How DESI’s star-forming galaxies map the cosmic web

When we look at the night sky, we see a scattered collection of stars and galaxies, but to an astrophysicist, these are merely the glowing foam on the crest of a much deeper, invisible ocean. The vast majority of the matter in our universe is dark matter, a substance that does not emit, absorb, or reflect light. It acts as the gravitational scaffolding for everything we see. To understand the evolution of the cosmos, we must understand the relationship between the visible galaxies and the invisible dark matter halos that host them. This relationship is what we call the galaxy–halo connection. A recent and significant study ¹ has utilized the Dark Energy Spectroscopic Instrument, known as DESI, to map this connection with unprecedented precision using a specialized group of galaxies called Emission-Line Galaxies, or ELGs.

DESI

The DESI survey is currently engaged in creating the largest 3D map of the universe ever constructed. To do this, it targets ELGs because they are prolific star-formers that glow brightly in specific wavelengths. These galaxies are particularly useful because their distinct signatures allow us to measure their distance, or redshift (z), across vast cosmic scales, reaching back to a time when the universe was only a fraction of its current age. However, simply seeing where these galaxies are is not enough. We need to know how they “choose” their homes within the dark matter landscape. This is where the SHAMe-SF2 model comes into play.

The SHAMe-SF model

SHAMe-SF stands for SubHalo Abundance Matching extended for Star Formation, and it represents a sophisticated way to bridge the gap between computer simulations and real-world observations. In basic subhalo abundance matching, we assume a simple correspondence where the brightest galaxies live in the most massive dark matter halos. Nature, however, is rarely that simple. The “SF” in the model name indicates that it specifically accounts for star formation properties. Because ELGs are defined by their active star formation, their presence in a halo might depend on more than just the mass of that halo. It might depend on the age of the halo, its environment, or how fast it is growing.

The role of assembly bias

One of the most fascinating aspects of this research is the investigation of “assembly bias.” This is the idea that the clustering of dark matter halos depends on properties other than their mass, such as when they were formed. For a long time, simpler models ignored this, assuming that if you knew the mass of a halo, you knew everything important about how it clustered with its neighbors. The SHAMe-SF model allows researchers to test if the ELGs observed by DESI are sensitive to this bias. By comparing the DESI data to high-resolution simulations, the researchers found that these star-forming galaxies do not just follow the mass. Their distribution is subtly influenced by the history of the dark matter structures themselves.

This is crucial because if we misunderstand how galaxies are biased tracers of dark matter, our measurements of the universe’s expansion and the growth of cosmic structure will be slightly off. If we want to use DESI to hunt for the secrets of dark energy, which is the mysterious force driving the accelerated expansion of the universe, we must have a “clean” understanding of this galaxy–halo connection. The study demonstrates that the SHAMe-SF model can reproduce the complex clustering patterns of ELGs across different scales, providing a robust framework for interpreting the massive amounts of data DESI continues to collect.

Closer to a standard model of galaxy-dark matter interaction

The success of this modeling tells us that we are getting closer to a “standard model” of how galaxies and dark matter interact. By successfully fitting the DESI ELG samples, the research confirms that we can use these energetic, blue galaxies as reliable probes of the large-scale structure. This is a foundational step for precision cosmology. The ability to account for the nuances of star formation and halo assembly will be the difference between a blurry snapshot of the universe and a high-definition map.

The researchers have shown that the connection between the light we see and the dark matter we cannot see is governed by a complex but ultimately trackable set of rules. This work ensures that when we measure the distance between millions of ELGs to determine the expansion rate of the universe, we are doing so with a corrected lens that accounts for the intricate dance between baryonic matter (the stuff we are made of) and the dark matter that dominates the gravitational landscape.

Author: César Tomé López is a science writer and the editor of Mapping Ignorance

Disclaimer: Parts of this article may have been copied verbatim or almost verbatim from the referenced research paper/s.