A galaxy can hide a large part of its mass in nearly invisible stars. This is demonstrated by a study conducted using James Webb and the Very Large Telescope.
When a galaxy is billions of light-years away, it is impossible to count its stars one by one. Astronomers must estimate its mass from the overall light reaching us. But this light mainly comes from the brightest stars.
An excerpt from the JADES survey obtained by James Webb’s NIRCam camera shows numerous distant galaxies.
Credit: JADES collaboration
Small stars therefore pose a problem. They shine faintly, but can be extremely numerous. Taken together, they represent a significant amount of matter without necessarily making the galaxy much brighter.
To move from light to mass, astronomers therefore assume a certain distribution between small and large stars. This proportion is generally based on that observed in the Milky Way. But there is no guarantee that it is identical everywhere, especially in the young Universe.
An international team sought to verify this in nine massive galaxies observed as they were several billion years ago. All had already stopped forming many new stars. This relative calm makes it easier to study older stellar populations.
The researchers combined highly detailed spectra obtained with James Webb and the Very Large Telescope in Chile. A spectrum breaks down a galaxy’s light according to its different wavelengths. Some small variations indirectly reveal which types of stars contribute to this light.
The result is quite clear for some galaxies in the sample: they appear to contain more small stars than expected. Their mass may therefore be underestimated when calculations use a distribution similar to that of the Milky Way.
The oldest galaxy studied is attracting particular attention. Its stars would have formed less than 1.5 billion years after the Big Bang. It could be a descendant of even earlier galaxies discovered by James Webb.
These very young galaxies have already surprised astronomers. Some appear to have assembled enormous amounts of matter in a very short time, more rapidly than several galaxy-formation models predict.
If these early galaxies also contained many more small stars, their actual mass would be even greater than current estimates. The researchers estimate this increase at about a factor of four.
The researchers now want to apply this method to galaxies formed even earlier. The goal will be to determine whether this abundance of small stars was common in the young Universe or specific to certain systems.