A team of astrophysicists from the University of Copenhagen has made an important finding regarding the populations of stars beyond the Milky Way. The result could change our understanding of a wide range of astronomical phenomena, including how black holes form, supernovae and why galaxies die.
For as long as humans have been studying the sky, what stars in distant galaxies look like has remained a mystery. In a study published today in The Astrophysical JournalA team of researchers from the Niels Bohr Institute at the University of Copenhagen is ending what we know about stars beyond our own galaxy.
Since 1955, the composition of stars in other galaxies in the Universe has been thought to be similar to hundreds of billions of stars in our own—a mix of high-mass, intermediate-mass, and low-mass stars. But using observations of 140,000 galaxies across the Universe and a wide range of advanced models, the team tested whether the same apparent stellar distribution in the Milky Way holds elsewhere. The answer is no. Stars in distant galaxies are generally more massive than those in our “local neighborhood”. The discovery has a major impact on what we think we know about the universe.
“The mass of the stars tells us astronomers a lot. If you change the mass, you also change the number of supernovae and black holes that form from massive stars. In this respect, our result means that we have to revise many things. We assumed earlier because distant galaxies look very different from ours,” says Albert Sneppen, PhD student at the Niels Bohr Institute and first author of the study.
Analysis of the light from 140,000 galaxies
The researchers assumed that the size and weight of the stars of other galaxies were similar to ours for the simple reason that they could not observe them through a telescope like the stars of our own galaxy.
Distant galaxies are billions of light years away. As a result, only the light from their strongest stars reaches Earth. This has puzzled researchers around the world for years, as they have never been able to precisely clarify the distribution of stars in other galaxies, an uncertainty that has forced them to assume they were distributed much like the stars in our Milky Way.
“We’ve only seen the tip of the iceberg, and we’ve known for a long time that expecting other galaxies to look like ours isn’t a particularly good guess. However, nobody has been able to prove that other galaxies form other stellar populations. This study enabled us to do exactly what could open the door to a deeper understanding of how galaxies form and evolve,” says Associate Professor Charles Steinhardt, co-author of the study.
In the study, researchers analyzed light from 140,000 galaxies using the COSMOS catalogue, a large international database of more than a million light observations from other galaxies. These galaxies are distributed from the nearest to the farthest reaches of the Universe, from where light traveled 12 billion years before it was observed on Earth.
Massive galaxies die first
According to the researchers, the new discovery will have a variety of implications. For example, we still don’t know why galaxies die and stop forming new stars. The new result suggests that this could be explained by a simple trend.
“Now that we can better decipher the mass of stars, we can see a new pattern; less massive galaxies continue to form stars, while more massive galaxies stop giving birth to new stars. This indicates a remarkably universal trend in galactic deaths,” concludes Albert Sneppen.
The research was carried out at the Cosmic Dawn Center (DAWN), an international center for basic astronomical research supported by the Danish National Research Foundation. DAWN is a collaboration between the Niels Bohr Institute at the University of Copenhagen and DTU Space at the Technical University of Denmark.
The center is dedicated to understanding when and how the first galaxies, stars and black holes formed and evolved in the early Universe, through observations with the largest telescopes, as well as theoretical work and simulations.
About the study
- The empirical function used to describe the mass distribution of a stellar population is known as the IMF – Initial Mass Function. It includes a distribution of low-mass, medium-mass, and high-mass stars that astronomers have observed throughout the Milky Way. In the past, researchers have assumed that the IMF is universal and also applies to other galaxies in the universe.
- In their analysis of the galaxies, the researchers looked at the amount of light that galaxies emit at different wavelengths. Large, massive stars are bluish, while small, low-mass stars are more yellow or red. This means that by comparing the distribution of blue and red in a galaxy, one can measure the distribution of large and small stars.
- Researchers have taken a closer look at 140,000 galaxies scattered across the universe over the last 12 billion years of the universe’s history.
- The results show that stars in distant galaxies are generally more massive than those in our local neighborhood, and that the further away researchers look, the more massive average stars become.
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