Dwarf galaxy gamma rays solve astronomical mystery

(Image credit: NASA Goddard Space Flight Center)

This article was originally published in The Conversation. (will open in a new tab) The publication published an article in Expert Voices: Op-Ed & Insights on

A luminous blob known as a “cocoon” that appears to be inside one of the huge gamma rays from the center of our galaxy, dubbed “Fermi Bubbles”, has puzzled astronomers since it was discovered in 2012.

In a new study (will open in a new tab) published in the journal Nature Astronomy, we show that the cocoon is caused by gamma rays emitted by rapidly rotating extreme stars called “millisecond pulsars” located in the Sagittarius dwarf galaxy that orbits the Milky Way. While our results clear up the mystery of the cocoon, they also cast a shadow over attempts to search for dark matter in whatever gamma radiation it might emit.

Seeing with gamma rays

Luckily for life on Earth, our atmosphere blocks gamma rays. These are particles of light with energies over a million times greater than the photons that we detect with our eyes.

Since our view from ground level is obscured, scientists had no idea of ​​the abundance of gamma rays in the sky until instruments were launched into space. But since the accidental discoveries made by the Vela satellites (launched into orbit in the 1960s to monitor the nuclear test ban), more and more of this wealth is being discovered.

The state-of-the-art gamma-ray instrument in operation today is the Fermi Gamma-ray Space Telescope, a major NASA mission in orbit for more than a decade. Fermi’s ability to distinguish fine details and detect faint sources has revealed a number of surprises about our Milky Way and the wider cosmos.

Mysterious Bubbles

One of these surprises appeared in 2010. (will open in a new tab), shortly after Fermi’s launch: something in the center of the Milky Way blows out what looks like a pair of giant gamma-ray bubbles. These completely unexpected “Fermi Bubbles” completely cover 10% of the sky.

The main suspected source of the bubbles is a supermassive black hole in the galaxy. This behemoth, 4 million times more massive than the Sun, hides in the core of the galaxy, in the region from which the bubbles originate.

Most galaxies contain such giant black holes at their centers. In some cases, these black holes are actively absorbing matter. As they feed in this manner, they simultaneously spew giant effluent “jets” visible in the electromagnetic spectrum.

Thus, the question asked by researchers after the discovery of the bubbles is: can we find a smoking gun linking them to our galaxy’s supermassive black hole? Soon the evidence of the assassination really surfaced: there was a hint (will open in a new tab)inside each bubble is a thin gamma-ray jet directed back towards the galactic center.

However, over time and new data, this picture has become clouded. While the jet-like feature of one of the bubbles was confirmed, the apparent jet in the other seemed to evaporate under close scrutiny. (will open in a new tab).

The bubbles looked strangely lopsided: in one of them there was an elongated bright spot – a “cocoon” that was not in the other bubble.

See also: Astronomers have discovered one of the largest black hole jets in the sky (will open in a new tab)

Cocoon and where does it come from

Our recent work (will open in a new tab) in Nature Astronomy is a deep study of the nature of the “cocoon”. Remarkably, we found that this structure has nothing to do with either the Fermi bubbles or the Galaxy’s supermassive black hole.

Rather, we discovered that the cocoon is actually something completely different: gamma rays from the Sagittarius dwarf galaxy, which lies beyond the southern bubble as viewed from Earth.

Diagram showing the Milky Way, the gamma-ray Fermi Bubbles (pink), and the Sagittarius dwarf galaxy and its tails (yellow/green). From the position of the Sun, we see the Sagittarius dwarf through the southern Fermi bubble. (Image credit: Aya Tsuboi, Kavli IPMU, Credit: Author)

So named because its position in the sky is in the constellation Sagittarius, the Sagittarius Dwarf is a satellite galaxy orbiting the Milky Way. This is the remnant of a much larger galaxy that was literally torn apart by the strong gravitational field of the Milky Way. Indeed, stars elongated from the Sagittarius dwarf can be found in the “tails” that wrap around the entire sky.

What creates gamma rays?

In the Milky Way, the main source of gamma rays are high-energy particles called cosmic rays that collide with the very rarefied gas between stars.

However, this process cannot explain the gamma rays emitted by the Sagittarius dwarf. She lost her gas long ago due to the same gravitational pull that swept away so many of her stars.

So where do gamma rays come from?

We’ve looked at several possibilities, including the exciting prospect that they are the signature of dark matter, an invisible substance known only for its gravitational effects, that astronomers believe makes up most of the universe. Unfortunately, the shape of the cocoon exactly matches the distribution of visible stars, which rules out dark matter as a source.

One way or another, the stars were responsible for the gamma rays. And one more thing: the stars of the Sagittarius dwarf are old and silent. What type of source among such a population produces gamma rays?

Millisecond pulsars

We are satisfied that there is only one possibility: rapidly rotating objects called “millisecond pulsars”. These are the remnants of certain stars, much more massive than the Sun, that also orbit another star.

Under the right circumstances, such binaries produce a neutron star — an object about the same weight as the Sun but only about 20 km across — that rotates hundreds of times per second.

Because of their rapid rotation and strong magnetic field, these neutron stars act like natural particle accelerators, launching extremely high-energy particles into space.

These particles then emit gamma rays. We found that millisecond pulsars in the Sagittarius dwarf were the main source of the mysterious cocoon.

Read more: This newly discovered neutron star could light the way for a whole new class of stellar objects (will open in a new tab)

Hunting for dark matter

Our findings shed new light – pun intended – on millisecond pulsars as sources of gamma rays in other old star systems.

At the same time, they also overshadow efforts to find evidence for the existence of dark matter through observations of other satellite galaxies of the Milky Way; unfortunately, there is a stronger “background” of gamma rays from millisecond pulsars in these systems than previously thought.

Thus, any signal they produce cannot be unequivocally interpreted as being related to dark matter.

The hunt for dark matter signals continues.

This article is republished from The Conversation (will open in a new tab) under a Creative Commons license. Read original article (will open in a new tab).

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