Vast oceans of mysterious dark matter found in huge primordial galaxies

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Artist’s impression of a pair of galaxies from the very early universe. Image: NRAO/AUI/NSF/D Berry

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Our most powerful telescopes have searched deep within the most massive of galaxies to find vast oceans of mysterious dark matter.

Using the world’s largest radio telescope – the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile – a team of researchers has uncovered evidence that massive galaxies not only form incredibly quickly, but are oozing with mysterious dark matter.

In a paper published to Nature, researchers from the University of Arizona revealed how ALMA surprised the team with examples of massive, star-filled galaxies seen when the cosmos was less than 1bn years old.

Suggesting that small galactic building blocks could assemble themselves quite quickly, this discovery went against common conviction that the first galaxies formed after the Big Bang would share many similarities with some of the dwarf galaxies we see in the nearby universe today.

It was thought that these early blocks would become incorporated into larger galaxies over a few billion years.

The two galaxies observed by ALMA were something quite spectacular, the team said, as they were seen when the universe was only 780m years old. These cosmic behemoths are nestled inside an even larger cosmic structure, a halo of dark matter several trillion times bigger than the sun.

“With these exquisite ALMA observations, astronomers are seeing the most massive galaxy known in the first billion years of the universe in the process of assembling itself,” said Dan Marrone, lead author on the paper.

ALMA shot of galaxies

A composite image showing ALMA data (red) of the two galaxies of SPT0311-58. These galaxies are shown over a background from the Hubble Space Telescope (blue and green). Image: ALMA (ESO/NAOJ/NRAO), Marrone et al; B Saxton (NRAO/AUI/NSF); NASA/ESA Hubble

‘Whopping large’ quantity of cosmic dust

This assembling period occurred during a time in cosmic history known as the ‘epoch of reionisation’, when most of intergalactic space was suffused with an obscuring fog of cold hydrogen gas.

When more stars and galaxies formed, their combined energy ionised the hydrogen between galaxies, resulting in the universe we see around us today.

Using some sophisticated computer models to shed the astrophysical phenomenon of gravitational lensing, the team was able to see that the larger of the two galaxies is forming stars at a rate of 2,900 solar masses per year. On top of that, it also contains about 270bn times the mass of our sun in gas, and nearly 3bn times the mass of our sun in dust – a “whopping large quantity”, according to the team.

The observations also allowed the researchers to get a better glimpse of the elusive and mysterious dark matter halo surrounding both galaxies. Thankfully, our ability to measure it isn’t so unknown.

Chris Hayward, one of those involved in the research, said: “Fortunately, we know very well the ratio between dark matter and normal matter in the universe, so we can estimate what the dark matter halo mass must be.”

Colm Gorey is a journalist with