Cocoon of Fermi bubbles may not be connected to the center of the Milky Way

(ORDO NEWS) — The region of particularly hard gamma radiation visible near the center of our Galaxy may be created by a dwarf companion galaxy located behind it.

At the center of the Milky Way is a supermassive black hole. It gains about 4.3 million solar masses and attracts large amounts of matter, which forms dense clouds.

Above and below the plane of the Galaxy, a pair of giant “Fermi bubbles” protrude above them, actively emitting in the X-ray and gamma-ray ranges.

They resemble an hourglass, stretched in both directions for 25 thousand light years – a quarter of the diameter of the entire Milky Way.

Fermi bubbles were first spotted by the Fermi Space Telescope in 2010, and scientists have been trying to figure out their nature ever since.

Not so long ago, they discovered a pair of “pipelines” leading to the center, into the turbulent and obscure surroundings of a supermassive black hole.

In the southern bubble, one can distinguish a ” cocoon ” – a small elongated region of especially active radiation.

However, this area does not seem to be associated with bubbles. The authors of a new paper published in the journal Nature Astronomy have shown that the position and orientation of the cocoon coincides with a slightly further dwarf elliptical galaxy in Sagittarius (Sgr dSph) – a small satellite galaxy of the Milky Way.

The answer to the question of the origin of certain structures in the Universe depends on where exactly they are located, and determining distances in astronomy is far from an easy task, especially when it comes to an object whose initial characteristics are unknown to us.

However, the Sgr dSph galaxy is located exactly in the region of the bright cocoon in the southern Fermi bubble, has the same dimensions and is oriented in the same way, which can hardly be a mere coincidence.

This conclusion was reached by an international team of astronomers led by Roland Crocker from the Australian National University and Oscar Macias from the University of Amsterdam.

Cocoon of Fermi bubbles may not be connected to the center of the Milky Way 2
Blue shows gamma radiation from Fermi bubbles; red – variable stars of the RR Lyrae type, which astronomers are guided by to estimate cosmic distances. A cluster of stars is visible in the dwarf galaxies the Large and Small Magellanic Clouds and Sgr dSph. Bright “cocoon” coincides with Sgr dSph

The dwarf galaxy Sgr dSph has been swallowed up by the more massive Milky Way for several billion years. It is ten times smaller than the Galaxy and is located on its side, opposite from us.

There is no supermassive black hole in Sgr dSph, and there is not enough material left for new stars to be actively born there.

So if the high-energy radiation seen as a cocoon in the Fermi bubbles comes from Sgr dSph, it is unlikely that it could come from processes that could operate in the Milky Way, say, supernova explosions.

The authors of the new paper point to two possible sources of radiation in Sgr dSph. It could be a massive annihilation of dark matter particles.

But then the shape and size of the cocoon would not match so exactly with the distribution of stars in the dwarf galaxy.

Another option is a large population of millisecond pulsars. Pulsars are fast-spinning neutron stars left over from the death of massive stars.

Possessing a colossal magnetic field, they emit powerful streams of particles and radiation, including those in the hardest X-ray and gamma-ray ranges.

The population of pulsars in Sgr dSph should be distributed in space in the same way as the stars that gave birth to them were distributed, which explains the shape and size of the cocoon that we observe in the southern Fermi bubble.

And the absence of gas and dust, long drawn out by the Milky Way, makes their radiation especially bright.

Scientists believe that the radiation is further enhanced by the appropriate age of pulsars – about seven to eight billion years – and their low content of heavy elements.


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