Studying the baryon-to-halo mass relation in galaxies with DESI and 4MOST

Understanding how the properties of galaxies depend on their dark matter halos is vital for the study of both galaxy evolution and cosmology. On the one hand, the growth of galaxies is regulated by their halos and their location in the cosmic web, and on the other, the galaxies themselves are often the beacons used by cosmologists to track the underlying (dark) matter distribution.

This connection between galaxies and their dark matter halos is most often studied through the stellar-to-halo mass relation. However, the stars are only one component of galaxies, with gas the other major reservoir of normal matter. The premise behind this proposal is that we could gain significant more insight into the galaxy-halo connection if we had access not just to the stellar mass, but to the entire normal matter (or "baryonic matter") contents of the galaxies.

We will use data from the DESI Bright Galaxy Survey (BGS) and from the 4MOST WAVES survey, and methodology developed by our group to retrieve the total cold gas mass of galaxies from information encoded in their optical spectra. Applying this gas mass predictor to stacked spectra from DESI-BGS and combining with stellar mass estimates will give us access to the total baryonic mass of well-selected galaxy sub-samples, selected for example by mass or environment} Halo masses will be measured via weak lensing for the same sub-samples using the overlap between DESI-BGS and both DES and KiDS.

In the second year of the project, the analysis will be extended to the 4MOST-WAVES sample which will probe even smaller halos, in the regime where gas mass dominates over stellar mass. The project brings together Prof. A.

Saintonge who has extensive experience with statistical studies of the cold gas contents of galaxies and Prof. B. Joachimi, an expert in the field of weak lensing.

This crucial consolidation of expertise and the access to all the survey data products required (DES, DESI, KiDS, and 4MOST) is what makes this exciting project possible.