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

Bo-Eun Choi, Jessica K. Werk, Kirill Tchernyshyov, Mary E. Putman, Yong Zheng, J. E. G. Peek, Hannah Bish, David Schiminovich

TL;DR

This study reports a serendipitous discovery of a highly ionized extension of the Magellanic Stream traced by C IV absorption, extending to the northern side of the Galactic plane by roughly $60^{\circ}$ beyond the previously known ionized extent. Using HST/COS G160M observations from the Plane Quasar Survey, C IV absorbers are detected in ten sight lines (with Si IV in some) and show kinematic alignment with the MS velocity gradient and no low ions, indicating a highly ionized gas phase. Ionization modeling favors collisional ionization or turbulent radiative mixing layers at $T \approx 10^{5.3}$ K, rather than pure photoionization, and TRMLs can plausibly reproduce part of the observed ratios. The estimated ionized mass of the extension is substantial (6–60% of the prior ionized MS mass), implying recent stripping on a timescale of a few hundred Myr and providing a constraint on the Magellanic Clouds’ orbital history.

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 HST/COS 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$^\circ$ 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 non-detection of low ions such as Al II and Si II, and the detection of C IV (and Si IV in some sightlines), indicates a highly ionized gas phase. The observed Si IV/C IV column density ratios suggest a gas temperature of $T \sim 10^{5.3}$~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.

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

TL;DR

This study reports a serendipitous discovery of a highly ionized extension of the Magellanic Stream traced by C IV absorption, extending to the northern side of the Galactic plane by roughly beyond the previously known ionized extent. Using HST/COS G160M observations from the Plane Quasar Survey, C IV absorbers are detected in ten sight lines (with Si IV in some) and show kinematic alignment with the MS velocity gradient and no low ions, indicating a highly ionized gas phase. Ionization modeling favors collisional ionization or turbulent radiative mixing layers at K, rather than pure photoionization, and TRMLs can plausibly reproduce part of the observed ratios. The estimated ionized mass of the extension is substantial (6–60% of the prior ionized MS mass), implying recent stripping on a timescale of a few hundred Myr and providing a constraint on the Magellanic Clouds’ orbital history.

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 HST/COS 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 non-detection of low ions such as Al II and Si II, and the detection of C IV (and Si IV in some sightlines), indicates a highly ionized gas phase. The observed Si IV/C IV column density ratios suggest a gas temperature of ~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.

Paper Structure

This paper contains 11 sections, 2 equations, 5 figures.

Table of Contents

  1. Introduction
  2. Observation & Data Analysis
  3. HST/COS G160M Observation
  4. Data Analysis
  5. Results
  6. C4 Absorbers Associated with the Stream
  7. Ionization State
  8. Mass Estimation
  9. Discussion
  10. Summary
  11. Voigt Profile Fitting

Figures (5)

  • Figure 1: The location of high-ion absorbers near the Stream in (a, top panel) Galactic coordinates and (b, bottom panel) MS coordinates. In this work, we detect C4 absorbers (circles), extending to the Northern sky (Galactic latitude of $b \sim 30^{\circ}$). We also include C4 (squares; fox14) and O6 (x markers; sembach03) absorbers from previous studies. The LSR velocity of the absorbers is color-coded. The all-sky H1 map from the HI4PI survey westmeier18 is shown with the same velocity color scale.
  • Figure 2: UV spectra and fitted Voigt profiles for Si2 $\lambda1527$ (left), Si4 $\lambda1394$ (center), and C4 $\lambda1548$ (right) absorptions in the QSO sight lines. The components associated with the MS are shown in purple. These components have distinct velocity offsets from other HVCs, such as Complex C (green), and are not detected in low ions like Si2.
  • Figure 3: Velocity centroid of C4 (half-top circles) and Si4 (half-bottom circles) MS absorbers in the LSR by MS Longitude ($L_{\rm MS}$). The C4 and Si4 from fox20 are also shown in half-top and half-bottom diamonds, respectively, and O6 absorbers from sembach03 are shown as x marks. The velocity gradient suggested by nidever10 is presented with dashed line, and our data follows this trend including velocity inflection at $L_{\rm MS} \sim -120^{\circ}$. The background H1 contours are adopted from nidever10 with permission.
  • Figure 4: The ion column density ratios of Si4 and C4 along the Stream. (Left) QSO sight lines with detected C4 absorption, shown in MS coordinates and color-coded by Si4/C4 column density ratio. Filled circles represent measurements from this work, and open circles are from fox14. Triangles denote sight lines where only upper limits on Si4 are available. (Right) Si4/C4 column density ratios along with MS Latitude ($B_{\rm MS}$).
  • Figure 5: (a) Observed ion column density ratios, $N_{\rm SiIV}/N_{\rm CIV}$ (black-hatched) and $N_{\rm OVI}/N_{\rm CIV}$ (blue-hatched), and comparison with ionization models. We present ion ratio predictions from photoionization (gray), collisional ionization equilibrium (CIE; black), radiative cooling (olive and brown), and turbulent radiative mixing layers models (black pluses). (b) The expected C4 fraction from collisional ionization models gnat07, collisional ionization equilibrium (black), time-dependent radiative cooling at constant pressure (isobaric; olive) and at constant volume (isochoric; brown). We present sub-solar metallicity ($0.1~Z_{\odot}$) for radiative cooling models. The shaded temperature ranges are derived from observed ion column density ratios, $N_{\rm SiIV}/N_{\rm CIV}$ (black) and $N_{\rm OVI}/N_{\rm CIV}$ (blue), respectively.