Ancient quasars spin at relativistic speeds

An article published in “The Astrophysical Journal” reports an observation of 5 quasars between 9.8 and 10.9 billion light years away from Earth with the help of gravitational lenses that offered multiple images of them. A team of astronomers used NASA’s Chandra X-ray Observatory to study the accretion disks around the supermassive black holes that power those quasars, discovering that one of them spins at a speed higher than 70% of the speed of light.

For a long time now astronomers have been exploiting gravitational lenses, the distortion of light that comes from a cosmic object caused by the very strong total gravity of a galaxy between it and the Earth. It’s a very useful phenomenon to observe objects billions of light years away, in this case quasars, a type of active galactic nuclei in which supermassive black holes are surrounded by large amounts of materials heated to the point of emitting electromagnetic waves that can be very energetic such as X-rays. For this reason it’s possible to use the Chandra X-ray Observatory to study those quasars.

The techniques to observe objects that are very far away using gravitational lenses have been perfected to take into account the distortions generated by the lenses but also by exploiting microlenses, individual stars that can provide a further amplification of the light coming from a quasar. The consequence is that a smaller region generating X-ray emissions is observed.

In their study of quasars, the astronomers exploited the ability of a spinning black hole to drag space around allowing materials to orbit closer to it than a non-spinning black hole. The consequence is that a smaller region from which X-ray emissions originate, which corresponds to a closer orbit, implies a black hole that spins more rapidly.

The quasars studied are cataloged as Q J0158-4325, HE 0435-1223, SDSS J1004+4112, HE 1104-1805 and Q 2237+0305 (image NASA/CXC/Univ. of Oklahoma/X. Dai et al.), which have masses between 160 and 500 million times the Sun’s. According to the results of the study of the one cataloged as Q 2237+0305 and nicknamed Einstein Cross, the supermassive black hole that powers it spins at a speed higher than 70% of the speed of light. The supermassive black holes that power the other four studied quasars spin at speeds that are on average half that of Einstein Cross.

This is not the first study concerning Einstein Cross: one about it only was published in December 2015 in the journal “The Astrophysical Journal Letters”. Thanks to the Chandra X-ray Observatory it was possible to estimate that X-ray emissions come from a part of the accretion disk that is less than 2.5 times the size of the event horizon, while for the other quasars subject of this study the X-rays come from a part of the disk that is between 4 and 5 times the size of the event horizon.

According to the authors of this study, the examined supermassive black holes spin so quickly probably because they grew up accumulating materials from accretion disks that spin with a very similar orientation and direction and not with random directions. That situation was compared to that of a merry-go-round, a carousel that keeps on being pushed in the same direction, increasing its speed.

The observations of these 5 quasars were the longest ever conducted with the Chandra X-ray Observatory as the total exposure times ranged from 1.7 to 5.4 days. The supermassive black holes that power them have masses far away from those of the most massive known, which reach some billion times the Sun’s but they’re still extreme objects and the ones object of this study are also very ancient. They have a considerable influence on the galaxies that host them, so these researches help to understand extreme phenomena but also the evolution of galaxies.

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