Off the baryonic Tully-Fisher relation: a population of baryon-dominated ultra-diffuse galaxies

TL;DR

This study reveals that HI-rich ultra-diffuse galaxies are outliers in the baryonic Tully-Fisher relation, with low circular velocities despite having baryon fractions consistent with cosmological expectations, challenging existing galaxy formation models.

Contribution

The paper provides the first detailed kinematic analysis of HI-rich UDGs showing they deviate from the baryonic Tully-Fisher relation and possess baryon fractions aligning with cosmological values.

Findings

UDGs are outliers in the baryonic Tully-Fisher relation.

Baryon fractions in UDGs match cosmological expectations.

Baryonic matter accounts for the observed rotation curves.

Abstract

We study the gas kinematics traced by the 21-cm emission of a sample of six HI$-$rich low surface brightness galaxies classified as ultra-diffuse galaxies (UDGs). Using the 3D kinematic modelling code $\mathrm{^{3D}}$Barolo we derive robust circular velocities, revealing a startling feature: HI$-$rich UDGs are clear outliers from the baryonic Tully-Fisher relation, with circular velocities much lower than galaxies with similar baryonic mass. Notably, the baryon fraction of our UDG sample is consistent with the cosmological value: these UDGs are compatible with having no "missing baryons" within their virial radii. Moreover, the gravitational potential provided by the baryons is sufficient to account for the amplitude of the rotation curve out to the outermost measured point, contrary to other galaxies with similar circular velocities. We speculate that any formation scenario for these objects will require very inefficient feedback and a broad diversity in their inner dark matter content.