Mixing between High Velocity Clouds and the Galactic Halo

TL;DR

This paper uses detailed 2D and 3D simulations to study how high velocity clouds in the Galactic halo mix with surrounding halo material, leading to metal enrichment before the clouds dissipate.

Contribution

It introduces comprehensive hydrodynamic simulations that track ionization and mixing processes, providing quantitative estimates of metal enrichment in HVCs.

Findings

Mixing occurs early in cloud-halo interactions.

Metallicity of HVCs increases significantly due to mixing.

Simulations show the evolution of mixing efficiency over time.

Abstract

In the Galactic halo, metal-bearing Galactic halo material mixes into high velocity clouds (HVCs) as they hydrodynamically interact. This interaction begins long before the clouds completely dissipate and long before they slow to the velocity of the Galactic material. In order to make quantitative estimates of the mixing efficiency and resulting metal enrichment of HVCs, we made detailed 2 and 3 dimensional simulations of cloud-ISM interactions. Our simulations track the hydrodynamics and time dependent ionization levels. They assume that the cloud originally has a warm temperature and extremely low metallicity while the surrounding medium has a high temperature, low density, and substantial metallicity, but our simulations can be generalized to other choices of initial metallicities. In our simulations, mixing between cloud and halo gas noticeably raises the metallicity of the high velocity material. We present plots of the mixing efficiency and metal enrichment as a function of time.