# The HI content of isolated ultra-diffuse galaxies: A sign of multiple formation mechanisms?

## TL;DR

This study investigates the atomic hydrogen content of four isolated ultra-diffuse galaxies, revealing diverse gas properties that suggest multiple formation mechanisms, including feedback-driven outflows and other unknown processes.

## Contribution

It provides new HI observations of isolated UDGs, highlighting the existence of both gas-rich and gas-poor types, implying multiple formation pathways.

## Key findings

- Gas-rich UDGs support feedback-driven formation models.
- Gas-poor UDGs challenge current understanding of isolated galaxy evolution.
- Evidence for multiple formation mechanisms of UDGs.

## Abstract

We report on the results of radio observations in the 21 cm emission line of atomic hydrogen (HI) of four relatively isolated ultra-diffuse galaxies (UDGs): DGSAT I, R-127-1, M-161-1, and SECCO-dI-2. Our Effelsberg observations resulted in non-detections for the first three UDGs, and a clear detection for the last. DGSAT I, R-127-1, and M-161-1 are quiescent galaxies with gas fractions that are much lower than those of typical field galaxies of the same stellar mass. On the other hand, SECCO-dI-2 is a star forming gas-rich dwarf, similar to two other field UDGs that have literature HI data: SECCO-dI-1 and UGC 2162. This group of three gas-rich UDGs have stellar and gaseous properties that are compatible with a recently proposed theoretical mechanism for the formation of UDGs, based on feedback-driven outflows. In contrast, the physical characteristics of R-127-1 and M-161-1 are puzzling, given their isolated nature. We interpret this dichotomy in the gaseous properties of field UDGs as a sign of the existence of multiple mechanisms for their formation, with the formation of the quiescent gas-poor UDGs remaining a mystery.