The Arecibo Galaxy Environment Survey XII : Optically dark HI clouds in the Leo I Group

TL;DR

This paper reports the discovery of five optically dark HI clouds in the Leo I group, challenging tidal origin explanations and suggesting possible primordial origins, with implications for understanding the Leo Ring's formation.

Contribution

It presents the first detailed detection and analysis of multiple optically dark HI clouds in the Leo I group, highlighting their properties and potential origins.

Findings

Five HI clouds without optical counterparts discovered

Some clouds follow the baryonic Tully-Fisher relation

Tidal origin explanation faces significant challenges

Abstract

Using data from the Arecibo Galaxy Environment Survey, we report the discovery of five HI clouds in the Leo I group without detected optical counterparts. Three of the clouds are found midway between M96 and M95, one is only 10$^{\prime}$ from the south-east side of the well-known Leo Ring, and the fifth is relatively isolated. HI masses range from 2.6$\times$10$^{6}$ - 9.0$\times$10$^{6}$M$_{\odot}$ and velocity widths (W50) from 16 - 42 km/s. Although a tidal origin is the most obvious explanation, this formation mechanism faces several challenges. For the most isolated cloud, the difficulties are its distance from neighbouring galaxies and the lack of any signs of disturbance in the HI discs of those systems. Some of the clouds also appear to follow the baryonic Tully-Fisher relation between mass and velocity width for normal, stable galaxies which is not expected if they are tidal in origin. Three clouds are found between M96 and M95 which have no optical counterparts, but have otherwise similar properties and location to the optically detected galaxy LeG 13. While overall we favour a tidal debris scenario to explain the clouds, we cannot rule out a primordial origin. If the clouds were produced in the same event that gave rise to the Leo Ring, they may provide important constraints on any model attempting to explain that structure