2024MNRAS.534..852C

The overall objective of this study is to investigate claims found in the literature that fossil groups, characterized by a large magnitude gap between their two brightest galaxies, are darker, i.e. exhibit higher mass-to-light (M/L) ratios, compared to regular groups. Specifically, we aim to measure the mass of these systems using the weak gravitational lensing technique. To achieve this, we obtained deep images of four fossil systems with the CFHT telescope in the $r$ and g bands. Through a careful process of weak gravitational shear measurement, including corrections for the point spread function (PSF) and contamination from group and foreground galaxies, we fit NFW models and obtained mass measurements as a result. Similarly, we quantified the light distribution produced by these groups, taking into account relevant data gaps in the images due to the presence of bright stars, as well as contamination from foreground and background galaxies. We obtained masses and M/L ratios that are consistent with previous results where mass was estimated from galaxy dynamics. Indeed, the four fossil systems studied here exhibit high M/L ratios compared to the general population of systems. Drawing more generalizable conclusions from a sample of only four systems is challenging. However, the procedure outlined in this study can be applied to large image surveys, allowing for a revisiting of this question with significantly reduced statistical uncertainties. This will enable a more homogeneous comparison between fossil groups and clusters and the overall population.