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Enlarge Image X-ray: NASA/CXC/Penn State/B. Luo et al

It looks like a bunch of slightly blurry, multicoloured fireflies.

The image may not seem impressive at first, but it certainly improves on more intimate acquaintance. It's actually a more than 1,000 black holes at the centres of galaxies, NASA's Chandra X-ray Observatory said last week when it released the image.

The black holes appear in a patch of sky around two-thirds the size of the full moon in the southern constellation of Fornax. A patch of sky the size of the full moon at this concentration would contain 5,000 black holes. The entire sky would hold a billion.

"With this one amazing picture, we can explore the earliest days of black holes in the universe and see how they change over billions of years," said study leader Niel Brandt, the Verne M. Willaman Professor of Astronomy and Astrophysics and professor of physics at Pennsylvania State University.

The colours represent X-rays. Red shows low-energy X-rays, green for medium-energy and blue for high-energy. These X-ray emissions come from over 2,000 galaxies between 11.9 billion and 12.9 billion light-years from Earth. Capturing the image took more than 7 million seconds of exposure time across 102 sessions, stacking the data and cross-referencing it against the Hubble Ultra Deep Field, which shows the same region.

The aim of this research is to study the formation of black holes in the early universe. This study, presented at the American Astronomical Society on Thursday and available to read on arXiv, reveals that early-universe black holes grew in short, powerful bursts rather than building slowly.

They also found that the "seeds" for these black holes are extremely massive, coming in at 10,000 to 100,000 times the mass of the sun. This could help solve the mystery of how black holes a billion times the mass of the sun can exist in the early universe. Previously, these supermassive black holes challenged the models for black hole growth. But if they start from a much larger seed, the growth rate doesn't need to be so steep.

Further research, using data from Chandra and future observations, will be required to solve this mystery once and for all.