Astrophysicists explain the ‘impossible’ brightness of the cosmic dawn

It is intense flashes of light, not mass, that solve the impossible brightness puzzle.

When scientists looked at James Webb Space TelescopeJWST’s first images of the universe’s first galaxies shocked them. The young galaxies seemed too bright, too massive, and too mature to have formed so soon after the Big Bang. It will be like a baby growing into an adult in just two years.

An amazing discovery even This led some physicists to question the standard model of cosmologywondering whether it should be flipped or not.

Galaxy brightness versus mass

Using a new simulation, a Northwestern UniversityA team of astrophysicists has now discovered that these galaxies are probably not very massive after all. Although a galaxy’s brightness is usually determined by its mass, new results suggest that less massive galaxies can glow just as brightly from irregular, brilliant bursts of star formation.

This discovery not only explains why small galaxies appear so deceptively small, but also fits within the standard model of cosmology.

The research was published on October 3 in Astrophysical Journal Letters.

Artist’s rendering of early stellar galaxies. The image is provided by the FIRE simulation data used in this research which could explain the recent JWST results. Stars and galaxies appear as bright white points of light, while dark matter and more diffuse gases appear in purple and red. Source: Aaron M. Geller, Northwestern, CIERA+IT-RCDS

“The discovery of these galaxies was a big surprise because they were much brighter than expected,” said Claude-André Foucher-Géguier, the study’s senior author from Northwestern University. “Normally a galaxy is bright because it is large. But because these galaxies formed at cosmic dawn, not enough time has passed since then.” the great explosion. How can these massive galaxies assemble so quickly? Our simulations show that galaxies have no problem shaping this brightness with the cosmic dawn.

“The key is to reproduce a sufficient amount of light in the system within a short period of time,” added Zhao Zhao Sun, who led the study. “This could happen either because the system is really massive or because it has the ability to produce a lot of light quickly. In the latter case, the system does not need to be that massive. If star formation occurs in bursts, it will emit flashes of light.” That’s why we see so many very bright galaxies.

Faucher Giguere is an associate professor of physics and astronomy at Northwestern University Weinberg College of Arts and Sciences And a member of Interdisciplinary Exploration and Research Center in Astrophysics (Walk). Sun is a CIERA postdoctoral fellow at Northwestern University.

Understanding the cosmic dawn

A period lasting from approximately 100 million years to a billion years after the Big Bang, the cosmic dawn is characterized by the formation of the first stars and galaxies in the universe. Before the James Webb Space Telescope was launched into space, astronomers knew very little about this ancient time period.

“The James Webb Space Telescope has brought us a lot of knowledge about the cosmic dawn,” Sun said. “Before the James Webb Space Telescope, most of our knowledge about the early universe was speculation based on data from very few sources. With the huge increase in observing power, we can see physical details about galaxies and use powerful observational evidence to study physics to understand what is happening.”

Advanced simulation and results

In the new study, Sun, Foucher-Géger, and their team used advanced computer simulations to model how galaxies formed immediately after the Big Bang. The simulations produced cosmic dawn galaxies that were just as bright as those observed by the James Webb Space Telescope. Simulations are part of Feedback from relative environments The FIRE project, which Faucher-Géger co-founded with collaborators at Caltech, Princeton University, and University of California, San Diego. The new study includes collaborators from the Flatiron Institute’s Center for Computational Astrophysics, the Massachusetts Institute of Technology, and the University of California, Davis.

FIRE simulations combine astrophysical theory with advanced algorithms to model galaxy formation. These models enable researchers to explore how galaxies form, grow and change shape, taking into account the energy, mass, momentum and chemical elements returning from the stars.

When Sun, Fouché-Géger and their team ran simulations to model early galaxies that formed at cosmic dawn, they discovered that stars formed in explosions — a concept known as “explosive star formation.” In massive galaxies such as milky wayStars form at a constant rate, with the number of stars gradually increasing over time. But so-called explosive star formation occurs when stars form in an alternating pattern — many stars at once, followed by millions of years of very few new stars, and then many stars again.

“Explosive star formation is particularly common in low-mass galaxies,” Faucher-Géger said. “The details of why this happens are still the subject of ongoing research. But what we think happens is that an explosion of stars forms, and then a few million years later, those stars explode as supernovas. The gas is expelled and then returns to form new stars, leading to Star formation cycle. But when galaxies become massive enough, they have much stronger gravity. When supernovae explode, it is not strong enough to force gas out of the system. Gravity holds the galaxy together and puts it in a stable state.

Bright galaxies and the universe model

The simulations were also able to produce the same abundance of bright galaxies detected by the James Webb Space Telescope. In other words, the number of bright galaxies predicted by the simulations matches the number of bright galaxies observed.

Although other astrophysicists have hypothesized that exploding star formation could be responsible for the unusual brightness of galaxies at cosmic dawn, researchers at Northwestern University are the first to use detailed computer simulations to prove that it is possible. They were able to do this without adding new factors that do not fit our standard model of the universe.

“Most of the light in the galaxy comes from the most massive stars,” Faucher-Géger said. “Because more massive stars burn at a higher speed, they are shorter-lived. They use up their fuel quickly in nuclear reactions. Therefore, the brightness of a galaxy is more directly related to the number of stars that have formed in the last few million years than to the mass of the galaxy as a whole.”

Reference: “Explosive star formation naturally explains the abundance of bright galaxies at cosmic dawn” by Juchao Sun, Claude Andre Faucher-Géguier, and Christopher C. Hayward, Xiujian Chen, Andrew Wetzel, and Rachel K. Cochrane, 3 October 2023, Available here. Astrophysical Journal Letters.
doi: 10.3847/2041-8213/acf85a

The study was previously supported NASA And the National Science Foundation.