Evolution of submillimeter galaxies across cosmic-web environments

Submillimeter galaxies (SMGs) provide valuable insights into galaxy formation and evolution and are likely influenced by their cosmic environment. However, their rarity makes environmental trends difficult to establish. We use the FLAMINGO simulation, which simultaneously reproduces the redshift distribution and number counts of SMGs. We use the DisPerSE to identify filamentary structures at $z=4$, 3, 2, 1.5, and 1. We define inner cluster-halo, outer cluster-halo, inner filament, outer filament, and void/wall environments at each redshift considering mass evolution of cluster-halos and density evolution of filaments. For a fixed stellar-mass cut of $M_* \geq 10^{9}$ M${\odot}$, the fraction of SMGs in the inner cluster-halo environment declines from $\sim30%$ at $z=4$ to $\sim3%$ by $z=1$, and similar trends are observed in other environments. The abundance of SMGs within a cluster-halo increases with halo mass, mirroring the increase in the total galaxy population. Consequently, the ratio of SMG halo occupation to that of all galaxies is largely insensitive to halo mass, but varies with redshift. In contrast, the ratio of the halo occupation of non-SMGs to that of all galaxies declines with halo mass and shows little redshift evolution. We show that the central and satellite SMGs form two distinct populations in inner cluster-halos. SMGs occupy the metal-rich side of the metallicity distribution, but rarely attain the highest metallicities because ongoing enrichment is limited by gas depletion. The brightest SMGs (S${850} > 10$ mJy) are found exclusively in inner cluster-halos, highlighting a strong connection between SMG luminosity and environmental density. Our results show that SMGs dominate star formation in dense environments, contributing up to $80%$ of the SFR in inner cluster-halos at $z=4$, but less than $50%$ in low-density regions.

💡 Research Summary

This paper investigates how submillimeter galaxies (SMGs) populate different cosmic‑web environments using the large‑volume FLAMINGO hydrodynamical simulation and the DisPerSE filament finder. The authors analyse five environments—inner and outer cluster‑halo, inner and outer filament, and void/wall—at redshifts z = 4, 3, 2, 1.5, and 1, defining them dynamically to account for the growth of cluster halos and the evolving density of filaments. Galaxies with stellar mass M* ≥ 10⁹ M⊙ are used as tracers of large‑scale structure, ensuring that the simulated stellar‑mass function matches observations.

SMG identification relies on the empirical relation from Hayward et al. (2013): S₈₅₀ = 0.81 (SFR/100 M⊙ yr⁻¹)^0.43 (Mdust/10⁸ M⊙)^0.54, where dust mass is derived from the metal mass of cold star‑forming gas assuming a fixed dust‑to‑metal ratio of 0.4. This approach reproduces the observed SMG number counts and redshift distribution within FLAMINGO, despite the simulation’s limited ISM resolution.

Key results:

1. The fraction of SMGs residing in the inner cluster‑halo drops dramatically from ~30 % at z = 4 to ~3 % by z = 1, while similar declines are seen in the other environments. This indicates that SMGs are most prevalent in the densest regions at early times but become rarer as gas is depleted and feedback processes suppress star formation.
2. Within a given cluster‑halo, the absolute number of SMGs rises with halo mass, mirroring the overall galaxy population. Consequently, the ratio of SMG halo occupation to total galaxy occupation is almost independent of halo mass but evolves strongly with redshift. By contrast, the occupation ratio for non‑SMG star‑forming galaxies declines with halo mass and shows little redshift evolution, reflecting stronger quenching in massive halos.
3. Central and satellite SMGs in inner cluster‑halos form two distinct sub‑populations. Centrals tend to be more massive and metal‑rich, while satellites have lower metallicities and higher gas fractions, suggesting different fueling mechanisms (e.g., rapid gas inflow and mergers for centrals versus filamentary accretion for satellites).
4. The brightest SMGs (S₈₅₀ > 10 mJy) appear exclusively in inner cluster‑halos, establishing a tight link between extreme sub‑mm luminosity and high‑density environments.
5. SMGs occupy the metal‑rich side of the star‑forming gas metallicity distribution but rarely reach the highest metallicities because ongoing enrichment is limited by rapid gas depletion.
6. In terms of star‑formation contribution, SMGs dominate the total SFR in dense environments, accounting for up to 80 % of the SFR in inner cluster‑halos at z = 4. In low‑density regions (filaments, voids/walls) their contribution falls below 50 %. This demonstrates that SMGs are the primary drivers of early stellar mass and metal buildup in proto‑clusters, while their relative importance declines toward lower redshift and lower density.

Methodologically, the paper showcases a robust pipeline: (i) high‑resolution cosmological simulation calibrated only on low‑z gas fractions and the z = 0 stellar‑mass function, (ii) a physically motivated sub‑grid model for dust production, and (iii) a dynamic, redshift‑dependent definition of cosmic‑web environments. By combining these elements, the authors overcome the traditional difficulty of low SMG statistics and provide statistically significant trends across cosmic time.

The study’s implications are multifold. First, it confirms that SMGs are excellent tracers of the most massive, rapidly assembling structures at early epochs, supporting the view that they are progenitors of today’s brightest cluster galaxies. Second, the environmental dependence of SMG luminosity and metallicity offers new constraints for semi‑analytic and semi‑empirical models that aim to reproduce SMG number counts without invoking a top‑heavy IMF. Third, the distinct central‑satellite dichotomy within clusters suggests that future high‑resolution observations (e.g., with ALMA or JWST) should target cluster cores to disentangle merger‑driven starbursts from filament‑fed accretion. Finally, the demonstrated dominance of SMGs in the SFR budget of dense regions underscores their role in early metal enrichment of the intracluster medium.

Overall, the paper provides a comprehensive, simulation‑driven picture of how SMGs evolve across the cosmic web, linking their extreme star‑forming properties to the underlying large‑scale structure and offering testable predictions for upcoming deep sub‑mm surveys.