Simulating the evolution of optically dark HI clouds in the Virgo cluster : will no-one rid me of this turbulent sphere ?

TL;DR

This study uses 3D hydro simulations to test if turbulent internal motions in optically dark HI clouds in the Virgo cluster can explain their broad line widths, finding that such clouds are inherently unstable and unlikely to be long-lived.

Contribution

The paper provides the first detailed hydrodynamical simulations of optically dark HI clouds in a cluster environment, challenging the turbulence hypothesis for their observed properties.

Findings

Simulations show clouds either collapse, disperse, or heat up rapidly.

Turbulent motions alone cannot sustain the clouds for more than 100 Myr.

Including the intracluster medium is crucial in modeling these objects.

Abstract

Most detected neutral atomic hydrogen (HI) at low redshift is associated with optically bright galaxies. However, a handful of HI clouds are known which appear to be optically dark and have no nearby potential progenitor galaxies, making tidal debris an unlikely explanation. In particular, 6 clouds identified by the Arecibo Galaxy Environment Survey are interesting due to the combination of their small size, isolation, and especially their broad line widths atypical of other such clouds. A recent suggestion is that these clouds exist in pressure equilibrium with the intracluster medium, with the line width arising from turbulent internal motions. Here we explore that possibility by using the FLASH code to perform a series of 3D hydro simulations. Our clouds are modelled using spherical Gaussian density profiles, embedded in a hot, low-density gas representing the intracluster medium. The simulations account for heating and cooling of the gas, and we vary the structure and strength of their internal motions. We create synthetic HI spectra, and find that none of our simulations reproduce the observed cloud parameters for longer than about 100 Myr : the clouds either collapse, disperse, or experience rapid heating which would cause ionisation and render them undetectable to HI surveys. While the turbulent motions required to explain the high line widths generate structures which appear to be inherently unstable, making this an unlikely explanation for the observed clouds, these simulations demonstrate the importance of including the intracluster medium in any model seeking to explain the existence of these objects.