Think inside the box: cosmic variance and large-scale conformity of high-redshift massive galaxies in the FLAMINGO simulations

Abstract

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 $(1\mathrm{cGpc}{)}^3$ volume with baryonic resolution sufficient to identify $\gtrsim 10^3$ independent JWST-like survey volumes of $(100\mathrm{cMpc}{)}^3$, providing unprecedented statistics to characterize the extremes of cosmic variance. At $z\simeq 6$, the total variance in the number of haloes with $M_{200}\simeq 10^{11.5}{\mathrm{M}}_{\odot }$ (or $M_{\ast }\simeq 10^{10}{\mathrm{M}}_{\odot }$) 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 \emph{conformity}: in volumes ranked by the stellar mass of their most massive galaxy ($M_{\ast ,\max }$), the stellar-to-halo mass relation and star-formation efficiency are coherently elevated or suppressed throughout the full $(100\mathrm{cMpc}{)}^3$ volume. When accounting for galaxies outside the volume, this signal persists only to radii $\lesssim 50\mathrm{cMpc}$, demonstrating that the detectable conformity is enhanced by the survey footprint. Moreover, $M_{\ast ,\max }$ 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 $f_{\ast }=M_{\ast }/(M_{200}{f}_{\mathrm{b},\mathrm{cosmic}})\simeq 0.2$ 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.

arXiv comment15 pages, 12 figures, submitted to MNRAS, comments are welcome