The growing pains of early galaxies

Simulations allow astronomers like Guochao Sun to study the early universe when the observational technologies aren’t yet available. One important property of the simulated early universe that caught the attention of Sun (Center for Interdisciplinary Exploration and Research in Astrophysics at Northwestern University) and his collaborators was the way luminous galaxies formed their stars in the early universe. For typical luminous galaxies in the present-day universe, like our Milky Way, star formation occurs at a fairly steady rate. Yet the simulations showed otherwise at z > 10, which corresponds to the universe’s first 500 million years.

Sun and his collaborators, including senior author Claude-André Faucher-Giguère, use the Feedback in Realistic Environments (FIRE) simulations to study the evolution of galaxies. FIRE is unique for the length of cosmic time that it covers given the resolution of physical details. FIRE allows astronomers to study not only galaxy formation just after the Big Bang but also the evolution of those early galaxies into what we see today. But a simulation isn’t the same as the real thing, and until the James Webb Space Telescope (JWST) launched, there were not enough observational data to compare against at z > 10.

Earlier this year, data from the telescope revealed an unexpectedly high abundance of UV-bright, luminous galaxies at redshift z > 10. Astronomers were initially puzzled by what they saw; so many luminous, young galaxies didn’t quite align with the predictions from galaxy-formation models. That potential violation is mainly based on the mass of the galaxies, however, which is nearly impossible to measure. To estimate galaxy mass, astronomers rely on the fact that the luminosity of a galaxy is primarily determined by its mass, creating a direct relationship between the two properties: An extremely luminous galaxy is understood to be massive. The overdensity of such seemingly massive galaxies in the early universe was puzzling with only the information available in the observational data.

The previous FIRE simulations provided the additional context and a likely answer: bursty star formation. A series of sporadic and extremely productive bursts of star formation would disrupt the simple scaling relationship between mass and luminosity. The mass–luminosity relationship assumes a slow and regular formation of stars, something that is more common in our present-day universe. A galaxy with a history of bursty star formation would appear brighter—due to an overabundance of short-lived luminous stars formed during a recent starburst—despite having less mass than an equally luminous galaxy with steady rates of star formation.

Bursty-star-formation histories had been proposed in response to the JWST observations, but Sun, Faucher-Giguère, and their collaborators are the first to demonstrate it in a simulation. That type of star-formation history is known to be common in low-mass galaxies, but astronomers are unsure why it occurs more often in certain types of galaxies. More work is in progress to better quantify the effects of different star-formation histories on galaxy statistics. (G. Sun et al., Astrophys. J. Lett. 955, L35, 2023 .)