The inclination of the dwarf irregular galaxy Holmberg II

TL;DR

This study constrains the inclination of Holmberg II's H I disk using stability analysis, aligning its properties with the baryonic Tully-Fisher relation and MOND predictions, while highlighting potential issues for MOND in dwarf galaxies.

Contribution

It provides a new estimate of Holmberg II's inclination angle based on stability analysis and discusses implications for MOND and galaxy dynamics.

Findings

Inclination angle of 27 degrees for Holmberg II

Consistent with the baryonic Tully-Fisher relation

Potential challenges for MOND in dwarf galaxies

Abstract

We provide constraints on the inclination angle of the H\,{\sc i} disk of the dwarf irregular galaxy Holmberg II (Ho II) from stability analysis of the outer gaseous disk. We point out that a mean inclination angle of 27 degrees and thus a flat circular velocity of ~60 km/s, is required to have a level of gravitational stability similar to that found in other galaxies. Adopting this inclination angle, we find that Ho II lies on the right location in the baryonic Tully-Fisher relation. Moreover, for this inclination, its rotation curve is consistent with MOND. However, the corresponding analysis of the stability under MOND indicates that this galaxy could be problematic for MOND because its outer parts are marginally unstable in this gravity theory. We urge MOND simulators to study numerically the non-linear stability of gas-rich dwarf galaxies since it may provide a new key test for MOND.