Matter and energy flowing from the centre of a galaxy are thought to play a key role in the formation and development of solar systems and other structures in the universe. Now, astronomers led by the Max Planck Institute for Extraterrestrial Physics in Garching have discovered two wide chimneys of gas in X-ray images. The chimneys connect inner regions of the Milky Way north and south of the galactic plane with structures far further out.
Our Milky Way is a fairly tranquil galaxy, and gigantic outbursts of energy from its centre are rare. Nevertheless, astronomers long ago detected bipolar bulges close to the Galactic Centre, which is identified by the radio source Sagittarius A*. These wings or lobes are visible in both the radio and X-ray spectral regions. They show outflows from the centre and only extend to distances of up to around 50 light years.
Far further out from the centre appear the two Fermi bubbles, which were also identified some time ago. These trace the radiation from relativistic particles as seen in gamma light and also stretch far beyond the galactic plane by some 25,000 light years. In the past, therefore, large quantities of energy must evidently have been released in our galaxy.
The two striking X-ray structures – known as “chimneys” – identified by the team appear to link the lobes in the inner regions of the Galactic Centre with the Fermi bubbles. “A few years ago, we discovered an overdense region of hot plasma directly above the Galactic Centre based on X-ray observations. For that reason, we wanted to scan a far larger region using the European X-ray satellite XMM-Newton,” says former Max Planck researcher Gabriele Ponti, lead author of the study, which was published in the journal Nature.
With their new measurements, the scientists actually succeeded not only in unambiguously confirming the existence of this hot plasma, but also in determining its shape and composition. They also discovered the chimneys, which stretch for hundreds of light years north and south of the Galactic Centre. The researchers consider this a clear indication that the two structures must have a common origin.
They are probably made of gas emitted in a fast and cold stream on both sides of, and perpendicular to, the galactic plane. This outflow could originate either from stars torn apart by the massive black hole at the heart of our galaxy or from supernova explosions in the central star cluster. Events of this kind could continuously blow energy and mass out of the Centre and transport them to the Fermi bubbles.
“The chimneys are approximately cylindrical in shape and have sharp vertical boundaries. They’re most likely contained by magnetic pressure,” says Florian Hofmann from the Max Planck Institute in Garching. Moreover, the two chimneys are not strictly symmetrical: “That’s presumably a consequence of galactic weather – essentially, an interplay with local clouds in the interstellar medium,” says Hofmann.
According to Eugene Churazov from the Max Planck Institute for Astrophysics, the Fermi bubbles are reminiscent of structures formed by highly energetic outflows from the supermassive black holes in galaxy clusters. “On the other hand, the appearance of the chimneys also leaves open the possibility that less energetic processes are involved,” says Churazov.
The researchers are still puzzling over the mechanism by which the two newly discovered chimneys were formed – and whether they are a continuation of the inner lobes. At any rate, they shed new light on how activity in the nucleus of the Milky Way relates to macroscopic structures at the galactic level and how it may well influence even the formation and development of our galaxy.