Dead Star 'Echos' Light Up Killer Black Holes' Dust
When black holes devour stars, the energy generated heats up a dusty sphere.
Imagine setting off a firework in a valley. The boom of the exploding gunpowder would likely be followed by one or more echos of the sound waves bouncing off the valley sides. Using a little knowledge about how fast sound waves travel through air, you could quickly calculate how far away the valley sides are from the exploding firework by simply timing how long it was between the firework going "boom" and the echo returning.
This may sound like a pretty basic physics lesson, but astrophysicists have applied a similar principal to studying the most extreme environment known to exist in our universe: supermassive black holes in the centers of galaxies. In this case, however, the "fireworks" are stars getting eaten by the black holes and the "valley sides" are the surrounding rings of dust, just beyond the reach of the black holes' gravitational grasp.
Supermassive black holes "weigh" in at millions to billions of times the mass of our sun and are highly efficient at devouring stars that come too close. When an unfortunate star does stray near, within a distance called the "Roche radius," the tidal shear exerted on that star will be so extreme that it will be stretched to oblivion -- this is known as "stellar tidal disruption." The star's matter stretch stretch and get pulled into the black hole's event horizon. When this happens, a powerful flare is generated, blasting surrounding space with extreme X-ray and ultraviolet light that can be observed from Earth.
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Should this ionizing radiation hit clouds of dust immediately surrounding the black hole, it will be vaporized, but further away the energy from this radiation will be absorbed and then re-emitted as infrared radiation. It is this re-emission of infrared radiation that acts like an echo, occurring some time after the original flare.
Now, using data from the Wide-field Infrared Survey Explorer (WISE), two groups of researchers were able to gain a surprising amount of information about the volume of space surrounding distant supermassive black holes, also revealing some of the secrets about these cataclysmic explosions.
Before the WISE mission was completed in 2011, the space telescope mapped the infrared universe every 6 months. After a selection of candidate stellar tidal disruption events were detected in the centers of galaxies, the WISE data could be used to see how the flare affected the black hole's dusty ring and how that heating evolved with time, using a technique known as "photo-reverberation" or "light echoes." With this information, the researchers were able to deduce how far from the central black hole the dust clouds were and how much energy was released by the flare.
"Our study confirms that the dust is there, and that we can use it to determine how much energy was generated in the destruction of the star," said Varoujan Gorjian, an astronomer at NASA's Jet Propulsion Laboratory, Pasadena, Calif.
In addition, they were able to gain a handle on the structure of the surrounding dust, revealing the dust has been blown out into a "patchy, spherical web of dust located a few trillion miles (half a light-year) from the black hole itself," according to a NASA news release.
"The black hole has destroyed everything between itself and this dust shell," said Sjoert van Velzen, of Johns Hopkins University, Baltimore. "It's as though the black hole has cleaned its room by throwing flames."