Revealing the Ionization Properties of the Magellanic Stream using Optical Emission

TL;DR

This study maps the ionization state of the Magellanic Stream using optical emission lines, revealing higher-than-expected ionization levels likely caused by shock processes and additional energetic sources.

Contribution

It provides the first extensive optical emission measurements along the Magellanic Stream, highlighting the role of shock ionization and other energetic processes in its ionization.

Findings

26 of 39 sight lines show detectable H-alpha emission

Regions with log NHI = 19.5 - 20.0 are 16-67% ionized

Shock cascade and energetic processes contribute to ionization

Abstract

The Magellanic Stream, a gaseous tail that trails behind the Magellanic Clouds, could replenish the Milky Way with a tremendous amount of gas if it reaches the Galactic disk before it evaporates into the halo. To determine how the Magellanic Stream's properties change along its length, we have conducted an observational study of the H-alpha emission, along with other optical warm ionized gas tracers, toward 39 sight lines. Using the Wisconsin H-alpha Mapper telescope, we detect H-alpha emission brighter than 30 - 50 mR in 26 of our 39 sight lines. This H-alpha emission extends more than 2-degree away from the HI emission. By comparing H-alpha and [OI] intensities, we find that regions with log NHI = 19.5 - 20.0 are 16 - 67% ionized. Most of the H-alpha intensities along the Magellanic Stream are much higher than expected if the primary ionization source is photoionization from Magellanic Clouds, the Milky Way, and the extragalactic background. We find that the additional contribution from self ionization through a "shock cascade" that results as the Stream plows through the halo might be sufficient to reproduce the underlying level of H-alpha emission along the Stream. In the sparsely sampled region below the South Galactic Pole, there exists a subset of sight lines with uncharacteristically bright emission, which suggest that gas is being ionized further by an additional source that could be a linked to energetic processes associated with the Galactic center.