Chandra Observatory spies densest-ever concentration of black holes

The universe is a big place, and we know from targeted observations that there are a lot of black holes out there. These collapsed husks of dead stars have such intense gravity that nothing escapes them, not even light. That makes it tricky to actually observe them, but NASA’s Chandra X-ray Observatory is uniquely suited to spot them. The Chandra team has just released a new image from a long-term study of the sky that shows the densest concentration of black holes ever seen. Every dot, smudge, and blur in this image is a black hole.

Of course, there’s nothing magical about the Chandra observatory — we still cannot see the singularity at the center of a black holes. They pull in (or at least deflect) electromagnetic waves like light and X-rays. However, the matter that’s spiraling into a black hole can still emit radiation into space as it collides and heats up (this is known as the accretion disk). Black holes are one of the most reliable x-ray radiation sources in the universe, so Chandra is perfect for spotting them. In the image, red dots are lower energy emissions and blue ones are higher energy. We can’t see x-rays, so the data has to be shifted down into the visual spectrum.

The newly released image represents a part of the Chandra Deep Field South Survey, which covers an area in the sky about two-thirds the size of the full moon as seen from Earth. Although, the edges of the image are less sensitive to x-rays, so they’ve been cropped out (you can see the high-res image on the Chandra site). There are still more than 5,000 objects visible here, each and every one is probably a black hole. It took over 7 million seconds of observation time (about 81 days total) to acquire the x-ray emissions that make up this image. Through stacking, the team was able to achieve the equivalent of 8 billion seconds of exposure (about 260 years).

Black hole spin

Chandra can detect black holes at various distances, and objects that are farther away are also seen as they were in the more distant past — it’s that pesky speed of light thing again. The smallest dots at the center of the image represent x-rays from black holes as they existed 12.5 billion years ago in the early eons of the universe.

Surveys like this are helpful in characterizing the nature of the early universe and how it has changed over time. The Chandra data shows that black holes in the early universe tended to grow in bursts rather than via a gradual accumulation of matter. Supermassive black holes (like those at the center of galaxies like ours) appear to start out with 1,000 to 100,000 solar masses. That means the growth curve isn’t as steep as if they began as more typical 100-solar-mass black holes. More observations are needed to nail down the specifics on black hole growth.

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