Remember those “impossibly massive galaxies” James Webb spotted recently? They may be even heavier than expected!

The issue is becoming increasingly puzzling. Because now the stellar mass of these strange galaxies appears to be another ten times (!) greater.

If you’ve been following the news about the James Webb telescope for a bit, you’ve probably heard of the ‘impossible discovery’. A few months ago, astronomers found six galaxies that were so massive very early in the universe that they should not exist according to current cosmological theories. They are simply way too heavy. And now the mystery gets even more puzzling. Because while astronomers are already stunned, a new analysis suggests that the size of these “impossibly massive galaxies” may also have been underestimated.

Six galaxies
Back to the beginning. Since James Webb’s first images were released, astronomers have been able to study increasingly distant galaxies. Disturbingly, several galaxies appeared “too massive.” Last February, for example, researchers came across six galaxies that existed some 500 to 700 million years after the Big Bang—or some 13 billion years ago. Researchers had expected to find only small, young baby galaxies at that point. But instead they found galaxies that look as mature as our own. And that in ‘the dawn’ of the universe! According to the currently accepted ΛCDM model (see box), these galaxies simply should not have had time to form so many stars. The discovery therefore defies current ideas about the origin of galaxies and our understanding of the universe.

More about the ΛCDM model
The ΛCDM model – pronounced Lambda-CDM – is currently the best model we have for describing the structure and evolution of our universe. The model is based on one of the best tested theories in physics, namely general relativity, which describes how matter affects space and how space affects matter. In this model, the universe is assumed to be composed primarily of an unknown substance known as dark energy—denoted by the Greek letter Λ—and “cold,” dark matter—CDM—where “cold” means it doesn’t move fast. Thanks to the ΛCDM model, astronomers have been able to successfully describe and predict a variety of phenomena. However, we still don’t know what dark matter and dark energy is. And we also know that general relativity, despite its success, is not a complete theory. Researchers therefore expect that ΛCDM model will eventually be extended or replaced by a better theory.

At present, the stellar mass of a galaxy is often estimated by measuring the amount of light emitted by the galaxy. Then researchers calculate how many stars it takes to emit this amount of light.

New analysis
But in a new analysis, researcher Clara Giménez Arteaga took a different tack. In her study, published in Astrophysical Journal, she studied five galaxies previously observed by James Webb. And instead of seeing each galaxy as one big ‘blob’ of light (as the usual method dictates), she saw each galaxy as a cluster of several clumps of light. “We calculated the masses of stars based on the images that James Webb took,” explains Giménez Arteaga. “But we didn’t do this by looking at the whole galaxy, but by studying every single pixel in the images.”

This image shows the galaxy cluster SMACS 072 and its surroundings. It was the first image released by the James Webb telescope. The photo shows the five galaxies studied in this study. We see these galaxies as they looked about 13 billion years ago. Image: NASA, ESA, CSA, STScI / Giménez-Arteaga et al. (2023), Peter Laursen (Cosmic Dawn Center

Pixel by pixel
Basically, to determine the total stellar mass, Giménez Arteaga calculated the mass of each pixel and then added up all the individual stellar masses. In principle, you would expect the results to be the same, clockwise or anticlockwise. But they are not. In fact, the derived stellar masses of the five studied galaxies now appear to be as much as 10 times (!) greater.

This figure shows the five galaxies with their stellar masses determined using the ‘usual’ method (blue numbers) and the new pixel-by-pixel method (red numbers). If the two different approaches agreed, all the galaxies would lie along the oblique line called “the same.” But as you can see, they are all above this line. Image: Cosmic Dawn Center

It means the “impossibly massive galaxies” spotted by James Webb are now even more massive than previously thought. It makes the issue a bit more complicated than it already was. But what exactly is the remarkable difference?

Giménez Arteaga thinks he can explain the mystery. “Stellar populations consist of small and faint stars on the one hand and bright, massive stars on the other,” she explains. “If we only look at the combined emitted light, the fainter stars disappear completely in the emitted light of the brighter stars, leaving them unnoticed. Our analysis shows that the clear, star-forming clumps are thus the most dominant. But it is precisely the small stars that take up most of the total mass.”

The stellar mass is one of the most important properties used to characterize a galaxy. But apparently we still don’t know how to calculate it for the most distant and fainter galaxies. However, Giménez Arteaga does not give up. “Other studies that have studied galaxies from later eras have also found this discrepancy,” she says. “If we can determine how big the difference is in these earlier eras, and then quantify it, we will hopefully be able to better calculate the stellar mass of distant galaxies. And that is one of the main challenges in studying galaxies in the early universe.”

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