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.

💡 Research Summary

This paper presents a systematic comparison of the stellar properties of brightest group galaxies (BGGs) across four state‑of‑the‑art cosmological hydrodynamic simulations—ROMULUS, SIMBA, SIMBA‑C, and OBSIDIAN—and contrasts them with observations of BGGs drawn from X‑ray‑selected galaxy groups in the COSMOS field. The authors focus on three key observables: stellar mass, specific star formation rate (sSFR), and mass‑weighted stellar age, which together encode the growth history, current activity, and quenching state of massive central galaxies.

The methodology section details the numerical setups and sub‑grid physics of each simulation. ROMULUS employs a smooth‑particle hydrodynamics (SPH) code (ChaNGa) with high mass resolution (∼3×10⁵ M⊙ for dark matter, ∼2×10⁵ M⊙ for gas) and a purely thermal AGN feedback model that injects 0.2 % of the accreted rest‑mass energy isotropically into the nearest 32 gas particles. Black holes are seeded based on local gas conditions and are allowed to orbit freely, with growth governed by a modified Bondi–Hoyle prescription that includes angular momentum support.

SIMBA, run with the meshless finite‑mass (MFM) solver in GIZMO, adopts a Chabrier IMF, H₂‑based star formation, and a two‑mode AGN feedback scheme: a high‑Eddington radiative/thermal mode and a low‑Eddington kinetic jet mode. Black holes are seeded once a galaxy reaches ∼10⁹ M⊙, are pinned to the potential minimum, and grow via a hybrid Bondi‑plus‑torque accretion model. The kinetic mode launches bipolar outflows that are hydrodynamically decoupled for a short period, mimicking powerful radio jets. SIMBA‑C is a calibrated variant of SIMBA with modest adjustments to jet efficiency, intended to explore the sensitivity of massive galaxy quenching to feedback strength.

OBSIDIAN introduces a more sophisticated three‑regime AGN feedback framework: (1) a radiative/thermal regime at high accretion rates, (2) a “maintenance” hot‑mode heating regime, and (3) a kinetic jet regime at low Eddington ratios. Transitions between regimes are governed by local gas density, temperature, and SMBH mass, allowing a smoother, more continuous regulation of cooling flows. This model also incorporates dynamical friction sub‑grid forces for SMBH orbital decay and a refined torque‑limited accretion prescription.

The observational comparison uses the COSMOS2020 BGG catalog, which provides stellar masses, SFRs, and mass‑weighted ages derived from multi‑wavelength photometry and X‑ray group identification. The authors apply consistent selection criteria (e.g., halo mass range, central galaxy identification) to the simulated catalogs to ensure a fair one‑to‑one comparison.

Results show stark differences driven primarily by the AGN feedback implementation. ROMULUS BGGs retain high sSFRs (often >10⁻¹⁰ yr⁻¹) even at M★ > 10¹¹ M⊙, exhibit low quenched fractions (~30 % at high mass), and have relatively young mass‑weighted ages (∼3–5 Gyr). This reflects the inefficacy of pure thermal feedback to halt cooling flows in massive halos. SIMBA and SIMBA‑C produce a rapid quenching transition: once SMBHs enter the low‑Eddington jet mode, sSFRs drop precipitously, yielding quenched fractions >80 % for M★ > 10¹¹ M⊙ and older stellar ages (>7 Gyr). SIMBA‑C’s reduced jet efficiency shifts the quenching threshold to slightly higher masses, but the overall trend remains steep.

In contrast, OBSIDIAN reproduces the gradual decline of sSFR with stellar mass observed in COSMOS. Its three‑regime feedback allows cooling to be moderated rather than abruptly shut off, leading to a smooth increase in quenched fraction from ~40 % at 10¹⁰.⁵ M⊙ to ~85 % at 10¹¹.⁵ M⊙, matching the observed distribution. The mass‑weighted age–mass relation also aligns well: more massive BGGs are older, with a slope comparable to the COSMOS data.

The authors discuss the physical implications of these findings. Thermal feedback alone cannot generate the hot, low‑density intragroup medium required to suppress cooling in group‑scale halos; kinetic jets are essential but must be calibrated to avoid over‑quenching. A multi‑phase approach, where AGN output transitions smoothly between radiative, maintenance, and kinetic modes based on the local environment, appears to capture the complex interplay between SMBH growth, gas cooling, and star formation in BGGs.

Finally, the paper emphasizes that BGGs serve as a stringent testbed for galaxy formation models because their evolution is tightly coupled to group‑scale processes such as mergers, tidal interactions, and the thermodynamics of the intragroup medium. The success of OBSIDIAN suggests that future simulations should prioritize physically motivated, environment‑dependent AGN feedback prescriptions and incorporate realistic SMBH dynamics to faithfully reproduce the observed properties of massive central galaxies.