In colliding galaxies, a pipsqueak shines brightly

The most amazing features of the Whirlpool galaxy – officially known as the M51a – are the two long "arms" filled with stars that wrap around the center of the galaxy like ribbons. The M51b, much smaller, clings like a goose to the edge of the Whirlpool. Collectively known as M51, the two galaxies merge.

At the center of each galaxy is a supermassive black hole, millions of times more massive than the Sun. Galactic fusion is expected to result in huge amounts of gas and dust in these black holes and their orbits. In turn, the intense gravity of the black holes should heat up and emit this material into orbit, forming shiny disks around each one that can overshadow all the stars of their galaxies.

But none of the black holes emit as many X-rays in the X-ray range as scientists might expect during a merger. Based on previous observations of low-energy X-ray satellites, such as NASA's X-ray observatory Chandra, scientists have estimated that layers of gas and dust around the black hole from the largest galaxy blocked additional broadcasts. But the new study, published in the Astrophysical Journal, used NuSTAR's high-energy X-ray vision to look beneath these layers and discovered that the black hole was always darker than expected.

"I am always surprised by this discovery," said lead author of the study, Murray Brightman, a Caltech researcher in Pasadena, California. "Galactic fusions are supposed to generate a growth of black holes, and the proof would be a strong emission of high-energy X-rays, but we do not see it here."

Brightman thinks the most plausible explanation is that black holes "twinkle" during galactic fusions rather than radiate with more or less constant brightness throughout the process.

"The flicker hypothesis is a new idea on the ground," said Daniel Stern, researcher at NASA's Jet Propulsion Laboratory in Pasadena and scientist of the NuSTAR project. "We used to think that the variability of black holes occurred over time scales of millions of years, but we now think that these time scales could be much shorter. an active field of study is short. "

Small but brilliant

In addition to the two black holes emitting less than the scientists anticipated in M51a and M51b, the first also contains an object millions of times smaller than one or the other of the holes but shines with the same intensity. The two phenomena are not related, but they create a surprising X-ray landscape in M51.

The small X-ray source is a neutron star, an incredibly dense nugget left behind by the explosion of a massive star at the end of its life. A typical neutron star is hundreds of thousands of times smaller in diameter than the Sun – as wide as a large city – and one to two times its mass. A teaspoon of neutron star material would weigh more than 1 billion tons.

Despite their size, neutron stars are often known through intense light emissions. The neutron star found in M51 is even brighter than the average and belongs to a newly discovered class called ultraluminous neutron stars. Brightman said that some scientists have suggested that the strong magnetic fields generated by the neutron star could be responsible for the light emission; A previous article by Brightman and his colleagues on this neutron star corroborates this hypothesis. Some of the other bright, high-energy X-ray sources observed in these two galaxies could also be neutron stars.

Explore further:
Cosmic collision forges a galactic ring in X-rays

More information:
Mr. Brightman et al. A long radiographic look at M51's dual active galactic nuclei with NuSTAR, The astrophysical journal (2018). DOI: 10.3847 / 1538-4357 / aae1ae