Galaxy Evolution with DESI: Mapping the Baryon Cycle Across the Cosmic Web

Galaxies grow and evolve by acquiring gas, converting it into stars, and redistributing the chemically enriched material within the vast dark matter framework that structures the Universe.

This continual baryon cycle is regulated by interconnected processes — including gas inflows and outflows, star formation, and environmental effects (Fig. 1). The remarkable diversity observed in today’s galaxy population, spanning morphologies, sizes, colours, and star-formation activity, reflects the cumulative outcome of this complex interplay.

Diagram showing Baryon Cycle
Figure 1: A representation of the baryon cycle, highlighting gas inflow, the processing of some of this gas into stars within the inter-stellar medium, and the stellar- and AGN-powered outflows. Image credit: L.Davies (ICRAR)/ M.Romano (MPIfR)/ M.Saraf (UCL)/ A.Vadolia/ NASA.

A central challenge in building a predictive theory of galaxy formation and evolution is to reconstruct the physical processes that give rise to the diverse structural and chemical properties observed across the galaxy population today. Addressing this requires not only detailed observations of individual systems but also statistically powerful datasets capable of isolating the primary drivers of galaxy growth while revealing the secondary dependencies that shape different evolutionary pathways.

At University College London (UCL), large-area surveys (Fig. 2) are leveraged to investigate how baryons cycle between galaxies and their surrounding large-scale structure, how this exchange regulates star formation, and how it imprints the distribution of galaxy properties observed today. Key questions include: How are baryons coupled to their host dark matter structures? Which properties — such as star-formation activity, structure, and environment — drive departures from average galaxy scaling relations and variations in galaxy mass functions? And how do these relationships evolve with cosmic time?

Graph showing Spectroscopic surveys, weak lensing surveys and atomic hydrogen surveys
Figure 2: Illustration of the sky coverage of large-area (a) spectroscopic, (b) weak lensing and (c) atomic hydrogen surveys, listed with estimated galaxy counts. Image credit: M.Saraf (UCL)/ A.Vadolia.

The Dark Energy Spectroscopic Instrument (DESI), a state-of-the-art spectroscopic survey, is mapping the three-dimensional positions of tens of millions of galaxies across a significant fraction of the observable Universe. Its spectral energy distribution (SED) measurements enable the estimation of stellar masses, star-formation rates, and chemical abundances, while simultaneously tracing the underlying cosmic web. By combining this statistically powerful spatial and redshift information with simulation-informed machine-learning algorithms, we are reconstructing the large-scale structure with high fidelity, classifying galaxies into voids, walls, filaments, and clusters (Fig. 3). This enables direct tests of environmental influences on galaxy properties beyond traditional density metrics.

Graphic map showing Inferred dynamical cosmic web environments of DESI galaxies with M* > 109M☉ using a GNN Classifier (Kololgi et al. 2025).
Figure 3: Inferred dynamical cosmic web environments of DESI galaxies with M* > 109M☉ using a GNN Classifier (Kololgi et al. 2025). Image credit: D. Kololgi (UCL)/M. Saraf (UCL).

As an additional and complementary measure of galaxy environments, dark matter halo masses are being estimated through weak gravitational lensing using overlapping datasets from the Dark Energy Survey (DES), Hyper Suprime-Cam Survey (HSC), Kilo-Degree Survey (KiDS), and Sloan Digital Sky Survey (SDSS). These measurements will provide a direct link between the visible galaxy population and their underlying dark matter haloes, enabling new constraints on the baryon–halo mass relation.

We are also integrating DESI spectroscopy with wide-area radio surveys, including the Arecibo Legacy Fast ALFA Survey (ALFALFA) and the FAST All Sky HI survey (FASHI), conducted with the Five-hundred-meter Aperture Spherical radio Telescope (FAST). These surveys probe atomic hydrogen (HI) — the cold gas that fuels future star formation and serves as a sensitive tracer of environmental processes. While ALFALFA established benchmark measurements of the local HI mass function and gas-fraction scaling relations, FASHI is extending this work by revealing, with FAST’s unparalleled sensitivity, previously undetected gas reservoirs in dwarf systems and low-density environments at greater distances.

Together, these complementary datasets — spanning optical spectroscopy, radio gas measurements, weak lensing constraints, and advanced structure reconstruction — allow us to examine the overall baryon–halo connection across a broad dynamic range in mass, from massive systems to dwarf galaxies. Looking ahead, our research will expand through emerging surveys, including the Wide Area VISTA Extragalactic Survey (WAVES), the 4MOST Hemisphere Survey (4HS), and the Widefield ASKAP L-band Legacy All-sky Blind surveY (WALLABY), further expanding our statistical reach and strengthening our ability to connect gas, stars, and dark matter across diverse galaxy environments.

By combining statistical power with physical insight, innovative analysis methods, and machine-learning techniques, we at UCL are translating large-scale multi-wavelength observations to inform a coherent framework for galaxy formation and evolution.

For further information or potential research opportunities, contact Amélie Saintonge (a.saintonge@ucl.ac.uk)