Astronomers exploring the distant universe with NASA’s most powerful telescope, the James Webb Space Telescope, have found a class of galaxies that defy even the most adept at mimicking, such as the octopus. The creature can impersonate other marine animals to avoid predators. Need to be a flounder? No problem Sea Snake? light.

When astronomers analyzed Webb’s first images of the distant reaches of the universe, they noticed a group of galaxies that had never been seen before. A few hundred of these galaxies, called Little Red Spots, are very red and compact and have only been seen for about 1 billion years of cosmic history. Like the mimic octopus, Little Red Spots have puzzled astronomers because they resemble a variety of astrophysical objects. They are either massive galaxies or modestly sized galaxies, each with a supermassive black hole at their center.

But we can say one thing for sure. The typical Little Red Spot is small, with a radius of only about 2% of the Milky Way galaxy. As an astrophysicist who studies distant galaxies and black holes, I am interested in understanding the nature of these small galaxies. What feeds their light and what are they really like?

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 their light to study Little Red Spots and find out whether they are made up of many stars or contain a black hole.

The wavelengths of light reaching our telescopes range from long radio waves to energetic gamma rays. Astronomers divide the light into different frequencies and visualize them using a diagram 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 to estimate its mass.

Similarly, studying the X-ray radiation of a galaxy can reveal the presence of a supermassive black hole. True masters of disguise, Little Red Dots appear as different astrophysical objects, depending on whether astronomers choose to study them with X-rays, emission lines, or something else.

Information gathered by astronomers from the spectra and emission lines of Little Red Dots has led to two different models explaining their nature: These objects are either extremely dense galaxies containing billions of stars, or they contain a supermassive black hole.

Two hypotheses

According to the all-star hypothesis, the Little Red Dots contain as many as 100 billion stars. That’s about the same number of stars as the much larger Milky Way. Imagine standing alone in a vast, empty room. This vast, silent space represents the region of the universe near our solar system where stars are sparsely scattered. Now imagine the same room filled with the entire population of China.

This crowded room is what the core of the densest little red dot feels like. These astrophysical objects may be the densest stellar environment in the entire universe. Astronomers aren’t even sure if such star systems physically exist.

There is also the black hole hypothesis. Many of the small red dots show clear signs of a supermassive black hole at their center. Astronomers can tell if a galaxy has a black hole by looking for large emission lines in the spectra produced by the gas around the rapidly spinning black hole.

In fact, astronomers believe that these black holes are very large compared to the size of compact galaxies.

Typically, black holes have a mass of about 0.1% of the stellar mass of their host galaxy. But some of these Little Red Dots contain a black hole nearly as massive as the entire galaxy. Astronomers call these supermassive black holes because their existence defies the traditional correlations normally observed in galaxies.

But there’s another problem. Unlike regular black holes, the ones likely to be found in the Little Red Spot show no signs of X-ray emission. Even the deepest high-energy images available, where astronomers can easily observe these black holes, show no sign of them.

Little solution and much hope

The fact that black holes are very large or supermassive may not be a problem for our understanding of the universe; on the contrary, it may be the best evidence yet of how the first black holes in the universe were born. In fact, if the first black holes to form were very large (about 100,000 times the mass of the Sun), theoretical models suggest that the ratio of the black hole mass to the mass of the host galaxy could remain high for a long time after formation.

So how can astronomers discover the true nature of these tiny specks of light that shone at the dawn of time? Like our master of disguise, the octopus, the secret lies in observing their behavior.

Using the Webb telescope and more powerful X-ray telescopes to make additional observations will eventually reveal a feature that astronomers can attribute to only one of two scenarios. For example, if astronomers could clearly detect X-rays or radio emissions or infrared light from the vicinity of a location where a black hole might be, they would know the black hole hypothesis is correct.