- Spiral galaxy NGC 4151 has a growing, giant black hole at its centre.
- Dubbed “The Eye of Sauron” for its resemblance to the “The Lord of the Rings” character
AT THE HEART OF MANY (perhaps most) galaxies there lives a dark, malevolent force—a black hole.
And they aren’t just ordinary black holes. They are giants…what astronomers call ‘supermassive’ black holes, which can have masses hundreds of millions or billions of times the mass of our Sun.
One such galaxy is NGC 4151. Located about 43 million light-years from Earth, it is one of the nearest galaxies to contain an actively growing black hole.
A new false-colour image put together using different wavelength data makes the local region surrounding the black hole look like the ‘Eye of Sauron’ from the Lord of the Rings movies.
In the ‘pupil’ of the eye, X-rays (coloured blue) detected by the Chandra X-ray Observatory are combined with visible light wavelengths (yellow) showing positively charged hydrogen atoms (from observations with the Jacobus Kapteyn Telescope in the Canary Islands).
The red surrounding the pupil shows neutral hydrogen detected by radio observations with the Very Large Array radio telescope in the USA.
Because it is so close (in astronomical terms), NGC 4151 offers one of the best opportunities to study the interaction between an active supermassive black hole and the surrounding gas of its host galaxy.
Such interaction, or ‘feedback’, is recognised to play a key role in the growth of both black holes and their host galaxies.
Gas falls into the black hole, feeding it and making it grow larger.
But as the gas approaches the black hole, it heats up…to the point where some of it shoots back into the galaxy in a process known as an outflow. That hot gas emits X-rays.
If the X-ray emission seen in the core of NGC 4151 indeed originates from hot gas heated by the outflow from the black hole, it would be strong evidence for black hole feedback occurring within individual galaxies.
Such feedback has already been seen on larger scales—in clusters of galaxies such as the Perseus Cluster, where active black holes interact with surrounding gas.
Adapted from information issued by Chandra X-ray Centre. Image credit: X-ray, NASA / CXC / CfA / J.Wang et al.; optical, Isaac Newton Group of Telescopes, La Palma / Jacobus Kapteyn Telescope; radio, NSF / NRAO / VLA.
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