(Almost) Dark Galaxies in the ALFALFA Survey: Isolated HI Bearing Ultra Diffuse Galaxies

TL;DR

This paper identifies a new class of isolated, HI-rich ultra-diffuse galaxies with irregular morphologies, low star formation efficiency, and potential origins in high spin parameter halos, expanding understanding of galaxy diversity.

Contribution

It introduces a sample of isolated, HI-rich ultra-diffuse galaxies with unique properties and explores their potential as progenitors to cluster UDGs, using detailed optical and HI observations.

Findings

These galaxies are bluer and more irregular than typical UDGs.

They have large HI diameters consistent with the HI mass-radius relation.

They may reside in high spin parameter halos, indicating a specific formation pathway.

Abstract

We present a sample of 115 very low optical surface brightness, highly extended, HI-rich galaxies carefully selected from the ALFALFA survey that have similar optical absolute magnitudes, surface brightnesses, and radii to recently discovered "ultra-diffuse" galaxies (UDGs). However, these systems are bluer and have more irregular morphologies than other UDGs, are isolated, and contain significant reservoirs of HI. We find that while these sources have normal star formation rates for HI selected galaxies of similar stellar mass, they have very low star formation efficiencies. We further present deep optical and HI synthesis follow up imaging of three of these HI-bearing ultra-diffuse sources. We measure HI diameters extending to ~40 kpc, but note that while all three sources have large HI diameters for their stellar mass, they are consistent with the HI mass - HI radius relation. We further analyze the HI velocity widths and rotation velocities for the unresolved and resolved sources respectively, and find that the sources appear to inhabit halos of dwarf galaxies. We estimate spin parameters, and suggest that these sources may exist in high spin parameter halos, and as such may be potential HI-rich progenitors to the ultra-diffuse galaxies observed in cluster environments.