From Clusters to Local Groups: Extending X-ray Scaling Relations with eROSITA.

Abstract: Low-mass galaxy groups are the most common environments for galaxies and serve as a key bridge between cosmological structure formation and galaxy evolution. Yet, their hot gas and baryonic content remain poorly characterized, largely due to their low surface brightness in X-rays. In this talk, I will show how spectral stacking of eROSITA data offers a powerful method to uncover the X-ray properties of galaxy groups and clusters identified through large spectroscopic surveys, including SDSS, GAMA, and DESI. Focusing on eRASS1 results, I will present stacked X-ray measurements for these systems and compare them with mock observations based on hydrodynamical simulations such as Magneticum and IllustrisTNG. This technique allows us to trace the intra-group medium from massive clusters down to group scales comparable to the Local Group, extending and validating key X-ray scaling relations into the low-mass regime. These results open new pathways to connecting observations and theory across the full mass spectrum of cosmic structures.