# Evolution of matter and galaxy clustering in cosmological hydrodynamical simulations

**Authors:** Jaan Einasto, Gert Hütsi, Lauri-Juhan Liivamägi, Changbom Park, Juhan Kim, Istval Szapudi, Maret Einasto  
**arXiv:** [2304.09035](https://arxiv.org/abs/2304.09035)  
**Date:** 2023-04-18  
**Summary page:** https://www.summarizepaper.com/en/arxiv-id/2304.09035v1/

## Key points

- The evolution of matter and galaxy clustering is a central problem in cosmology
- Galaxies have a filamentary distribution with large regions devoid of galaxies
- Numerical simulations predicted the filamentary character of particles but showed the presence of a rarefied population of particles in voids
- Galaxies are biased tracers of matter density fields
- The authors quantify the evolution of matter and galaxy clustering via correlation and bias functions using cosmological hydrodynamical simulations TNG100 and TNG300 with epochs from $z=5$ to $z=0$
- Bias parameters decrease during the evolution, confirming earlier results
- At low and medium luminosities, bias parameters of galaxies ($b_0$) are equal, suggesting that dwarf galaxies reside in the same filamentary web as brighter galaxies
- Bias parameters of the lowest luminosity galaxies estimated from CFs are lower relative to CFs of particle density-limited clustered samples of DM
- Bias parameters $b_0$, estimated from CFs of clustered DM, agree with expected values from the fraction of particles in the clustered population ($b=1/F_c$)
- The cosmic web contains filamentary structures of various densities, and fractions of matter in clustered and unclustered populations are both less than unity. Thus, the CF amplitude of clustered matter is always higher than for all matter; i.e., bias parameter must be $b>1$
- Differences between CFs for galaxies and clustered DM suggest that these functions describe different properties of the cosmic web

## Summary

The evolution of matter and galaxy clustering in cosmological hydrodynamical simulations is a central problem in cosmology. Early studies showed that galaxies have a filamentary distribution with large regions devoid of galaxies, while numerical simulations predicted the filamentary character of particles but showed the presence of a rarefied population of particles in voids. A physical mechanism for the formation of galaxies in dark matter (DM) halos was suggested by White & Rees, and more detailed studies were presented by Dekel & Silk, Dekel, Dekel & Rees, and Bond et al. Thus galaxies are biased tracers of matter density fields. In this study, the authors quantify the evolution of matter and galaxy clustering via correlation and bias functions using cosmological hydrodynamical simulations TNG100 and TNG300 with epochs from $z=5$ to $z=0$. They calculate spatial correlation functions of galaxies ($\xi(r)$) for simulated galaxies and DM particles to characterize the evolving cosmic web. The authors find that bias parameters decrease during the evolution, confirming earlier results. At low and medium luminosities, bias parameters of galaxies ($b_0$) are equal, suggesting that dwarf galaxies reside in the same filamentary web as brighter galaxies. Bias parameters of the lowest luminosity galaxies estimated from CFs are lower relative to CFs of particle density-limited clustered samples of DM. The authors find that bias parameters $b_0$, estimated from CFs of clustered DM, agree with expected values from the fraction of particles in the clustered population ($b=1/F_c$). The cosmic web contains filamentary structures of various densities, and fractions of matter in clustered and unclustered populations are both less than unity. Thus, the CF amplitude of clustered matter is always higher than for all matter; i.e., bias parameter must be $b>1$. Differences between CFs for galaxies and clustered DM suggest that these functions describe different properties of the cosmic web. Overall, this study provides insights into the evolution of matter and galaxy clustering in cosmological hydrodynamical simulations and highlights the importance of bias parameters in characterizing the cosmic web.