The Webb Discovers a Rich Population of Brown Dwarfs Outside the Milky Way

This stunning image of a star cluster in the Small Magellanic Cloud (SMC) is more than just a pretty picture. It’s part of a scientific effort to understand star formation in an environment different from ours. The young star cluster is called NGC 602, and it’s very young, only about 2 or 3 million years old.

This image lives up to the standard the JWST has set. NGC 602 is inside a nebula of multi-coloured gas and dust. The many energetic stars in the cluster light the nebula up from within, while its outer edges are dark. The cluster is rich in ionized gas, which indicates that star formation is still taking place.

The cluster is different from our region of space. It’s a low-density environment and has lower metallicity than our region. Metallicity affects the heating and cooling of gas, and in general, the more metals there are, the more they absorb heat, keeping the star-forming gas cooler. Since stars form from cooler gas, metallicity is expected to enhance star formation.

But there are many questions, including how brown dwarfs fit into this scenario. Do they form like other stars do, from the collapse of giant molecular clouds? Or do they form like planets from the fragmentation of circumstellar disks?

New research in The Astrophysical Journal examined NGC 602 with the JWST and reported the first detection of a brown dwarf population outside the Milky Way. It’s titled “Discovering Subsolar Metallicity Brown Dwarf Candidates in the Small Magellanic Cloud.” The lead author is Peter Zeidler of AURA/STScI for the European Space Agency.

Brown dwarfs are sometimes called planetars or hyperjovians because they’re more massive than planets but not massive enough to be stars. They’re also often called sub-stellar mass objects. For some reason, during formation, they fail to attract enough mass to trigger fusion and become full-blown stars. Identifying them in a low-metallicity environment is a chance to understand brown dwarfs and star formation in general in a different environment.

“Only thanks to the incredible sensitivity and resolution in the right wavelength range we are able to detect these objects at such great distances,” shared lead author Zeidler. “This has never been possible before and also will remain impossible with telescopes on the ground for the foreseeable future.”

“Until now, we’ve known of about 3000 brown dwarfs, but they all live inside our own galaxy,” added team member Elena Manjavacas of AURA/STScI for the European Space Agency.

The Hubble space telescope played a role in this work, and it’s not the first time the pair of space telescopes have created valuable scientific synergy by working together.

“This discovery highlights the power of using both Hubble and Webb to study young stellar clusters,” explained team member Antonella Nota, executive director of the International Space Science Institute in Switzerland and the previous Webb Project Scientist for ESA. “Hubble showed that NGC602 harbours very young low-mass stars, but only with Webb can we finally see the extent and the significance of the substellar mass formation in this cluster. Hubble and Webb are an amazingly powerful telescope duo!”

The researchers found 64 brown dwarf candidates in the cluster. They ranged from 0.05 to 0.08 solar masses (50-84 Jupiter masses) and are co-located with main sequence stars. The low stellar density in the cluster helped the JWST resolve individual objects. The observations are important for studying the sub-solar mass function at low metallicities.

The concept of the Initial Mass Function (IMF) is central to star formation theory. It’s like a recipe that tells us how many stars of different masses will form in a star-forming region. The IMF usually follows a power law, meaning that more low-mass stars will form than higher-mass stars. It generally features a broad peak centred at the mass of the mean mass star.

Usually, stars lower than one stellar mass make up about 70% of the initial mass budget in a region. But even small deviations in the mean mass can have large effects on the evolution of a star cluster. Stellar radiation from young stars can affect the mean mass by raising the temperature of the star-form gas. There’s some evidence that the mean mass shifts to higher masses when the initial temperature is higher.

The data from this work shows that the low-mass objects in NGC 602 are well below the characteristic mass. The brown dwarfs have masses between 0.048 and 0.08 solar masses or 50 and 84 Jupiter masses. Since these brown dwarfs are co-located with the cluster’s young pre-Main Sequence Stars, it suggests they formed synchronously. This indicates that the stellar mass function continues into the substellar mass regime.

Unlike other similar research, the team was able to accurately measure the ages of the brown dwarfs. Typically, it’s difficult to study the IMF below the hydrogen-burning limit because objects without fusion are constantly cooling down. That makes it difficult for astronomers to estimate an object’s mass because the effective temperature keeps changing.

But by finding these brown dwarfs co-located with hydrogen-burning stars, Zeidler and his co-researchers found a way around the problem. It shows that the brown dwarfs are roughly the same age as the stars. That means the brown dwarfs and the main sequence stars all provide insight into the IMF and the sub-stellar IMF.

This first study is just their first step, and they intend on digging deeper.

“The accurate selection of ages, together with the superb sensitivity and calibration of JWST, will allow us, in a forthcoming paper, to reliably study the substellar mass function, well below the turnover of the IMF,” the authors write.

It’s all aimed at understanding how brown dwarfs form. If they can study the sub-stellar IMF in detail, they can determine whether it’s a continuation of the stellar IMF. Then, the researchers can answer an important unanswered question: do these objects form from the fragmentation and collapse of giant molecular clouds like stars do? Or do they form from the fragmentation of circumstellar disks like planets do?

As of now, they have only a partial answer.

“From this work, the colocation with the PMS suggests that the formation channel of the cBDs is the same as the one for their more massive stellar counterparts, as expected from solar neighbourhood studies: the fragmentation and collapse of the GMC,” the authors conclude.