In the center of a typical galaxy lies a “supermassive” black hole, a beast several millions of times more massive than our own Sun. While light that is fully engulfed by one of these monsters will never escape, hence the name “black hole”, the surrounding neighborhood can in some cases be dramatically bright. 

In these so-called “active” black holes, infalling gas that is destined to be consumed swirls about the black hole in a rapidly spinning disk, heating to such hot temperatures that it gives off high energy X-rays. Fast-moving gas clouds less than a lightyear from the black hole are illuminated by the light from this disk.

These nearby gas clouds then send out their own glow at wavelengths indicative of elements like Hydrogen, orbiting at breath-taking speeds of 1000s of kilometers per second. In more distant, tenuous, slower-moving gas clouds, collisions between electrons and atomic nuclei lead to light at wavelengths characteristic of Oxygen, Nitrogen, and Sulfur.  In short, the presence of a hungry, supermassive black hole paradoxically leads to a dramatic amount of light spilling outward into the universe and eventually to our waiting telescopes here on Earth.

Now, of course, this light does not arrive at our telescopes immediately.  While the speed of light—about 300,000 kilometers per second (or 186,000 miles per second)—may seem impossibly fast to a human observer, it is not infinitely fast. This means that it always takes some amount of time for light to travel from one region of the universe to another; the more distance the light has to travel, the more of a wait there will be. This simple fact has powerful implications when mapped out onto the vastness of outer space.  

First and foremost, the very fast but finite speed of light means that the farther away we look in the Universe, the further back in time we can see. In that sense, doing astronomy is like having a time machine, where all we have to do is sit back and wait for the light to reach us from the most distant galaxies and the earliest chapters of the Universe’s history. In addition, however, light’s finite speed can also give us a way to weigh black holes in the centers of galaxies, and even peer back in time at how actively and voraciously they were feeding in the past. 

To do all this, astronomers have spent decades carefully watching the light output of active supermassive black holes, leading to powerful new insights into the inner workings of these mysterious beasts.  It turns out that the disks around active black holes not only emit copious amounts of light, they also flicker, sending out varying amounts of light over timescales as short as days or even hours. Nearby glowing gas clouds that are lit up by such a flickering disk will flicker as well, but because they are some distance away, their flickering will be slightly delayed in time, perhaps by days to months.  In a technique called “reverberation mapping”, astronomers record the variations in brightness coming from the black hole’s disk as well as their echoes in the nearby glowing gas clouds. 

The amount of delay is a direct measure of how long it takes for light to travel from one to the other, and therefore, how far away the gas clouds are from the black hole. Combining this distance with measurements of how fast these same gas clouds are moving leads to an estimate of the mass of the black hole itself.

Active supermassive black holes impact not only their local neighborhood but in some cases also reach far outside their own home galaxy. Gas that surrounds a galaxy, some 10,000-100,000 lightyears away, can be lit up by the copious radiation from the supermassive black hole, leading to enormous glowing nebulae.

The huge sizes of these gaseous nebulae means that it takes a significant amount of time for light leaving the supermassive black hole to reach their most distant outskirts. 

In the meantime, if the black hole activity calms down and its powerful radiation fades away, the surrounding nebula will be left behind as a glowing echo, tracing what the black hole was up to, and how voraciously it was feeding, some 10,000-100,000 years in the past. Thus, astronomers have discovered that active supermassive black holes not only flicker over hours, days, or months, they can also finish their cosmic meal and fade away over timescales approaching the roughly 200,000 year timespan of modern human history.

Moire Prescott is an associate professor of Astronomy at New Mexico State University. She can be reached at [email protected].