New insights into galaxy growth and dark matter from James Webb

Over the last couple of years, the James Webb Space Telescope has constantly broken its own record in discovering the most distant, and hence earliest, galaxies. But while exploring the very first galaxies was indeed a prime science goal of Webb, the telescope originally set out three years ago with three additional goals:

As Webb aficionados may know, two of these goals are the investigation of exoplanets and newborn stars, respectively. The fourth, however, also deals with galaxies, namely their evolution through cosmic time.

Growing pains by gravity

Galaxies form and grow by gravity withstanding the expansion of the Universe, causing sufficiently dense clumps of gas to collapse and start forming stars. Small galaxies merge to form larger galaxies, and thus the magnificent structures of stars, black holes, planets and other wonders slowly build up. An important ingredient in this tale is the mysterious dark matter which helps speed up the process.

While this general narrative is more or less universally accepted, many details are still unclear. But a new study, led by researchers at the Cosmic Dawn Center (DAWN) at the Niels Bohr Institute and DTU Space in Copenhagen, has now brought us a large step closer to the understanding of these fascinating building blocks of the Universe:

Utilizing data from one of the largest James Webb programs conducted, “COSMOS-Web”, with support data from the Hubble Space Telescope as well as multiple ground-based telescopes, the astronomers surveyed a single large patch of the sky known as “the COSMOS field”. Collecting data from all these telescopes allowed them to observe over half a million galaxies at all distances. Because the light from more distant galaxies has traveled for a longer time, this effectively means that the galaxies are distributed not only through space, but also through time — in fact across most of the history of the Universe, all the way back to less than 400 million years after the Big Bang.

Marko Shuntov, postdoc at DAWN and leader of the study, explains: “By gauging how much light the galaxies emit in different wavelength regions, we can get a census of the mass of all their stars; a prime diagnostic of a galaxy. Although we can’t see how any individual galaxy grows, this tells us in a statistical sense how, and how fast, their stellar mass increases.”

Discords and concords

Perhaps not surprisingly, galaxies increase their stellar mass throughout their lives. Although some stars burn out, birth rates vastly outstrip death rates. But how they do it depends strongly on their mass, and with the new results, Shuntov and hos colleagues were able to quantify accurately this growth came about.

Interestingly, when they compare their results to older measurements of how efficient galaxies are at forming stars, they find a strong tension in how many stars the galaxies should have spawned at late epochs. If the old results are correct, galaxies should have ended up with much more stars than Shuntov and his collaborators find. The reason for this tension is poorly understood, but could indicate that earlier models need to be re-calibrated.

On the other hand, the new results are in remarkable agreement with more recent James Webb observations of galaxies in the very early Universe. This corroborates the emerging picture that galaxies formed more rapidly than previously thought, with an unexpected profusion of bright, massive galaxies shortly after the Big Bang.

The dark side of galaxies

While stars are the most prominent asset of a galaxy, by far their dominant component is dark matter. Typically there is ten times as much of this peculiar ingredient as “regular” matter — distributed in an extended “halo” around the galaxy itself — but this varies a lot from galaxy to galaxy. Understanding the exact relationship is another important way to specify galaxies, and indeed one of the prime goals of the COSMOS-Web survey is to link the dark matter to the visible matter.

But dark matter is, well, dark. How, then, can the astronomers see it?

“We can’t,” states Marko Shuntov. “But there are nevertheless many ways to infer its existence. In our study we compare our observations to detailed computer simulations that include both the stuff we can see — stars, gas, dust, etc. — and the stuff we don’t see, that is dark matter. This approach lets us deduce not only the amount of dark matter in galaxies of different sizes, but also how this fraction changes through time.”

In this way the astronomers paint an interesting picture of the galaxies’ evolution:

Although galaxies grow more massive with time, both in terms of stars and in terms of dark matter, the two ingredients do not increase at the same rate. Initially, dark matter halos grow faster than the stellar component, but after some two billion years, star formation catches up and outpaces halo growth.

Exactly why is still an open question, although the authors speculate that it is a consequence of the galaxies slowly accreting less matter from their surroundings, while still being able to form stars from previously accreted gas.

Luckily, the prospects of soon understanding the exact mechanism better are bright:

Firstly, future spectroscopic observations will provide more accurate distance to the galaxies. More importantly, however, Shuntov and his colleagues are currently undertaking a much larger survey — the so-called Cosmic Dawn Survey — with a multitude of telescopes, most notably the recently launched Euclid spacecraft.

“In the Cosmic Dawn Survey, we’re probing en enormous field, almost 40 times larger than the COSMOS-Web survey. The first data set is already acquired, and be released publicly within the next few months, so we hope to be able to soon understand better the fascinating co-evolution between galaxies and their dark matter halos,” Marko Shuntov concludes.

The study has just been published in the scientific journal Astronomy & Astrophysics.