Kavli Affiliate: Roberto Maiolino
| First 5 Authors: Seunghwan Lim, Seunghwan Lim, , ,

| Summary:
We use the highest-resolution FLAMINGO hydrodynamical simulation to quantify cosmic variance and large-scale coherence in the evolution of massive galaxies at high redshift. FLAMINGO combines a $(1,mathrmcGpc)^3$ volume with baryonic resolution sufficient to identify $gtrsim,10^3$ independent JWST-like survey volumes of $(100,mathrmcMpc)^3$, providing unprecedented statistics to characterize the extremes of cosmic variance. At $z,simeq,6$, the total variance in the number of haloes with $M_200,simeq,10^11.5,mathrmM_odot$ (or $M_ast,simeq,10^10,mathrmM_odot$) is 2–3 times the Poisson expectation, while this ratio decreases with redshift. Similarly, at $z,gtrsim,4$, the variance in the most massive halo per JWST-like field is twice the Poisson prediction. We find a pronounced large-scale emphconformity: in volumes ranked by the stellar mass of their most massive galaxy ($M_ast,mathrmmax$), the stellar-to-halo mass relation and star-formation efficiency are coherently elevated or suppressed throughout the full $(100,mathrmcMpc)^3$ volume. When accounting for galaxies outside the volume, this signal persists only to radii $lesssim 50,mathrmcMpc$, demonstrating that the detectable conformity is enhanced by the survey footprint. Moreover, $M_ast,mathrmmax$ is a better predictor of the volume-wide efficiency of massive galaxies than the total number counts, which mainly trace clustering. Finally, the stellar fraction of the most massive galaxies peaks at $f_ast,=,M_ast,/,(M_200f_rm b,cosmic),simeq,0.2$ at $z,simeq,5$, with a narrower dispersion in $f_ast$ at fixed redshift and stronger redshift evolution than commonly assumed. These results show that both cosmic variance and footprint-confined conformity must be modelled when interpreting early massive galaxy populations in JWST fields.
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