The S-PLUS Fornax Project (S+FP): Fornax-like clusters in cosmological hydrodynamical simulations

Technical Summary: The S-PLUS Fornax Project (S+FP): Fornax-like clusters in cosmological hydrodynamical simulations

Problem and Context
The Fornax cluster, the second-nearest rich galaxy cluster, serves as a critical laboratory for studying galaxy evolution in dense environments. Recent observations by the Southern Photometric Local Universe Survey (S-PLUS) have provided unprecedented multi-band photometric data for Fornax, revealing new features regarding its galaxy populations and surrounding regions. However, interpreting these observational findings requires a theoretical framework to trace the origin and evolution of these systems. While cosmological simulations have previously modeled aspects of Fornax, there is a need for a robust catalog of simulated "Fornax-like" systems that can be directly compared with S-PLUS data using consistent photometric methodologies.

Methodology
The authors analyzed analogs of the Fornax cluster extracted from two state-of-the-art cosmological hydrodynamical simulation suites: eagle (specifically the RefL0100N1504 run) and IllustrisTNG (specifically the TNG50, TNG100, and TNG300 runs).

1. Selection Criteria:

   • Global Constraints: Initial candidates were selected based on a virial mass range of $10^{13} M_\odot \le M_{200} \le 10^{14} M_\odot$ at redshift $z=0$, consistent with literature estimates for Fornax.
   • Central Galaxy Constraints: To refine the sample, the central galaxy of each candidate was filtered against observed properties of NGC 1399 (Fornax's central galaxy). Criteria included:
     • Stellar mass: $1 \times 10^{11} M_\odot \le M_\star \le 5 \times 10^{11} M_\odot$.
     • Stellar half-mass radius: $20 \text{ kpc} \le R_{h,\star} \le 40 \text{ kpc}$.
     • Visual morphology: Spherical morphology confirmed via visual inspection of edge-on and face-on renderings.
   • Exceptions: For the TNG50 simulation (highest resolution but smallest volume), the radius and morphology constraints were relaxed to include the most similar systems, as no clusters met all strict criteria.

2. Synthetic Observation Generation:

   • The skirt radiative transfer code was employed to generate synthetic observations for every galaxy in the selected clusters.
   • Configuration: The code was configured to match the S-PLUS survey's 12-band filter system (5 broad-band and 7 narrow-band filters).
   • Assumptions: To reduce computational cost, dust was neglected in the radiative transfer calculations; thus, the resulting synthetic quantities represent observations corrected for internal and external extinction.
   • Stellar Sources: Star particles were treated as Simple Stellar Populations (SSPs). Young populations ($t \le 10$ Myr) were modeled using MAPPINGS-III libraries, while older populations used GALAXEV models with a Chabrier IMF.
   • Outputs: The process generated synthetic spectral energy distributions (SEDs), magnitudes, colors, and mock images (data cubes) for all galaxies in the selected clusters.

Key Contributions

• Catalog Creation: The paper delivers a catalog of simulated Fornax-like clusters from eagle and IllustrisTNG, specifically curated to match the mass and central galaxy properties of the real Fornax cluster.
• Methodological Implementation: It introduces a specific implementation of the skirt code configured for the 12 S-PLUS bands, a dataset not natively available in the simulation databases. This allows for a direct, apples-to-apples comparison between simulation outputs and S-PLUS observations.
• Best Candidates: Four "best" Fornax-like clusters were identified (one from each simulation run) based on their spatial distribution, particularly the separation between the central galaxy and the second most massive galaxy, mimicking the Fornax/NGC 1316 structure.

Results

• SED and Photometric Agreement: The synthetic SEDs and photometry derived from the simulations show good agreement with S-PLUS observations, particularly for the central galaxies (analogous to NGC 1399). The simulated spectra match the observed shape of NGC 1399's SED when normalized.
• Color-Magnitude Diagrams (CMDs): The simulated CMDs reproduce the observed red sequence of galaxies. However, the simulations show a sparser "blue cloud" compared to observations and an excess of bright blue galaxies in some cases (notably TNG300 and TNG50).
• Spatial Distribution: The best candidates, particularly from TNG300, reproduce the spatial substructure between the cluster core and the Fornax A (NGC 1316) region, with the second most massive galaxy located outside the virial radius.
• Radial Trends: Simulated galaxies exhibit a trend of becoming bluer at higher cluster-centric distances, consistent with S-PLUS observations, although the simulated colors are generally $\sim 0.2$ mag redder than observed values at fixed locations.

Significance and Claims
The paper claims that its selection criteria and synthetic photometry methodology are suitable for creating reliable analogs of the Fornax cluster. The results support the reliability of current cosmological simulations (eagle and IllustrisTNG) in reproducing key general features of the Fornax cluster, such as the red sequence and spatial substructure.

The authors modestly note that discrepancies exist, particularly regarding the underpopulation of the blue cloud in simulations and the excess of bright blue galaxies, which they attribute to potential over-efficiency of quenching mechanisms or AGN feedback modeling in the simulations. They emphasize that this work serves as a foundational step: the primary goal is to establish a robust sample and methodology for future, more detailed physical interpretations (e.g., star formation histories, chemical enrichment, and kinematics) which will be presented in subsequent papers within the S-PLUS Fornax Project. The generated mock data cubes and synthetic photometry are presented as a valuable resource for rigorous comparisons with S-PLUS observations to trace the formation and evolution of the Fornax cluster.