The Dependence of Halo Mass on Galaxy Size at Fixed Stellar Mass and Colour using Galaxy-Galaxy Lensing

Abstract

To advance our holistic understanding of galaxy formation physics, we must examine the relationship between baryonic matter and dark matter (DM) within the universe. In this thesis, we investigate the correlation between dark matter halo mass and galaxy size at fixed stellar mass and colour. Using galaxy-galaxy lensing, we measure excess surface density (ESD) profiles for red, early-type galaxies from the Dark Energy Spectroscopic Instrument (DESI) Legacy Imaging survey, with shape measurements from the Ultraviolet Near Infrared Optical Northern Survey (UNIONS). We model the lensing signal using a conditional stellar mass function (CSMF) halo occupation distribution (HOD) calibrated on the AbacusSummit simulations and adopt a power-law relation between halo mass and galaxy size at fixed stellar mass: Mh ∝ r^ηeff . To fit the HOD parameters to the observationally measured ESD vectors, we train and utilize neural-network emulators, a form of non-linear interpolators. The analysis performed in this work follows a two-step process. First, we fit the HOD parameters to the ESD vectors corresponding to three stellar mass bins, not split further into two size bins. We freeze the best-fit HOD parameters and generate the corresponding best-fit mock catalogue. Using this best-fit mock catalogue, we apply our halo mass-size model and fit to the size-split observational ESD measurements. For central galaxies with stellar mass 10.5 ≤ log10(M⋆/M⊙) < 11.2, we find no significant correlation. At higher stellar masses, 11.2 ≤ log10(M⋆/M⊙) < 11.78, we detect a positive correlation with ηcent = 0.51 ± 0.14. A linear fit as a function of the logarithm of stellar mass yields a slope of sηcent = 0.66 ± 0.28, indicating that the halo mass-size correlation strengthens with increasing stellar mass. For satellite galaxies, we observe a negative correlation at low and intermediate stellar masses for the host halo mass-galaxy size relation of ηsat = −0.19 ± 0.05 and ηsat = −0.09 ± 0.05, respectively. The correlation is consistent with zero within the stellar mass range of 11.2 ≤ log10(M⋆/M⊙) < 11.78.