In 2010, astronomers announced the discovery of two vast — and very mysterious — bubbles of gamma-ray emissions towering above and below our galaxy’s disk. Four years on, after oodles of analysis, the source of these bubbles is as mysterious as ever.
The scale of these gamma-ray structures is truly mind-blowing. Apparently originating directly from the galactic core, the two lobes extend tens of thousands of light-years into intergalactic space. They both generate gamma-ray radiation at an astonishing luminosity, “like two 30,000-light-year-tall incandescent bulbs screwed into the center of the galaxy,” according to a Stanford University news release.
The discovery was made by NASA’s Fermi Gamma-Ray Observatory that orbits the Earth away from our planet’s gamma-ray absorbing atmosphere. Without Fermi, we wouldn’t have even been aware of these giant structures.
Since their discovery by Fermi’s Large Area Telescope (LAT), it was assumed that an ancient eruption by the Milky Way’s supermassive black hole may have energized galactic matter, inflating these two energetic bubbles. But since astronomers have been studying the nature of these features, their origin is as foggy as ever.
After subtraction of extragalactic gamma-ray sources, the bubbles have very clearly defined edges and, closest to the galactic plane, are associated with microwave emissions. However, the microwave emissions appear to fade away — the gamma-ray emissions glow uniformly throughout. This is a peculiar and vexing problem for astronomers.
“Since the Fermi bubbles have no known counterparts in other wavelengths in areas high above the galactic plane, all we have to go on for clues are the gamma rays themselves,” said postdoctoral researcher Anna Franckowiak of the Kavli Institute for Particle Astrophysics and Cosmology.
A few models have been put forward, but none fully explain the shape, scope or luminosity of these gamma-ray factories. Could the bubbles be expanding from black hole jets? Or could it be that a cluster of young massive stars formed and exploded at the same time, producing the huge bubbles like supernova exhaust? In short, we still have no idea.
In a paper published in the Astrophysical Journal today, Stanford University researchers say they need a better view of the gamma-ray radiation near the core of the Milky Way before we can better understand the bubbles’ source, but it will be a very hard task to subtract other gamma-ray sources from the region.
“What would be very interesting would be to get a better view of them closer to the galactic center,” said Franckowiak, “but the galactic gamma ray emissions are so bright we’d need to get a lot better at being able to subtract them.”
So, we may be getting up-close and personal with our galaxy’s gamma-ray bubbles, but we’re still completely baffled as to their origin.