Detection of a Supervirial Hot Component in the Milky Way Circumgalactic Medium Along Multiple Sight Lines by Using the Stacking Technique

The study of the elusive hot component ( T ≳ 10 ^7 K) of the Milky Way circumgalactic medium (CGM) is a novel topic for understanding Galactic formation and evolution. In this work, we use the stacking technique through 46 lines of sight with Chandra ACIS-S HETG totaling over 10 Ms of exposure time and nine lines of sight with ACIS-S LETG observations totaling over 1 Ms of exposure time, to study in absorption the presence of highly ionized metals arising from the supervirial temperature phase of the CGM. Focusing in the spectral range 4–8 Å, we were able to confirm the presence of this hot phase with high significance. We detected transitions of Si XIV K α (with a total significance of 6.0 σ ) and, for the first time, S XVI K α (total significance of 4.8 σ ) in the rest frame of our own galaxy. For S XVI K α we found a column density of ${1.50}_{-0.38}^{+0.44}\times {10}^{16\,}{\mathrm{cm}}^{-2}$ . For Si XIV K α we measured a column density of 0.87 ± 0.16 × 10 ^16 cm ^−2 . The lines of sight used in this work are spread across the sky, probing widely separated regions of the CGM. Therefore, our results indicate that this newly discovered hot medium extends throughout the halo and is not related only to the Galactic bubbles. The hot gas location, distribution, and covering factor, however, remain unknown. This component might contribute significantly to the missing baryons and metals in the Milky Way.