Black holes are incredibly messy eaters. As matter falls into a spinning black hole, that matter can be accelerated to incredible velocities and launched out the poles. In the case of the supermassive black holes at the centers of galaxies, these are the most energetic events in the universe since the Big Bang.

The exact mechanism for the creation of these jets is unknown. There are two competing theories, one called the Blandford-Payne mechanism, and one called the Blandford-Znajek mechanism. The details are too fiddly to get into here, but the former has more to do with the in-falling matter and the latter has to do with how magnetic fields interact with the spinning black hole.

The image above is of the galaxy Centaurus A and the jets produced by its super-massive black hole, which is fifty five million times the mass of our sun. The white glow and brown disk are the galaxy itself and associated dust cloud respectively. The blue line is the ultrarelativistic jet of material emitted by the black hole. (Actually, it’s the X-rays emitted by the fast-moving matter in the jet.)

You can’t see the black hole at all. Even on the scale of a galaxy, it’s just a dot, smaller than a pixel. But it has a wide wide reach, extending far beyond the galaxy and influencing the growth and evolution of the galaxy profoundly.

(The image is actually the composite of three images. From Wikipedia: This is a composite of images obtained with three instruments, operating at very different wavelengths. The 870-micron submillimetre data, from LABOCA on APEX, are shown in orange. X-ray data from the Chandra X-ray Observatory are shown in blue. Visible light data from the Wide Field Imager (WFI) on the MPG/ESO 2.2 m telescope located at La Silla, Chile, show the background stars and the galaxy’s characteristic dust lane in close to “true colour”.)

Image Source: https://en.wikipedia.org/wiki/Centaurus_A

Related Reading

These objects are called active galactic nulcei. See: https://en.wikipedia.org/wiki/Active_galactic_nucleus

Black holes glow for other reasons too. They have so-called accretion disks, which glow incredibly brightly. I wrote about this a while back: http://www.thephysicsmill.com/2013/11/09/accretion-disk/