Overestimated inclinations of Milgromian disc galaxies: the case of the ultradiffuse galaxy AGC 114905

TL;DR

This study uses hydrodynamical simulations within the MOND framework to show that inclination overestimation can explain the apparent discrepancy of AGC 114905's rotation curve with MOND predictions.

Contribution

It demonstrates that external fields and inclination misestimations can reconcile the observed rotation curve of AGC 114905 with MOND predictions.

Findings

Inclination overestimation can lead to underestimating rotation curves.

External gravitational fields influence galaxy isophotes and perceived inclinations.

Correcting for inclination can align observations with MOND expectations.

Abstract

We present hydrodynamical star-forming simulations in the Milgromian dynamics (MOND) framework of a gas-rich disc galaxy with properties similar to AGC 114905, which has recently been argued to have a rotation curve (RC) that is inconsistent with the MOND prediction. Our first model considers the galaxy in isolation, while our second model includes an external field of $0.05 \, a_{_0}$, the estimated gravitational field from large-scale structure. We show that isophotes in the face-on view can differ from circular at the 50% level. This could mislead observers into overestimating the inclination $i$ between disc and sky planes. Because RCs require a correction factor of $1/\sin i$, the actual RC could be much higher than reported by observers. This plausibly reconciles AGC 114905 with MOND expectations.