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At colliding galaxies, a pipsqueak lights up

Light green sources of high-efficiency X-ray lights captured by NASA's NuSTAR mission are superimposed on an optical image of the Whirlpool Galaxy (in the center of the image) and its companion galaxy, M51 b (the bright green-white spot above the Whirlpool), taken by Sloan Digital Sky Survey. Credit: NASA / JPL-Caltech, IPACIn the nearby Whirlpool galaxy and its companion galaxy, M51b, two super-massive black holes heat up and dry surrounding material. These two monsters should be the brightest X-ray sources in sight, but a new study of NASA's Nuclear Spectroscopic Telescope Array (NASA) observations shows that a much smaller object competes with the two behemoths. The most amazing features of the Whirlpool Galaxy – officially known as the M51a – are the two long star-filled "arms" that revolve around the galactic center as a band. The much smaller M51b clings like a jaw shell to the edge of the Whirlpool. In summary, known as the M51, the two galaxies merge. In the middle of each galaxy, a supermassive black hole is millions of times more massive than the sun. The galactic fusion should drive huge amounts of gas and dust in the black holes and in orbit around them. In turn, the intense center of gravity of the black holes should cause it to revolve the material to warm up and radiate, forming light plates around each that can boast all the stars in their galaxies. But neither black hole radiates so strongly in the X-ray range that researchers would…



Light green sources of high-efficiency X-ray lights captured by NASA’s NuSTAR mission are superimposed on an optical image of the Whirlpool Galaxy (in the center of the image) and its companion galaxy, M51

b (the bright green-white spot above the Whirlpool), taken by Sloan Digital Sky Survey. Credit: NASA / JPL-Caltech, IPAC

In the nearby Whirlpool galaxy and its companion galaxy, M51b, two super-massive black holes heat up and dry surrounding material. These two monsters should be the brightest X-ray sources in sight, but a new study of NASA’s Nuclear Spectroscopic Telescope Array (NASA) observations shows that a much smaller object competes with the two behemoths.

The most amazing features of the Whirlpool Galaxy – officially known as the M51a – are the two long star-filled “arms” that revolve around the galactic center as a band. The much smaller M51b clings like a jaw shell to the edge of the Whirlpool. In summary, known as the M51, the two galaxies merge.

In the middle of each galaxy, a supermassive black hole is millions of times more massive than the sun. The galactic fusion should drive huge amounts of gas and dust in the black holes and in orbit around them. In turn, the intense center of gravity of the black holes should cause it to revolve the material to warm up and radiate, forming light plates around each that can boast all the stars in their galaxies.

But neither black hole radiates so strongly in the X-ray range that researchers would expect in a merger. Based on previous observations from satellites that detect low energy x-rays, such as NASA’s Chandra X-ray Observatory, scientists believed that gas and dust layers around the black hole in the larger galaxy blocked extra emissions. But the new study, published in Astrophysical Journal used NuSTAR’s high-efficiency X-ray vision to be compared during these layers and found that the black hole is still dimmer than expected.

“I’m still surprised at this discovery,” said study director Murray Brightman, a researcher at Caltech in Pasadena, California. “Galactic mergers should generate black hole growth, and the evidence for this would be strong emission of high energy X-rays. But we don’t see it here.”

Brightman thinks that the most likely explanation is that black holes “flicker” during galactic fusions instead of being radiated with a more or less constant brightness throughout the process.

“The flickering hypothesis is a new idea in the field,” said Daniel Stern, a researcher at NASA’s Jet Propulsion Laboratory in Pasadena, and project scientist for NuSTAR. “We used to think that the black hole variation occurred in time frames of millions of years, but now we think that the timetables can be much shorter. To determine how short is an area of ​​active study.”

Small but Brilliant

Along with the two black holes that radiate less than researchers expected in the M51a and M51b, the former host is an object that is millions of times less than a black hole but shines with equal intensity. The two phenomena are not connected, but they create a surprising X-ray landscape in the M51.

The small x-ray source is a neutron star, an incredibly dense nugget of material left behind by a massive star exploding at the end of its life. A typical neutron star is hundreds of thousands of times smaller in diameter than the sun’s only as wide as a large city, but has one to two times the mass. A teaspoon of neutron star material would weigh more than 1 billion tons.

Despite their size, neutron stars often make themselves known by intense light emissions. The neutron star of the M51 is smoother than average and belongs to a newly discovered class called ultraluminous neutron stars. Brightman said some researchers have suggested that strong magnetic fields generated by the neutron star could be responsible for the light emission. An earlier paper from Brightman and colleagues about this neutron star supports that hypothesis. Some of the other bright, high-efficiency X-ray sources seen in these two galaxies can also be neutron stars.


Explore further:
Cosmic collision erodes galactically a ring in x-rays

More information:
M. Brightman et al. A Long Hard X-ray Look at the Dual Active Galactic Nuclei of M51 with NuSTAR, The Astrophysical Journal (2018). DOI: 10.3847 / 1538-4357 / aae1ae

Journal reference:
Astrophysical magazine

Provided by:
Jet Propulsion Laboratory

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