Spectral analysis of nine massive galaxies using the James Webb Space Telescope and the Very Large Telescope suggests an excess of small, faint stars. If a similar population existed in their early universe ancestors, their masses could be up to four times larger than previously estimated—exacerbating tensions with models of galaxy formation.
galaxies Massive galaxies may contain many more small, faint stars than astronomers have previously assumed. The finding could also significantly increase estimates of the mass of their progenitors in the early universe, making it even more difficult for models to explain how large, mature galaxies formed so soon after the Big Bang.
The study, published in the journal Nature Astronomy, is based on extremely deep spectra of nine massive galaxies that have already stopped forming stars. The researchers combined observations from an instrument NIRSpec on the James Webb Space Telescope with shorter wavelength data collected using The very large telescope of the European Southern Observatory.
It is important to be precise: the galaxies examined are not themselves among the first galaxies in the universe. They were observed at a redshift of about 0.7, that is, as they appeared about six to seven billion years ago. However, the oldest stars in them formed much earlier. One of the galaxies contains a population of stars that probably formed at a redshift greater than five, when the universe was less than a billion and a half years old.
Light doesn't tell the whole story of matter.
When astronomers estimate the mass of stars in a distant galaxy, they can't count each star individually. Instead, they measure the total light and run models of stellar populations.
But the amount of light does not necessarily represent the majority of the mass. A relatively small number of large, bright stars can dominate the spectrum of a galaxy, while a vast population of small, faint stars contribute little light but a lot of mass.
Chloe Cheng of Leiden University, the lead author of the study, compared the situation to a city seen from afar: the skyscrapers immediately catch the eye, but many low-rise buildings are hidden among them. Similarly, the bright stars are clearly visible, while the smaller stars almost disappear in the combined light of the galaxy.
To estimate the number of faint stars, the researchers looked for subtle absorption signatures in the spectra that depend on the mass and surface gravity of the stars. The quality of Webb's observations, combined with ground-based data, allowed them to constrain the ratio of small to large stars in a way that had not been possible before in galaxies so distant.
A fundamental assumption in estimating the mass of galaxies
The distribution of the masses of stars at the time of their formation is called "The initial mass function", or IMF. In mass calculations of distant galaxies, it is generally assumed that this function is similar to that measured for the Milky Way.
This is a convenient assumption, but not necessarily a law of nature. The results of the study show that the most massive galaxies in the sample probably had a "bottom-heavy" mass function: they formed more small stars than large stars, compared to a typical population in the Milky Way.
low mass stars They live a long time and emit little light. Therefore, if their number is greater than estimated, the ratio between the galaxy's mass and light intensity increases – and accordingly, the estimate of the total mass trapped in stars also increases.
“These galaxies are different,” said Joel Legge of Penn State University, one of the study’s authors. He said that in some cases, the change in the mass function could lead to mass estimates that are three or four times larger than estimates based on a Milky Way-like distribution of stars.
The connection to Webb's "too large" galaxies
Since the James Webb Space Telescope began operating, bright, massive, and apparently mature galaxies have been discovered that existed only hundreds of millions of years after the Big Bang. Some of the initial estimates of their masses have been revised by spectroscopic measurements and a better understanding of their dust, gas, and stars, but there are still galaxies whose formation rates are difficult to explain within the framework of accepted models.
The new study does not directly measure the initial mass function of those first galaxies. The researchers suggest that the extinguished galaxies they examined are the descendants of massive systems that formed very early. If their ancestors also had an exceptionally rich population of small stars, the masses of the early galaxies could be about (4\pm1) times larger than estimates based on a mass function similar to that of the Milky Way.
Instead of solving the problem of galaxies that appear too big and too early, this possibility actually exacerbates it: models of Formation of galaxies They will have to explain how not only a population of bright stars, but also a huge mass of small, faint stars, was rapidly formed.
However, this is still an inference from later populations. To determine whether the mass function was also heavy at the bottom in the first galaxies, it would be necessary to obtain spectra of sufficient quality from even more distant and ancient galaxies.
More small stars – and perhaps more planetary systems
Mariska Crick of the Leiden Observatory, who led the study, noted that the change in the number of small stars may have implications beyond estimates of the mass of galaxies. Small, long-lived stars are common hosts for planets. So a larger population of them could also suggest that the number of planetary systems formed in the early universe was also greater than estimated.
This is only a possibility at this point, and not a direct result of the study. The observations do not detect planets in those galaxies, nor do they determine whether the conditions around the ancient stars allowed for planet formation similar to the one we are familiar with.
The researchers plan to apply the analysis to older galaxies. Such observations could determine whether the excess of small stars was a feature of extreme star-forming environments in the early universe, or a phenomenon that developed later within massive galaxies.
For the scientific article: Opening the scientific article
Questions and Answers
What did the researchers discover?
Analysis of the spectra of nine massive galaxies revealed an excess of small, faint stars compared to the usual distribution for the Milky Way.
How do faint stars affect the estimate of galaxy mass?
Small stars emit little light but contribute a lot of mass. So a larger than estimated number of them could increase the estimate of the stellar mass in galaxies by a factor of three or four.
Did researchers directly observe the first galaxies in the universe?
No. The galaxies examined were observed as they were about six to seven billion years ago. The connection to the older galaxies is based on the possibility that they are descendants of massive systems that formed early in the universe.
Does the finding solve the mystery of large galaxies in the early universe?
On the contrary. If those galaxies contained more small stars than estimated, their mass would be even greater—and it would be harder for models to explain how they formed so quickly.
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