Extending the Magellanic Stream: A Hidden Ionized Trail Across the Galactic Plane

Bo-Eun Choi and collaborators present a study revealing a surprising discovery: a large, ionized extension of the Magellanic Stream (MS), a river of gas pulled from the Large and Small Magellanic Clouds, stretching far above the Milky Way’s disk. Using ultraviolet data from the Plane Quasar Survey (PQS) with the Hubble Space Telescope’s Cosmic Origins Spectrograph (HST/COS), the team detected signatures of highly ionized carbon (C IV) in quasars located near the Galactic plane. These new detections extend the known reach of the Stream by about 60 degrees farther north than previously observed, suggesting the Stream’s influence spans well into the Milky Way’s upper halo.

Mapping the Invisible Stream

The Magellanic Stream traces the gravitational and gaseous interactions between the Large and Small Magellanic Clouds (LMC and SMC) and the Milky Way. It is already known as the Milky Way’s most massive halo gas structure, stretching more than 150° across the southern sky. Earlier studies found most of its gas to be ionized rather than neutral. Choi and colleagues used ultraviolet absorption lines in distant quasars’ light to trace the Stream’s faint, ionized component. By focusing on light absorbed by elements like carbon (C IV) and silicon (Si IV), they could detect gas invisible to optical telescopes. The absence of low-ionization species (like Si II or Al II) showed that this newly found region is composed of highly ionized, hot gas rather than cooler, neutral clouds.

How the Data Were Collected

The researchers examined HST/COS G160M spectra of quasars selected as part of the PQS, targeting directions near Galactic longitudes 90° and 270° and latitudes below 30°. Each observation covered ultraviolet wavelengths sensitive to a range of metal ions. To refine their results, the team used software to coadd exposures and identify absorption features based on velocity relative to the local standard of rest (vₗₛᵣ), which helps astronomers separate the Stream’s motion from that of the Milky Way. Table 1 in the paper lists ten quasar sight lines where C IV was detected at velocities between −200 and −400 km/s, consistent with the Magellanic Stream’s known flow pattern.

Linking the Absorbers to the Magellanic Stream

Choi’s team compared the measured C IV velocities with existing maps of the Stream’s neutral hydrogen (H I) gas. The new detections align with the Stream’s established velocity gradient, suggesting they belong to the same structure rather than unrelated Galactic high-velocity clouds like Complex C or the Outer Arm. In addition, the ratio of Si IV to C IV column densities in these sight lines mirrors trends seen farther south, confirming the gas shares similar ionization properties. This evidence strongly supports the idea that the detected gas is a northern continuation of the Stream rather than a separate feature.

Understanding the Gas: Collisions, Not Starlight

To uncover how the gas became ionized, the team compared their results with theoretical models. They found that photoionization, ionization caused by radiation from stars or the Milky Way, could not reproduce the observed ratios of different ions. Instead, the data are best explained by collisional ionization, where gas is heated through interactions such as shocks or mixing with the hot Galactic halo. The ratios of Si IV/C IV and O VI/C IV indicate high gas temperatures, consistent with turbulent or radiatively cooling layers around the Stream. This suggests that the gas is still evolving as it interacts with the Milky Way’s outer atmosphere.

Estimating the Stream’s Extended Mass

Using the measured C IV column densities and models of the gas’s ionization, the team estimated the mass of the new ionized extension. Depending on the assumed conditions, this addition could account for 6–60% of the Stream’s previously known ionized mass, a substantial increase. If confirmed, the total mass of the Stream, including its ionized and neutral components, would further reinforce its role as a key conduit of matter between the Magellanic Clouds and the Milky Way’s halo.

Implications for the Milky Way and the Magellanic Clouds

The discovery of this ionized northern extension reshapes the understanding of how gas stripped from the Magellanic Clouds disperses around our Galaxy. The fact that this gas retains coherent motion suggests it was stripped within the last few hundred million years, before it could mix with the hot Galactic halo. Because the newly found region lies above the Galactic plane, it also provides new clues to the orbit of the Magellanic Clouds, potentially indicating their path as they interact with the Milky Way.

Source: Choi