The universe is full of strange ‘little red dots’. A new study shines light on their real nature

Andrew Battisti, University of Western Australia, Australian National University

In 2023, the James Webb Space Telescope revealed something strange: the distant reaches of the universe are filled with “little red dots” that astronomers still can’t quite explain.

But a new paper I coauthored, published today in Nature Astronomy, might have found the key to understanding their nature.

It turns out these little red dots might actually be evidence of the earliest supermassive black holes. And they could hold clues to how stars and galaxies formed in the earliest years of the universe.

Discovering the unexpected

The James Webb Space Telescope is the most powerful telescope ever made and has enabled the discovery of things we had never imagined.

This is the case for little red dots. Their name is a simple derivation of their appearance: they appear as very small compact dots that are “red” in colour (noting the James Webb Space Telescope sees in the infrared).

These objects are extremely distant, and are seen at an epoch when the universe is only around one billion years old (based on the time it takes the light to reach us).

Their compact size and extreme distance mean that most of their light comes from an area smaller than 2% of the size of our Milky Way galaxy.

Deciphering the dots

Despite finding many of these distant objects, astronomers are still unclear what they might be. The current favoured theory is that most little red dots are young supermassive black holes, with some consuming matter at such a feverish rate that their “surface” acts like the surface of a star (termed “black hole stars”).

Another theory is that some are regions of extreme star formation that form the most massive galaxies in the universe.

To help unravel the mystery of little red dots, astronomers have been looking to the host galaxies they reside in for answers, aiming to understand how galaxies and supermassive black holes grow in tandem with each other.

Our new paper, led by Yiyang Zhang from Wuhan University in China, provides key insights astronomers have been looking for.

Analysing more than 200 little red dots

We used high-definition James Webb Space Telescope imaging of the distant universe over a region of space about three times the size of the Moon, known as the COSMOS-Web region (which you can view online).

More than 400 little red dots are found in this region, a large enough number that astronomers can now unveil the secrets of the galaxies they reside in.

Measuring the size of little red dot host galaxies is not a simple task.

Imagine trying to see the colour details in a painting with a flashlight at its centre that shines bright, red light directly into your eyes. This work uses precise modelling of the optics of the James Webb Space Telescope to remove the bright central light in each image.

The technique is essentially like blocking this flashlight to see the painting clearly. The team combined 217 of these images to see the light from the faint host galaxy hiding within.

The results indicate that at the red wavelengths measured by the telescope, the host galaxies only account for about 10% of the total light of little red dots, suggesting the central region is far outshining the galaxy itself.

The host galaxies are also very small, only 40% of the size of typical galaxies at this early stage in the universe (or about 4% of the size of our Milky Way today). This suggests that little red dots can only form under very specific conditions that are atypical for most galaxies.

Measuring the number of stars

The most abundant stars that form are smaller in mass and size than our Sun and have a cooler surface temperature.

Cooler objects emit redder light than hotter objects, just as a red flame is cooler than a blue flame.

As a result, the most abundant stars emit most of their light in red optical and near-infrared wavelengths. Using this, astronomers can determine the total number of stars in a galaxy by measuring the amount of red optical and near-infrared light they emit.

This is a challenge for little red dots because the supermassive black holes in their centres also emit a lot of light at these wavelengths.

By removing this central light, this study found that the host galaxies of dots contain around 1 billion stars (just 4% as many stars as our Milky Way).

There is still considerable uncertainty about what is the mass of the supermassive black holes in little red dots, but studies suggest they may range between 10 million and 1 billion times the mass of our Sun. This implies these supermassive black holes grow most of their mass before the galaxy forms most of its stars.

This could be a key to understanding how supermassive black holes and galaxies co-evolve.

While this research has begun to unveil the nature of little red dots and their host galaxies, the exact manner by which they form and evolve remains a mystery.

This article is republished from The Conversation under a Creative Commons license. Read the original article.