by Astronomers exploring the distant universe with the James Webb Space Telescope, NASA’s most powerful telescope, have discovered a class of galaxies that challenge even the most skilled creatures in mimicry – like the fake octopus. This creature can mimic other marine animals to avoid predators. Need to be a flat? No problem. A sea serpent? Easy.
When astronomers analyzed the first Webb images of remote parts of the universe, they saw a group of galaxies that had never been seen before. These galaxies – some hundreds of them known as Little Red Dots – are very red and dense, and have only been visible during about 1 billion years of cosmic history. Like the octopus mimic, the astronomers Little Red Dots, because they resemble different astrophysical objects. They are either supermassive or moderately large galaxies, each of which has a supermassive black hole at its core.
However, one thing is certain. The typical Red Dot is small, with a radius of only 2% of the Milky Way galaxy. Some are even smaller.
As an astrophysicist who studies distant galaxies and black holes, I am interested in understanding the nature of these small galaxies. What powers do they have and what are they, really?
The imitation competition
Astronomers analyze the light our telescopes receive from distant galaxies to estimate their physical properties, such as the number of stars they contain. We can use the properties of light to study the Little Red Dots and determine whether they are made up of many stars or contain a black hole inside.
The wavelengths of light that reach our telescopes range from long radio waves to energetic gamma rays. Astronomers break the light down into different frequencies and visualize them with a chart, called a spectrum.
Sometimes, the spectrum contains emission lines, which are frequency ranges where more intense light emission occurs. In this case, we can use the shape of the spectrum to predict whether the galaxy has a supermassive black hole and estimate its mass.
Similarly, the presence of a supermassive black hole can be revealed by studying X-ray emission from a galaxy.
Being the ultimate masters of disguise, the Little Red Dots appear as different astrophysical objects, depending on whether astronomers choose to study them using X-rays, emission lines or something else.
The information astronomers have gathered so far from the spectrum of Red Dots and emission lines has led to two different models that explain their nature. These objects are either very dense galaxies containing billions of stars or contain a supermassive black hole.
Both hypotheses
In the stars alone hypothesis, the Little Red Dots contain massive amounts of stars – up to 100 billion stars. That’s about the same number of stars as the Milky Way – a much larger galaxy.
Imagine standing alone in a huge, empty room. This vast, silent space represents the region of the universe in the vicinity of our solar system where the stars are sparsely scattered. Now, picture that same room, but packed with the entire population of China.
This packed room feels like the heart of the Red Dots most intimately. These astrophysical objects may be the densest stellar environments in the entire universe. Astronomers are not even sure if such star systems can physically exist.
Then there is the black hole hypothesis. Most Red Dots show clear signs of having a supermassive black hole at their center. Astronomers can tell if a galaxy has a black hole by looking for large emission lines in its spectrum, created by gas around the black hole swirling at high speeds.
Astronomers consider these black holes too massive, compared to the size of their close host galaxies.
Black holes typically have a mass of about 0.1% of the mass of their host galaxy. But some of these Red Dots contain a black hole almost as massive as their entire galaxy. Astronomers call these supermassive black holes, because their lives exceed the normal ratio usually seen in galaxies.
There is another catch, however. Unlike normal black holes, those in the Little Red Dots probably show no sign of X-ray emission. Even in the deepest, high-energy images available, where astronomers should be able to easily see these black holes, there is no sign of them.
A few solutions and plenty of hope
So are these astrophysical curiosities massive galaxies with far too many stars? Or do they host supermassive black holes at their center that are too massive to emit enough X-rays? What their answer.
With more observations and theoretical modeling, astronomers are starting to come up with some possible solutions. The Little Red Dots may not be just stars, but these stars are so dense and compact that they mimic the emission lines typically seen from a black hole.
Or perhaps supermassive – even supermassive – black holes that lie at the cores of these Little Red Dots. If that is the case, two models can explain the lack of X-ray emission.
First, huge amounts of gas could float around the black hole, blocking some of the high-energy radiation emitted from the center of the black hole. Second, the black hole could be pulling in gas much faster than normal. This process would produce a different spectrum with fewer X-rays than what astronomers normally see.
The fact that black holes are too big, or too massive, for our understanding of the universe may not be a problem, but it is the best indication of how the first black holes in the universe were born. In fact, if the first black holes ever formed were very massive – about 100,000 times the mass of the Sun – theoretical models suggest that their ratio of black hole mass to host galaxy mass could remain high for a long time after after formation. .
So how can real-life astronomers detect these tiny specks of light that shine in the beginning of time? As with our master in disguise – the octopus – the secret is to observe their behavior.
Using the Webb telescope and more powerful X-ray telescopes to make additional observations will finally reveal a feature that astronomers can attribute to only one of the two scenarios.
For example, if astronomers could clearly detect X-ray or radio emission, or infrared light emitted from where the black hole is, they would know that the black hole hypothesis is the correct one.
As our sea friend can pretend to be a starfish, eventually it will move its tentacles and reveal its true nature.
This article is republished from The Conversation, a non-profit, independent news organization that brings you reliable facts and analysis to help you make sense of our complex world. It was written by: Fabio Pacucci, Smithsonian Institution
Read more:
Fabio Pacucci receives funding from NASA and SI and is on the AXIS Science Team.
