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The Doomed Hitomi Telescope Had a Spectacular View of the Universe As It Died

It viewed a monster tangle of galaxies known as Perseus.
The Perseus cluster, captured by NASA's Chandra X-ray telescope. Image credit: X-ray: NASA/CXC/SAO/E.Bulbul, et al.

The Hitomi X-ray astronomy satellite, which met its untimely end in March, didn't die in vain. It turns out that, in the five weeks or so that it was alive and whole up in space, Hitomi managed to do some pretty amazing science.

The view it took in before it died was spectacular: that of the Perseus cluster, a tangle of hundreds of galaxies that are bound together by gravity, 240 million light years away. It's mindbogglingly enormous, one of the largest known structures in the universe.


"Perseus was an ideal target, and the science was just exquisite," said Brian McNamara, University Research Chair in Astrophysics at the University of Waterloo, who's an author of a new paper, out Wednesday in Nature, that represents the first published findings from Hitomi. It could help explain how supermassive black holes regulate their galactic neighbourhoods. (The Japan-led mission had collaborators from all around the world, including Canada, the US and Europe, and this paper has dozens of authors.)

The Perseus cluster of galaxies. The image on the left shows a close up image of active galaxy NGC 1275, the central, dominant member of the Perseus cluster (Image: Data - Hubble Legacy Archive, ESA, NASA; Processing - Al Kelly). The image on the right shows the cluster using an X-ray telescope revealing the atmosphere of plasma enveloping the whole galaxy cluster.

Perseus isn't not just made up of galaxies. It has an "atmosphere" of hot plasma that's heated up to tens of millions of degrees, McNamara told me, and an invisible halo of dark matter. Plasma is invisible to the human eye (and to optical telescopes), but with its ultra-sensitive instruments, Hitomi could detect the X-rays it emits.

And hot plasma is extremely important. There's more of it, by mass, "than there are all the stars in all the galaxies," he said. "It's all over the place." It also contains the raw ingredients for stars, galaxies, planets, and ultimately, all of us.

So why didn't all this gas cool down, and form more of everything?

A montage consisting of a (background) Chandra X-ray image of the core of the Perseus cluster with the field viewed by the Hitomi X-ray Spectrometer outlined in yellow. The X-ray spectrum is also shown with emission from helium-like iron (ie iron with only 2 electrons) on the left and hydrogen-like iron (iron with only one electron) on the right. The narrownness of the emission lines shows that random motions in the 50 MK hot gas in the Perseus cluster core are only 164+/-10 km/s. Image: Hitomi Collaboration/JAXA, NASA, ESA, SRON, CSA

What seems to happen is a sort of feedback loop. As the supermassive black hole chows down on surrounding matter, it gives off huge amounts of energy. Heat pours out from outside the black hole's event horizon, "creating bubbles that float through the gas, keeping it hot," McNamara said. These course through hot plasma like champagne bubbles, he continued, and prevent the hot plasma from cooling down to form other galaxies.

In that way, the supermassive black hole seems to be acting as a sort of great galactic regulator. Over the next billions of years, hot plasma will become the next generation of stars and planets, but how quickly that happens is controlled by the black hole.

As for Hitomi's fate, well, these findings "emphasize the tragedy" of losing it so abruptly, McNamara said. Although scientists were initially tight-lipped about what happened in the early days after it went dark, they now say that the doomed satellite seems to have been the victim of human error, and a faulty command that sent it spinning out of control.

All isn't lost. McNamara predicts there could be 10 or 15 papers to come out of the mission, and this is just the first. "There's so much information even from the short time it was there," he said. "It's going to take a long time to puzzle through what it all means."