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

Abstract: The study of the elusive hot component ($T \gtrsim 10^7$ K) of the Milky Way circumgalactic medium (CGM) is a novel topic to understand Galactic formation and evolution. In this work, we use the stacking technique through 46 lines of sight with Chandra ACIS-S HETG totaling over 10Ms of exposure time and 9 lines of sight with ACIS-S LETG observations totaling over 1Ms of exposure time, to study in absorption the presence of highly ionized metals arising from the super-virial temperature phase of the CGM. Focusing in the spectral range $4 - 8$ $\r{A}$, we were able to confirm the presence of this hot phase with high significance. We detected transitions of Si XIV K$\alpha$ (with total significance of 6.0$\sigma$) and, for the first time, SXVI K (total significance 4.8$\sigma$) in the rest frame of our own Galaxy. For S XVI K$\alpha$ we found a column density of $1.50^{+0.44}_{-0.38} \times 10^{16} \mathrm{cm}^{-2}$. For Si XIV K$\alpha$ we measured a column density of $0.87\pm{0.16} \times 10^{16} \mathrm{cm}^{-2}$. 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.