The Plane Quasar Survey: An Ionized Extension of the Magellanic Stream on the Northern Side of the Galactic Plane

Abstract The Magellanic Stream (MS) is a vast gaseous structure in the Milky Way halo, containing most of its mass in ionized form and tracing the interaction between the Large and Small Magellanic Clouds and the Galaxy. Using Hubble Space Telescope/Cosmic Origin Spectrograph G160M spectra from the Plane Quasar Survey, we detect C iv absorbers likely associated with the MS, extending to the northern side of the Galactic plane, approximately 60 ∘ beyond its previously known ionized extent. These absorbers exhibit position and kinematic alignment and show consistent ionization trends with previously studied MS sight lines. The nondetection of low ions such as Al ii and Si ii , and the detection of C iv (and Si iv in some sight lines), indicates a highly ionized gas phase. The observed Si iv /C iv column density ratios suggest a gas temperature of T ∼ 10 5.3 K and favor collisional ionization over photoionization. We estimate the newly detected extension increases the previous ionized gas mass of the MS, and its coherent kinematics suggest that it was stripped within the past few hundred Myr and has not yet mixed with the Milky Way halo. The existence of highly ionized MS gas at a location above the Galactic plane may constrain the orbital direction of the Magellanic Clouds.