When you look at the sky, how do you determine if the specks of light that you admire are bright and far away or fainter but closer?
One way to determine that is by analyzing the quantity of light an object emits regarding how bright it appears.
The difference between apparent brightness and actual luminosity helps determine an object’s exact distance from the observer.
Determining a celestial object’s luminosity is tricky, especially in the case of black holes, as they don’t emit light.
However, some supermassive black holes that lie in the middle of most galaxies have a loophole:
They tend to pull vast amounts of matter around them, forming hot disks that typically radiate brightly.
Measuring a bright disk’s luminosity would help astronomers determine the distance to the black hole and its host galaxy. Distance measurements also help scientists build a 3D map of the universe; they can again return valuable information about how and when some objects formed.
In a recent study, astronomers used a technique called “echo mapping” to determine the luminosity of black hole disks in more than 500 galaxies. The study was published last month in the Astrophysical Journal.
The study is proof that echo mapping might be the next best way to measure distances between Earth and faraway galaxies.
How Does It Happen?
Echo mapping, also known as reverberation mapping, commences when the disk of hot plasma (atoms that lost electrons) next to the hole increases in brightness, at times even releasing short flares of visible light (wavelengths visible to the naked eye).
The light runs into an enormous cloud of dust in the shape of a doughnut around the black hole.
The torus brightens up at some point, in direct response (or “echo) to the changes that took place in the disk.