A Distant Echo of Milky Way Central Activity closes the Galaxy's Baryon Census

TL;DR

This paper reveals extensive million-degree gas in the Milky Way's interstellar and circum-galactic medium, indicating the galaxy's baryon content is complete and suggesting a past active phase of the central black hole about 6 million years ago.

Contribution

It provides new evidence of hot gas distribution extending beyond 200 kpc and links a central cavity to historical black hole activity.

Findings

Hot gas extends beyond 200 kpc, closing the baryon gap.

A central cavity shows evidence of past black hole activity.

The cavity's structure suggests a strong nuclear event 6 million years ago.

Abstract

We report on the presence of large amounts of million-degree gas in the Milky Way's interstellar and circum-galactic medium. This gas (1) permeates both the Galactic plane and the halo, (2) extends to distances larger than 60-200 kpc from the center, and (3) its mass is sufficient to close the Galaxy's baryon census. Moreover, we show that a vast, $\sim 6$ kpc radius, spherically-symmetric central region of the Milky Way above and below the 0.16 kpc thick plane, has either been emptied of hot gas or the density of this gas within the cavity has a peculiar profile, increasing from the center up to a radius of $\sim 6$ kpc, and then decreasing with a typical halo density profile. This, and several other converging pieces of evidence, suggest that the current surface of the cavity, at 6 kpc from the Galaxy's center, traces the distant echo of a period of strong nuclear activity of our super-massive black-hole, occurred about 6 Myrs ago.