas component, MOND overboosts its rotation speed even if the mass of the stellar disk is taken negligibly small. We conclude the rotation curve of HoII is probably inconsistent with MOND, unless the inclination and distance are both fine-tuned to very unlikely values.