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‘Echo mapping’ in faraway galaxies could measure vast cosmic distances

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'Echo mapping' in faraway galaxies could measure vast cosmic distances
A disk of hot material around a supermassive black hole emits a burst of visible light, which travels out to a ring of dust that subsequently emits infrared light. The blue arrows show the light from the disk moving toward the dust and the light from both events traveling toward an observer. Credit: NASA/JPL-Caltech

When you look up at the night sky, how do you know whether the specks of light that you see are bright and far away, or relatively faint and close by? One way to find out is to compare how much light the object actually emits with how bright it appears. The difference between its true luminosity and its apparent brightness reveals an object’s distance from the observer.


Measuring the luminosity of a celestial object is challenging, especially with black holes, which don’t emit light. But the supermassive black holes that lie at the center of most galaxies provide a loophole: They often pull lots of matter around them, forming hot disks that can radiate brightly. Measuring the luminosity of a bright disk would allow astronomers to gauge the distance to the black hole and the galaxy it lives in. Distance measurements not only help scientists create a better, three-dimensional map of the universe, they can also provide information about how and when objects formed.

In a new study, astronomers used a technique that some have nicknamed “echo mapping” to measure the luminosity of black hole disks in over 500 galaxies. Published last month in the Astrophysical Journal, the study adds support to the idea that this approach could be used to measure the distances between Earth and these faraway galaxies.

The process of echo mapping, also known as reverberation mapping, starts when the disk of hot plasma (atoms that have lost their electrons) close