Three years ago, the James Webb Space Telescope detected "little red dots". Their existence is only ephemeral, as they appear less than a billion years after the Big Bang and almost completely disappear after two billion years.
To explain their particularly intense brightness, researchers put forward two main ideas: exceptionally star-rich galaxies or supermassive black holes. However, these two scenarios seemed improbable because they would imply the presence of objects too massive to form so early in cosmic history.
Six James Webb images showing 'Little Red Dots' in the early Universe.
Credit: NASA, ESA, CSA, STScI, D. Kocevski (Colby College)
To settle the matter, a team analyzed twelve of these objects, the youngest of which existed about 840 million years after the Big Bang. Their work, published in
Nature, provides new elements to clarify this question.
The results show that these bright sources are equivalent to more than 250 billion suns, but measure less than a third of a light-year in diameter. Such compactness rules out the hypothesis of a concentration of stars.
The spectrum of the emitted light indicates that it is scattered by electrons in dense clouds of ionized gas at the center of these dots. It turns out that this type of envelope captures most of the radiation produced near the black holes, masking the usual signals like X-rays or radio waves.
The measurement of the gas speed, estimated at about 671,000 miles per hour (1.08 million km/h), allows scientists to deduce that the objects are certainly black holes with masses between 100,000 and 10 million times that of the Sun. These values correspond to what is expected from young black holes.
This advance paves the way for a better understanding of the birth of supermassive black holes, whether through gradual growth or direct formation.
Ionized gas clouds in astronomy
In space, ionized gas is a plasma where atoms have lost or gained electrons, becoming electrically charged. These regions are common around hot stars or energetic objects like black holes.
These clouds can influence how light reaches us. For example, they scatter radiation, altering its path and intensity. This sometimes creates masking effects, hiding certain characteristic emissions from astrophysical objects.
In the case of the little red dots, the ionized gas clouds act like cocoons that trap the light emitted near the black holes. This process explains why the usual signals, such as X-rays, are not detected, making these objects difficult to identify.
The study of these clouds helps astronomers interpret observations and better understand the extreme environments of the Universe. They play an important role in many cosmic phenomena.