A gamma ray liar has just been spotted near the Milky Way.
The high-energy radiation previously associated with structures called Fermi bubbles erupting from the galactic center actually appears to be coming from something farther away.
Instead, their origin is thought to be a millisecond pulsar in a small dwarf galaxy orbiting ourselves.
The discovery has important implications for our understanding of Fermi bubbles, but it could also have implications for broader areas of research, such as the search for galactic dark matter.
Fermi bubbles were discovered in 2010, and it was literally a huge surprise. They are huge bubbles of high-energy gas emanating from the center of the Milky Way, extending above and below the galactic plane for a total distance of 50,000 light-years, expanding at millions of miles per hour.
Whatever created them — the Milky Way’s supermassive black hole is a prime candidate — did so millions of years ago, and the bubbles have been blowing up and out ever since. They are brighter in high-energy gamma radiation than the rest of the galaxy.
Not all radiation from Fermi bubbles is uniformly distributed. In particular, there is a “cocoon” described as a newly accelerated cosmic ray in the southern lobe, which was explained as part of the superbubble environment when it was discovered in 2011.
Now, a team of astronomers led by Australian National University astrophysicist Roland Crocker has noticed something interesting.
The cocoon’s location directly coincides with that of another object – the core of the Sagittarius Dwarf Spherical Galaxy, a satellite of the Milky Way that is being torn apart and incorporated by a larger galaxy.
On its own, this would be a fairly large common denominator, with a very low probability of about 1%. But it gets more interesting. The Cocoon and Sagittarius galaxies also have similar shapes and orientations.
Of course, distances in space are difficult to measure. Unless you know exactly how much light something is emitting, it’s hard to know how far away it is.
If you see something that emits gamma radiation in a larger gamma radiation structure, it’s natural to think the two are related. But two things with similar shapes and orientations lined up directly in our line of sight would be quite peculiar.
Not impossible, but there may be more likely explanations – such as a connection between the two objects.
So the researchers decided to revisit the cocoon to see if dwarf galaxies might be an alternative explanation for the gamma radiation observed in them.
They modeled the emission through a range of interpretations, including the cocoon within the bubble and the Sagittarius galaxy, and found that the Sagittarius galaxy is, in a considerable sense, the most likely emitter of gamma radiation in the Fermi cocoon.
The next question, of course, is what happens. In the Milky Way, gamma rays are primarily produced by collisions between cosmic rays and gas in the interstellar medium.
This is impossible for the Sagittarius galaxy. Smaller satellite galaxies are gravitationally falling into the Milky Way, and have been for some time; as a result, their gas has been neatly stripped away between 2 and 3 billion years ago.
Nor have any massive, short-lived stars died in spectacular supernovae. These are produced from gas, and, well. Empty.
The most likely explanation, the team found, is millisecond pulsars. These are neutron stars (the ultra-dense cores of collapsed, dead, massive stars) that spin extremely fast, measured in milliseconds; as they spin, they emit jets of radiation—including gamma radiation—from their poles.
These will be compatible with recent star formation events in the Sagittarius galaxy and have the same spatial distribution as other stellar groups.
While the gamma radiation appears brighter compared to other galaxies like Andromeda, it would be possible if the pulsar was 7 to 8 billion years old and had low metallicity — a similar feature to Sagittarius, the researchers said. other members agree.
This finding suggests that dwarf spherical galaxies like Sagittarius may be producing more gamma radiation than expected.
If so, they could confound the search for dark matter signals, one of which is hypothesized about excess gamma radiation emitted when dark matter particles and antiparticles annihilate each other.
That possibility should prompt us to look more closely at these small, faint galaxies to see if we need to revise our understanding of dwarf spherical galaxies and the old groups of stars they contain, the researchers say.
The research has been published in natural astronomy.