What Do You Feed Your Inner Supermassive Black Hole?

Remember that black hole at the center of our galaxy I talked about in the last entry? It feeds off of stars, gas and dust that happens to get too close. This happens at the center of any galaxy with a supermassive black hole. Where does all that black hole “food” material come from? And, if a supermassive black hole at the center of a galaxy has a supermassive appetite to match, won’t it clear out its neighborhood quickly? And if so, and it continues to grow, what’s feeding it?

In the case of some types of galaxies that are different from ours–such as those with extra-bright cores (called Seyfert galaxies), their central black holes may be getting the cosmic equivalent of a Big Mac with fries whenever two of these galaxies interact with each other. When such a close encounter takes place, the interaction stirs up gas, which brings more material within the reach of the central black hole of one of the galaxies.

Now, this isn’t something you can see easily with an optical telescope. But, if you scan the sky with a radio telescope and trace out the emissions from hydrogen gas in the interacting galaxies, you find something interesting: a transfer of material from one galaxy to the other’s supermassive black hole. That’s what a Cheng-Yo Kuo, a graduate student at University of Virginia did, using the Very Large Array (VLA) radio telescope in Socorro, New Mexico. He looked at several sets of nearby Seyfert galaxies that appear to be snacking on each other. As the galaxies interact, the gas and dust goes goes from one to the other, on a sort of death spiral into the hungry maw of the black hole. The material don’t go quietly; it’s heated (ionized) by friction and radiation, and slapped around by twisted magnetic fields. Those actions produce energy as the material is consumed. Depending on how rapidly the black hole is eating up its neighbor’s contribution to the cosmic picnic, the galaxy can show a wide range of activity. The most action is at the center, which is the brightest (and quite “loud” in radio wavelengths). Seyfert galaxies have the mildest version of this activity, while quasars and blazars are hundreds of times more powerful.

Astronomers can trace the flow of material between galaxies by looking for neutral hydrogen, which is pretty abundant in galaxies. It gives off emissions at a wavelength of 21 centimeters, which radio telescopes can detect. It also traces out the route that material is taking between the two galaxies into the heart of the black hole. That’s what this image from the NRAO press release illustrates, showing what a good tool neutral hydrogen can be when it comes to diagnosing the inevitable “heartburn” that follows when two galaxies interact and feed a supermassive black hole.