The Galactic fountain as an origin for the Smith Cloud

TL;DR

This paper models the Smith Cloud as a product of the galactic fountain process, explaining its properties and morphology through interactions with the galactic halo, and explores scenarios involving inclined outflows and fast winds.

Contribution

It introduces a dynamical model of the galactic fountain that accounts for the Smith Cloud's observed properties and morphology, including new scenarios for its formation.

Findings

A simple vertical outflow model reproduces the cloud's kinematics and distance.

Inclined outflows and fast winds can explain the cloud's cometary morphology.

All scenarios suggest the cloud is in the ascending phase of its trajectory with feasible energy requirements.

Abstract

The recent discovery of an enriched metallicity for the Smith high-velocity HI cloud (SC) lends support to a Galactic origin for this system. We use a dynamical model of the galactic fountain to reproduce the observed properties of the SC. In our model, fountain clouds are ejected from the region of the disc spiral arms and move through the halo interacting with a pre-existing hot corona. We find that a simple model where cold gas outflows vertically from the Perseus spiral arm reproduces the kinematics and the distance of the SC, but is in disagreement with the cloud's cometary morphology, if this is produced by ram-pressure stripping by the ambient gas. To explain the cloud morphology we explore two scenarios: a) the outflow is inclined with respect to the vertical direction; b) the cloud is entrained by a fast wind that escapes an underlying superbubble. Solutions in agreement with all observational constraints can be found for both cases, the former requires outflow angles >40 deg while the latter requires >1000 km/s winds. All scenarios predict that the SC is in the ascending phase of its trajectory and have large - but not implausible - energy requirements.