Black hole pinball

The black hole at the centre of our galaxy has been found lying at the end of the road when the route of a hypervelocity star was traced back.

Hypervelocity stars, of which there are sixteen known, are stars travelling at tremendous velocities, such speeds that are required to leave the gravitational pull of the galaxy. It is believed that there is one hypervelocity star for every 100 million normal velocity stars, meaning about one thousand stars have been sent out by interactions with the dark heart of the Milky Way.

This particular star, HE 0437-5439, lies 200,000 light years from the centre of the galaxy, having been travelling for 100 million years at its present velocity. It is a large (~9 solar masses) blue star, suggesting a relatively short lifetime of around 20 million years. This has lead to two competing theories for its origin. Either the star was ejected by the Large Magellanic Cloud, at 65,000 light years it is nearer to the star’s present position, or it is a ‘Blue Straggler’, a star formed from the merger of two older stars.

Using the Hubble Space Telescope, astronomers looked at the spectral Doppler shift as well as using astrometry (precision measurements of position) to determine the star’s three dimensional direction. The result was a conclusive vector pointing towards the centre of the Milky Way. This was no single star ejected from the galactic core, but a pair that flew off gravitationally bound together. As one of them grew old and expanded into a red giant, it swallowed the other and together they formed a new bright blue star.

But the story doesn’t end there as for a star to have been ejected in this way requires a complex gravitational dance between it, the supermassive black hole and a star gravitationally bound to the one that is ejected. As this second star is swallowed, the first star meets a gravitational slingshot and is sent out on its journey. As we’re talking about a one time binary star here, this means the bright blue star was once part of a triple system that encountered the black hole at the centre of our galaxy and was kicked out never to return. But that doesn’t mean its story is over as the motion of hypervelocity stars will be affected by clumps of gravitating matter or even Dark Matter in the halo, making their velocities probes of the halo of the galaxy.

Another unusual probe of the Dark Side has emerged. Hydrogen gas formed from the plasma that composed the early universe. It should retain wave patterns prevalent in the plasma due to soundwaves and quantum fluctuations in it (like the slight temperature changes in the CMB). As the universe expands, so do these waves and they have a maximum size, meaning we can estimate how the universe is expanding (constantly, exponentially etc) by examining waves close by and far away and measuring their sizes. Normally this is done by looking at what the hydrogen has formed – galaxies, which congregate around the rims of vacancies formed by the waves, giving the universe a honeycomb superstructure. But by using radio wavelengths at which hydrogen is relatively bright, the translucent cosmic web can be mapped. Astronomers using the the Robert C Byrd Green Bank Telescope in West Virginia, USA, have mapped this tangle as far back as 1.12 redshifts (a higher redshift indicates faster acceleration and hence farther distance) or 6.5 billion light years, further than previous observations had reached. The technique used is called intensity mapping and tries to understand the intensity of light from each direction in terms of a translucent, glowing substance. A similar method has been used to map the Milky Way using hydrogen’s 21cm line, an emission line that is only slightly absorbed by hydrogen, but even more slightly absorbed by the rest of the universe.

But back to black holes and they don’t always get their own way. These galactic bullies may enjoy flinging the odd star here and there, but what happens when they meet something their own size? They get spun round appears to be the answer. When galaxies collide, black holes can be knocked out of position, or even out of their galaxy, but some stick to their guns. That in galaxy 4C +00.58 is one of those that hung onto its position by the tips of its fingers. Initially, it sat happily in place in one galaxy while another began to feed into it. The feeding process gave the black hole material to produce jets, these punched holes through the gas in the galaxy, leaving cavities when the galaxy is now viewed in the x-ray. But the new gas source wasn’t aligned with the spin-axis of the black hole, making transferring material from the accretion disc to the black hole complex, so the black hole found its axis changing. As a result, the jets now blasted two new cavities as seen in the x-ray. Astronomers observed this galaxy using the Chandra X-ray Space Telescope and found it to have the X shape caused by the blasting of the jets. Then a radio telescope was honed onto the target to view emissions from particles accelerated by the jets directly. They showed something surprising, the black hole appeared to have changed spin axis again, with the new jets joined by an arc of gas to the position of the old jets. This, researchers believe, is due to the dynamics of the black holes of the two colliding galaxies merging.


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