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43 million light years away, the galaxy we see in this picture has been called the "Eye of Sauron" for the similarity of the image to the evil eye of the "Lord of the Rings" character.
It is actually a composite, false colour image of the central regions of the spiral galaxy NGC 4151, combining X-rays imaged by the Chandra Observatory (blue), with an HII (ionized gas) image from the Jacobus Kapteyn Telescope on La Palma (yellow), and a neutral hydrogen image made with the Very Large Array (red).
The neutral hydrogen appears distorted due to its interaction with the rest of the galaxy, and near its inner edge turbulent motions have triggered star formation, visible as small yellow spots. Some of the neutral gas also seems to be falling toward the centre of the galaxy, where a super-massive black hole awaits its arrival.
NGC 4151 is actually one of the closest galaxies that show activity related to the presence of a central black hole, and the X-rays (blue) emanating from the centre are probably from a peak of activity in the recent past.
To explain the energetic emission imaged by Chandra two possibilities have been put forward. The first hypothesis is that rapid fueling of the black hole about 25 thousand years ago resulted in the emission of so much radiation that it ionized the surrounding gas, and as the atoms recombine they emit X-rays to the present day.
The second hypothesis also involves the fall of material onto the black hole, but this time it is the interaction of this material with the accretion disk, around the black hole, that causes a flux of hot gas that interacts with its surroundings, heating it and causing X-ray emission. In this case the emission would last for about 100,000 years. In both cases, the relative brevity of the activity implies that the black hole spends only about 1% of its lifetime in an active state.
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