Forged by Feedback: Stellar Properties of Brightest Group Galaxies in Cosmological Simulations

We investigate how different galaxy formation models impact the stellar properties of brightest group galaxies (BGGs) in four cosmological simulations: ROMULUS, SIMBA, SIMBA-C, and OBSIDIAN. The stellar masses, specific star formation rates, and mass-weighted stellar ages of the simulated BGGs are analysed alongside those of observed BGGs from X-ray-selected galaxy groups in the COSMOS field. We find that the global properties and underlying evolutionary pathways of simulated BGG populations are strongly impacted by the strength and mechanism of their respective active galactic nucleus (AGN) feedback models, which play a critical role in regulating the growth of massive galaxies. OBSIDIAN's sophisticated three-regime AGN feedback model achieves the highest overall agreement with COSMOS observations, matching stellar property distributions, quenched fractions, and the evolution of star formation in increasingly massive systems. We find evidence suggesting that BGG populations of OBSIDIAN and COSMOS undergo a gradual decline in star formation with stellar mass, in contrast to SIMBA and SIMBA-C, which display rapid quenching linked to the onset of powerful AGN jet feedback. By comparison, ROMULUS produces highly star-forming, under-quenched BGGs due to the inefficiency of its thermal AGN feedback in preventing cooling flows from fuelling BGG growth. The success of the OBSIDIAN simulation demonstrates the importance of physically motivated subgrid prescriptions for realistically capturing the processes that shape BGGs and their dynamic group environments.