How does one weigh a supermassive black hole that is anywhere between a million and a billion times the mass of the Sun? The answer could be as easy as taking a snapshot of its surrounding galaxy.
A team of astronomers has concluded that the larger the black hole at the centre of a spiral galaxy, the tighter the galaxy鈥檚 arms wrap around itself. If correct, the simple relationship would give researchers an easy way to learn about black holes up to 8 billion light years away 鈥� thousands of times farther than most black hole masses can be resolved today.
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Marc Seigar of the University of Arkansas in Little Rock, US, and colleagues studied 37 spiral galaxies, including the Milky Way and our neighbour, Andromeda.
Those with the smallest black holes had their arms outstretched at angles of as much as 43掳 (scroll down for image), while those with the biggest black holes hugged themselves much more tightly, with as few as 7掳 separating the galaxies鈥� arms from their cores.
It鈥檚 not the first time that astronomers have noticed a link between supermassive black holes, which they can鈥檛 see, and their surrounding galaxies, which they can.
Speedy stars
Previously, researchers have observed that the bulge of stars forming the galaxy鈥檚 centre is larger for more massive black holes. But 鈥渢o estimate the bulge mass at very high distances, there would be a lot of uncertainty,鈥� Seigar told New 杏吧原创.
Astronomers also weigh black holes by studying the motions of stars that orbit around them 鈥� the bigger the black hole, the faster the stars move. But the stars can only be resolved in galaxies a few tens of millions of light years away.
鈥淎s you get farther away, basically that method gets harder to use 鈥� even with the Hubble Space Telescope 鈥� because you don鈥檛 have the resolution to do it,鈥� Seigar told New 杏吧原创. 鈥淭he spiral arms are just easier to detect at very high distances.鈥�
Indeed, the most distant black holes hold the most allure for astronomers because they are being seen as they were in the early universe, Seigar says.
Dark matter
As a result, they can shed light on how galaxies evolve, as well as on a cosmological component that is even harder to pin down 鈥� dark matter. Galaxies are thought to take shape in giant cradles of the mysterious matter, which is detected by its gravitational pull on visible matter.
The gravitational attraction of dark matter may also dictate the reach of a galaxy鈥檚 arms, the heft of its central bulge and the mass of its black hole, Seigar says, although the theory has yet to be proven.
Seigar鈥檚 team will next study more distant active black holes, which are in the process of devouring their surroundings. These are easier to measure than their tranquil cousins but rarer, and currently can only be weighed if they are within about 5 billion light years from Earth.
The researchers want to make sure that the simple relationship they found between spirals and black hole mass does not change as the galaxies evolve over time. Says Seigar: 鈥淲e have to make sure the correlation itself doesn鈥檛 evolve.鈥�
Journal reference: (forthcoming)
