We study the gas kinematics traced by the 21-cm emission of a sample of six H i-rich low surface brightness galaxies classified as ultra-diffuse galaxies (UDGs). Using the 3D kinematic modelling code 3D Barolo we derive robust circular velocities, revealing a startling feature: H i-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.

Ultra-diffuse galaxies have generated significant interest due to their large optical extents and low optical surface brightnesses, which challenge galaxy formation models. Here we present resolved synthesis observations of 12 H i-bearing ultra-diffuse galaxies (HUDs) from the Karl G. Jansky Very Large Array, as well as deep optical imaging from the WIYN 3.5 m telescope at Kitt Peak National Observatory. We present the data processing and images, including total intensity H i maps and H i velocity fields. The HUDs show ordered gas distributions and evidence of rotation, important prerequisites for the detailed kinematic models of Mancera Piña et al. We compare the H i and stellar alignment and extent, and find that H i extends beyond the already extended stellar component and the H i disk is often misaligned with respect to the stellar one, emphasizing the importance of caution when approaching inclination measurements for these extreme sources. We explore the H i mass–diameter scaling relation, and find that, although the HUDs have diffuse stellar populations, they fall along the relation with typical global H i surface densities. This resolved sample forms an important basis for more detailed study of the H i distribution in this extreme extragalactic population.

We present results from deep H i and optical imaging of AGC 229101, an unusual H i source detected at v helio =7116 km s−1 in the Arecibo Legacy Fast ALFA (ALFALFA) blind H i survey. Initially classified as a candidate “dark” source because it lacks a clear optical counterpart in Sloan Digital Sky Survey (SDSS) or Digitized Sky Survey 2 (DSS2) imaging, AGC 229101 has 109.31±0.05 M ⊙ of H i, but an H i line width of only 43 ± 9 km s−1. Low-resolution Westerbork Synthesis Radio Telescope (WSRT) imaging and higher-resolution Very Large Array (VLA) B-array imaging show that the source is significantly elongated, stretching over a projected length of ∼80 kpc. The H i imaging resolves the source into two parts of roughly equal mass. WIYN partially populated One Degree Imager (pODI) optical imaging reveals a faint, blue optical counterpart coincident with the northern portion of the H i. The peak surface brightness of the optical source is only μ g ∼ 26.6 mag arcsec−2, well below the typical cutoff that defines the isophotal edge of a galaxy, and its estimated stellar mass is only 107.32±0.33 M ⊙, yielding an overall neutral gas-to-stellar mass ratio of M/M * = 98 − 52 + 111 . We demonstrate the extreme nature of this object by comparing its properties with those of other H i-rich sources in ALFALFA and the literature. We also explore potential scenarios that might explain the existence of AGC 229101, including a tidal encounter with neighboring objects and a merger of two dark H i clouds.

We present deep optical imaging and photometry of four objects classified as “Almost-Dark” galaxies in the Arecibo Legacy Fast Arecibo L-band Feed Array (ALFALFA) survey because of their gas-rich nature and extremely faint or missing optical emission in existing catalogs. They have H i masses of 107–109 M ⊙ and distances of ∼9–100 Mpc. Observations with the WIYN 3.5 m telescope and One Degree Imager reveal faint stellar components with central surface brightnesses of ∼24–25 mag arcsec − 2 in the g band. We also present the results of H i synthesis observations with the Westerbork Synthesis Radio Telescope. These Almost-Dark galaxies have been identified as possible tidal dwarf galaxies (TDGs) based on their proximity to one or more massive galaxies. We demonstrate that AGC 229398 and AGC 333576 likely have the low dark matter content and large effective radii representative of TDGs. They are located much farther from their progenitors than previously studied TDGs, suggesting they are older and more evolved. AGC 219369 is likely dark matter dominated, while AGC 123216 has a dark matter content that is unusually high for a TDG, but low for a normal dwarf galaxy. We consider possible mechanisms for the formation of the TDG candidates such as a traditional major merger scenario and gas ejection from a high-velocity flyby. Blind H i surveys like ALFALFA enable the detection of gas-rich, optically faint TDGs that can be overlooked in other surveys, thereby providing a more complete census of the low-mass galaxy population and an opportunity to study TDGs at a more advanced stage of their life cycle.