Tracing Huge, Distant Structures in the Universe

Galaxies are arrayed along strands of dark matter. New research uses cosmological simulations to explore which galaxies are the best tracers of these dark-matter filaments.

Seeking the Invisible Trees

Trying to figure out the large-scale structure of the universe is a bit like trying to map a forest from within it — except most of the trees are invisible. The luminous galaxies that populate our universe tend to be arranged along strands of invisible dark matter: a hypothetical form of matter that appears to interact with normal matter only gravitationally. While we can readily spot the galactic beacons, the underlying structure is more difficult to determine.

Mapping out the large-scale dark-matter structure of the universe is critical for being able to test our theories of cosmology. What’s the best way to map the dark-matter distribution in our universe?

Simulating Structure

Sang Hyeok Im (Seoul National University) and collaborators investigated the dark-matter-tracing abilities of two types of galaxies: Lyman-alpha emitters and Lyman-break galaxies. These are high-redshift galaxies that can be identified using photometry rather than spectroscopy, requiring (relatively) little telescope time to map their positions.

To determine how well the locations of these galaxies match the hidden dark-matter distribution, Im’s team used the results of the Horizon Run 5 cosmological simulation, which gave them access to both the underlying dark-matter structure and the overlying galaxy distribution. They collected samples of Lyman-alpha emitters and Lyman-break galaxies at redshifts of z = 2.4, 3.1, and 4.5, corresponding to roughly 2.7, 2.1, and 1.3 billion years after the Big Bang, respectively. These are also the target redshifts of the One-hundred-deg² DECam Imaging in Narrow-bands (ODIN) survey, an ongoing effort to map the large-scale structure of the universe using Lyman-alpha emitters.

Promising Probes

Using a structure-finding code, Im’s team first extracted the simulated three-dimensional dark-matter structure — the “invisible trees” that astronomers hope to probe through observations. Then, they determined the distributions of Lyman-alpha emitters and Lyman-break galaxies around those dark-matter filaments. As a comparison, they also calculated how concentrated the dark-matter particles themselves were around the dark-matter filaments.

This analysis showed that Lyman-alpha emitters and Lyman-break galaxies both follow the filamentary structure of the dark matter. Remarkably, both types of galaxies are more concentrated around the underlying dark-matter filaments than the dark-matter particles themselves. This may be because galaxies develop only in the most dark-matter-dense regions, leading to a narrow distribution along the spine of the dark-matter filaments, while dark-matter particles have no such restriction.

Critically, Im’s team found that Lyman-alpha-emitting and Lyman-break galaxies mirror the distribution of the general galaxy population, showing that the galaxy types studied here are an appropriate tracer of the underlying dark-matter distribution. Im and collaborators suggest that Lyman-alpha emitters are an especially promising way to trace fine filamentary structures, since the narrow-band observations used to select these galaxies allow for a more refined redshift estimate.

This work has determined that two types of commonly detected high-redshift galaxies trace the three-dimensional structure of the cosmic web. Future work will examine the two-dimensional distributions of the different galaxy types to provide a way to compare the statistical properties of the distributions against observations, such as those from the ODIN survey.

Citation

“Testing Lyα Emitters and Lyman-Break Galaxies as Tracers of Large-Scale Structures at High Redshifts,” Sang Hyeok Im et al 2024 ApJ 972 196. doi:10.3847/1538-4357/ad67d2