Mass, Morphing, Metallicities: The Evolution of Infalling High Velocity Clouds

TL;DR

This study uses hydrodynamical simulations to assess how contamination and viewing angles affect metallicity estimates of infalling high velocity clouds, revealing significant evolution and complexity in their properties.

Contribution

It provides a detailed analysis of how ambient gas contamination and viewing geometry influence metallicity measurements of high velocity clouds, highlighting their dynamic and evolving nature.

Findings

Metallicity estimates vary significantly with cloud evolution and position.

Cloud contamination by ambient gas exceeds 90%, affecting original material detection.

Clouds survive infall due to continuous condensation and cooling in their wake.

Abstract

We revisit the reliability of metallicity estimates of high velocity clouds with the help of hydrodynamical simulations. We quantify the effect of accretion and viewing angle on metallicity estimates derived from absorption lines. Model parameters are chosen to provide strong lower limits on cloud contamination by ambient gas. Consistent with previous results, a cloud traveling through a stratified halo is contaminated by ambient material to the point that <10% of its mass in neutral hydrogen consists of original cloud material. Contamination progresses nearly linearly with time, and it increases from head to tail. Therefore, metallicity estimates will depend on the evolutionary state of the cloud, and on position. While metallicities change with time by more than a factor of 10, well beyond observational uncertainties, most lines-of-sight range only within those uncertainties at any given time over all positions. Metallicity estimates vary with the cloud's inclination angle within observational uncertainties. The cloud survives the infall through the halo because ambient gas continuously condenses and cools in the cloud's wake and thus appears in the neutral phase. Therefore, the cloud observed at any fixed time is not a well-defined structure across time, since material gets constantly replaced. The thermal phases of the cloud are largely determined by the ambient pressure. Internal cloud dynamics evolve from drag gradients caused by shear instabilities, to complex patterns due to ram-pressure shielding, leading to a peloton effect, in which initially lagging gas can catch up to and even overtake the head of the cloud.