Fifteen years ago, astronomers looking through data from the Fermi Gamma-ray Space Telescope noticed odd bubbles that extended above and below the Milky Way’s disc.
Stretching tens of thousands of lightyears across and apparently centred on the Galactic centre, these features were recently joined by larger ‘superbubbles’, spotted in the data obtained by the eROSITA X-ray satellite.
But what are they?
The fact that they seem to be aligned with the Galactic centre suggests the supermassive black hole that lurks there – or the activity that happens when material accretes onto it – might be responsible.
But there isn’t enough energy being pumped into the Galaxy today from this source to get anywhere close to what’s needed.
Any explanation needs to account for both energetic gamma-ray bubbles and the larger X-ray features.
Most attempts at an explanation assume that, in the recent past, the black hole at the centre of the Galaxy was growing rapidly.
If so, the Milky Way would have resembled nearby galaxies that have active galactic nuclei surrounded by a hot accretion disc and powering jets of rapidly moving material stretching out into the Galaxy.
Enormous burst of energy
But could the Milky Way really have gone from being in such an active state to today’s somnolence in at most a few million years?
This is controversial, but to many it seems unlikely.
Worse, to match the bubbles, these jets – presumably ejected perpendicularly to the accretion disc around the black hole – would have to been aligned with the Galactic poles.
But observations with the Event Horizon Telescope, which provided images of the disc that exists around the black hole today, show the disc is tilted.
A solution at last?
This month’s paper, therefore, provides simulations to support another hypothesis.
The authors still blame the black hole, but consider what happens when it consumes a star, a phenomenon occasionally spotted elsewhere and known as a Tidal Disruption Event, or TDE.
In a galaxy the mass and shape of the Milky Way, an unlucky star should wander past the central black hole roughly once every 10,000–100,000 years, producing a burst of light and radiation as it is ripped apart, but leaving little long-term trace.
Each of these events produces an enormous burst of energy, which the researchers inject into a simulation of the Milky Way, capable of tracing the behaviour of our Galaxy’s gas and particles.
The result is surprisingly convincing replicas of the Milky Way’s bubbles, with shocks spreading outwards from the galactic centre gathering up and exciting material on their way.
Repeated TDEs make the bubbles long-lived features, renewed by successive episodes of snacking on stars.
So is the mystery solved? The details aren’t quite right, and the shape of the gamma-ray bright bubbles produced by the simulation doesn’t really match what Fermi sees.
The authors argue that there are many variables – from the frequency and brightness of each TDE through to the structure of the Galaxy itself that the model starts with – that could be tweaked.
That seems like a reasonable argument to me, and I’d bet that what we’ve been staring at for the last 15 years is leftovers from a black hole’s habit of snacking on stars.
Chris Lintott was reading Tidal Disruption Events as the Origin of the eROSITA and Fermi Bubbles by Tassilo Scheffler et al. Read it online at: arxiv.org/abs/2501.18713
This article appeared in the April 2025 issue of BBC Sky at Night Magazine
