The origin of the high-velocity cloud complex C

TL;DR

This paper demonstrates that high-velocity cloud complex C originated from a galactic fountain event caused by an explosion in the Cygnus-Outer arm, leading to gas condensation and accretion onto the Milky Way's disc.

Contribution

It introduces a combined model showing that both gas accretion and galactic fountain processes contribute to high-velocity cloud formation, specifically explaining complex C's origin.

Findings

Complex C was produced by an explosion 150 million years ago.

The fountain-driven process triggers condensation of circumgalactic gas.

This mechanism supplies low-metallicity gas to the Milky Way's disc.

Abstract

High-velocity clouds consist of cold gas that appears to be raining down from the halo to the disc of the Milky Way. Over the past fifty years, two competing scenarios have attributed their origin either to gas accretion from outside the Galaxy or to circulation of gas from the Galactic disc powered by supernova feedback (galactic fountain). Here we show that both mechanisms are simultaneously at work. We use a new galactic fountain model combined with high-resolution hydrodynamical simulations. We focus on the prototypical cloud complex C and show that it was produced by an explosion that occurred in the Cygnus-Outer spiral arm about 150 million years ago. The ejected material has triggered the condensation of a large portion of the circumgalactic medium and caused its subsequent accretion onto the disc. This fountain-driven cooling of the lower Galactic corona provides the low-metallicity gas required by chemical evolution models of the Milky Way's disc.