A story caught my eye last week about a supermassive black hole that may have been stirring some trouble in its host galaxy fairly recently, astronomically speaking. It has been dubbed the "Eye of Sauron" because, well …

Since I couldn't find a public-domain image from the movies …

You see it, don't you? Assuming that the Dark Lord himself hasn't taken up residency in a nearby galaxy, this multi-wavelength image has an interesting astrophysical story to tell. We are actually looking into the center of the galaxy NGC 4151, which has taught astronomers a little bit about the temper tantrums of black holes.


The yellow emission in the image represents visible light, specifically, the light coming from ionized hydrogen, which suggests a region where new stars are being born. The red color indicates radio emission from neutral hydrogen gas surrounding the region. In the center is the bright emission from the supermassive black hole. The light comes from extremely hot material spinning ever faster in a disk around the black hole as it falls toward its eventual oblivion.

The new discovery is the emission of diffuse X-ray light, colored in blue. This was imaged by the Chandra X-Ray Observatory, an orbiting telescope that gets above the pesky (and protective) atmosphere of the Shire … I mean, Earth.

This emission extends about 6,500 light-years from the central black hole. From this, astronomers can glean some information about the recent history of this black hole.

The X-ray emission could have formed in one of two ways. In one scenario, the black hole flared up in intensity, heating gas that couldn't expand, since it was confined within the neutral hydrogen shell. In another scenario, the electrons of atoms in the gas were violently stripped away by a massive outburst. The latter requires the black hole's disk to emit at its theoretical maximum, likely scaring the pants off any hobbits in this galaxy.

Both scenarios would have had to have occurred less than 100,000 years ago. Considering that galaxies exist for billions of years, that is a very short time ago. The discovery collaboration, led by Junfeng Wang, estimates that supermassive black holes in the centers of galaxies spend about 1 percent of their already active lifetime in such violent bursts.

It is thought that such black holes only spend a fraction of their entire lifetime being active, so this is a rare event. Keep your eyes ever-watchful for more discoveries about this nearby, active black hole!

Images: Top – The center of NGC 4151 in X-ray (blue), optical (yellow) and radio (red). 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. Middle – Cell phone wallpaper maker at RedDodo.com

This work was published in Astrophysical Journal Letters, and a preprint is available at arxiv.org.

With apologies to J. R. R. Tolkien.