Attack of the Flying Snakes : Formation of Isolated HI Clouds By Fragmentation of Long Streams

TL;DR

This study uses numerical simulations to explore whether isolated HI clouds can form from the fragmentation of long HI streams, finding such formation is extremely rare, and discusses the stability of dark galaxies and their possible link to ultra diffuse galaxies.

Contribution

The paper provides the first detailed simulation-based analysis of HI cloud formation via stream fragmentation and assesses the stability of dark galaxies in cluster environments.

Findings

High velocity width HI clouds are rarely formed through fragmentation (<0.2%).

Genuine dark galaxies can remain stable for over 5 Gyr in clusters.

Fragmentation is an unlikely primary formation mechanism for isolated HI clouds.

Abstract

The existence of long (> 100 kpc) HI streams and small (< 20 kpc) free-floating HI clouds is well-known. While the formation of the streams has been investigated extensively, and the isolated clouds are often purported to be interaction debris, little research has been done on the formation of optically dark HI clouds that are not part of a larger stream. One possibility is that such features result from the fragmentation of more extended streams, while another idea is that they are primordial, optically dark galaxies. We test the validity of the fragmentation scenario (via harassment) using numerical simulations. In order to compare our numerical models with observations, we present catalogues of both the known long HI streams (42 objects) and free-floating HI clouds suggested as dark galaxy candidates (51 objects). In particular, we investigate whether it is possible to form compact features with high velocity widths (> 100 km/s), similar to observed clouds which are otherwise intriguing dark galaxy candidates. We find that producing such features is possible but extremely unlikely, occurring no more than 0.2% of the time in our simulations. In contrast, we find that genuine dark galaxies could be extremely stable to harassment and remain detectable even after 5 Gyr in the cluster environment (with the important caveat that our simulations only explore harassment and do not yet include the intracluster medium, heating and cooling, or star formation). We also discuss the possibility that such objects could be the progenitors of recently discovered ultra diffuse galaxies.