Home Radiation After all, mysterious gamma rays may not emanate from Fermi bubbles: ScienceAlert

After all, mysterious gamma rays may not emanate from Fermi bubbles: ScienceAlert


A gamma-ray trickster has just been discovered near the Milky Way.

The energetic radiation previously associated with erupting structures in the Milky Way’s galactic center called Fermi Bubbles actually appears to be coming from something farther away.

Instead, the origins are believed to be millisecond pulsars in a small dwarf galaxy orbiting our own.

The discovery has implications for our understanding of Fermi bubbles, but it could also impact broader areas of research, such as the search for galactic dark matter.

Fermi Bubbles were discovered in 2010 and came as a huge surprise, literally. They are gargantuan bubbles of high-energy gas emanating from the galactic center that extend above and below the galactic plane, a total distance of 50,000 light-years, expanding at a speed of millions of miles at time.

A visualization of Fermi bubbles. (NASA Goddard Space Flight Center)

What created them – the Milky Way’s supermassive black hole being a top candidate – did so millions of years ago, and the bubbles have been popping up and out ever since. They are brighter in high-energy gamma radiation than the rest of the Milky Way’s disk.

Not all Fermi bubble radiation is evenly distributed. In particular, there is what is described as a “cocoon” of freshly accelerated cosmic rays in the southern lobe, interpreted when it was discovered in 2011 as part of the superbubble environment.

Now a team of astronomers, led by astrophysicist Roland Crocker of the Australian National University in Australia, have noticed something interesting.

The location of the cocoon directly coincides with the location of another object – the core of the Sagittarius dwarf spheroidal galaxy, a Milky Way satellite that is being torn apart and subsumed by the larger galaxy.

This, on its own, would be a pretty big co-inky-dink, with a very low probability of around 1%. But it gets even more interesting. The Cocoon and the Sagittarius Galaxy also have similar shapes and orientations.

Of course, distance in space can be extremely difficult to gauge. Unless you know precisely how much light something emits, it’s hard to know how far away it is.

If you see something emitting gamma radiation within a larger gamma radiation structure, it’s probably natural to assume that the two are related. But two things with similar shape and orientations lined up directly in our line of sight would be, well, really special.

Not impossible, but there might be a more likely explanation – like a connection between these two objects.

The researchers therefore decided to revisit the cocoon and see if the dwarf galaxy could possibly be an alternative explanation for the gamma radiation observed there.

They modeled the emission over a range of explanations, including the intra-bubble cocoon and the Sagittarius galaxy, and found that, to some extent, the Sagittarius galaxy was the most likely emitter of gamma radiation. in Fermi’s cocoon.

The next question, of course, was what could produce it. In the Milky Way, gamma rays are mainly generated by collisions between cosmic rays and gas in the interstellar medium.

This is not possible for the Sagittarius galaxy. The smaller satellite galaxy is gravitationally falling into the Milky Way, and has been for some time; as such its gas was carefully extracted, probably around 2-3 billion years ago.

Similarly, no massive short-lived stars have died in spectacular supernovae; these are born of gas, and well. There are not any.

The most likely explanation, according to the team, is millisecond pulsars. They are neutron stars (the collapsed, ultra-dense cores of dead massive stars) with extremely fast, millisecond-scale spin rates; as they spin, they emit jets of radiation from their poles, including gamma radiation.

These would be consistent with the most recent episodes of star formation in the Sagittarius galaxy, and would have the same spatial distribution as the rest of the stellar population.

Although gamma radiation appears bright compared to other galaxies such as Andromeda, this would be possible if the pulsars were 7-8 billion years old and had a low metal content, which is consistent with the rest of the population of Sagittarius, according to the researchers.

This discovery suggests that dwarf spheroidal galaxies like Sagittarius may produce more gamma radiation than expected.

If so, they could confuse searches for dark matter signals, one of which is thought to be excess gamma radiation emitted when dark matter particles and antiparticles annihilate each other.

The possibility, the researchers say, should prompt a closer look at these small faint galaxies, to see if we need to revise our understanding of dwarf spheroidal galaxies and the ancient star populations they contain.

The research has been published in natural astronomy.