Enigmatic Velocity Dispersions of Ultra-Diffuse Galaxies in Light of Modified Gravity Theories and Radial Acceleration Relation

TL;DR

This paper tests modified gravity theories against velocity dispersion data of two ultra-diffuse galaxies, finding that one galaxy's data aligns with the Radial Acceleration Relation while the other's does not, challenging certain gravity models.

Contribution

It extends previous analyses by evaluating three popular modified gravity theories against new velocity dispersion measurements of two UDGs, highlighting their differing compatibilities with observational data.

Findings

DF44's dispersion data is consistent with the Radial Acceleration Relation.

DF2's dispersion data is inconsistent with the Radial Acceleration Relation.

Modified gravity theories face challenges explaining the extreme velocity dispersions of these galaxies.

Abstract

Recent observations of anomalous line-of-sight velocity dispersions of two ultra-diffuse galaxies (UDGs) provide a stringent test for modified gravity theories. While NGC 1052-DF2 exhibits an extremely low dispersion value ($\sigma \sim 7.8_{-2.2}^{+5.6}$ km/s), the reported dispersion value for NGC 1052-DF44 is quite high ($\sigma \sim 41.0 \pm 8$ km/s). For DF2, the dynamical mass is almost equal to the luminous mass suggesting the galaxy have little to no `dark matter' in $\Lambda$CDM whereas DF4 requires a dynamical mass-to-light ratio of $\sim 30$ making it to be almost entirely consists of dark matter. It has been claimed that both these galaxies, marking the extreme points in terms of the estimated dynamical mass-to-light ratio among known galaxies, would be difficult to explain in modified gravity scenarios. Extending the analysis presented in \cite{islam2019modified}, we explore the dynamics of DF2 and DF44 within the context of three popular alternative theories of gravity [Modified Newtonian Dynamics (MOND), Weyl Conformal gravity and Modified gravity (MOG)] and examine their viability against the dispersion data of DF2 and DF44. We further show that the galactic `Radial Acceleration Relation' (RAR) is consistent with DF44 dispersion data but not with DF2.