A Compact High Velocity Cloud near the Magellanic Stream: Metallicity and Small-Scale Structure

TL;DR

This study analyzes a compact high velocity cloud near the Magellanic Stream, revealing its metallicity and internal structure, and suggesting it originates from the Stream and interacts with the Galactic halo.

Contribution

It provides the first detailed metallicity and structural analysis of a CHVC near the Magellanic Stream, linking it to the Stream's origin.

Findings

The cloud shows uniform ionization structure across small scales.

Elemental abundances match the Magellanic Stream's 0.1 solar metallicity.

The cloud likely originated from the Magellanic Stream and interacts with the Galactic halo.

Abstract

The Magellanic Stream (MS) is a well-resolved gaseous tail originating from the Magellanic Clouds. Studies of its physical properties and chemical composition are needed to understand its role in Galactic evolution. We investigate the properties of a compact HVC (CHVC 224.0-83.4-197) lying close on the sky to the MS to determine whether it is physically connected to the Stream and to examine its internal structure. Our study is based on analysis of HST/COS spectra of three QSOs (Ton S210, B0120-28, and B0117-2837) all of which pass through this single cloud at small angular separation (\lessim 0.72{\deg}), allowing us to compare physical conditions on small spatial scales. No significant variation is detected in the ionization structure from one part of the cloud to the other. Using Cloudy photoionization models, toward Ton S210 we derive elemental abundances of [C/H] = -1.21 +/- 0.11, [Si/H] = -1.16 +/- 0.11, [Al/H] = -1.19 +/- 0.17 and [O/H] = -1.12 +/- 0.22, which agree within 0.09 dex. The CHVC abundances match the 0.1 solar abundances measured along the main body of the Stream. This suggests that the CHVC (and by extension the extended network of filaments to which it belongs) has an origin in the MS. It may represent a fragment that has been removed from the Stream as it interacts with the gaseous Galactic halo.