Clustering of emission line galaxies with IllustrisTNG -- II. cosmology challenge with anisotropic correlation functions and ELG-halo connections

TL;DR

The paper assesses cosmological inferences from ELG clustering using high-fidelity mock ELG catalogs from IllustrisTNG, focusing on anisotropic redshift-space distortions and the ELG–halo connection. By modelling the anisotropic correlation function with the eTNS framework and comparing ELG samples to mass-matched controls, it reveals a suppression of the quadrupole moment and a biased underestimation of the linear growth rate $f$ due to ELG intra-halo infall and velocity bias. The authors demonstrate that restricting to large scales ($s\gtrsim 15$–$20\,h^{-1}$ Mpc) can mitigate these biases, while the ELG-halo connection (phase-space distributions and galactic conformity) explains the origin of the dynamical differences. The work underscores the necessity of ELG-specific dynamical modelling for unbiased cosmology in Stage-IV surveys and points to future large-volume hydrodynamical simulations to robustly capture ELG clustering across scales.

Abstract

Emission line galaxies (ELGs) are the primary tracers of the large-scale structures of the Universe in ongoing Stage-IV cosmological spectroscopic surveys, which aim to measure the clustering statistics at higher redshifts $z \simeq 1.5 \text{--} 2$ with unprecedented precision. In this study, we construct realistic mock ELG samples with IllustrisTNG hydrodynamical simulations and stellar population synthesis framework. In order to validate the modelling of clustering, we measure the anisotropic correlation functions of mock ELGs and infer the linear growth rate, which is one of key cosmological parameters in galaxy clustering. As a control sample, we construct the mass-limited subhalo samples with the same number density as ELGs. The isotropic correlation functions in real space for both samples do not differ significantly. However, the quadrupole moment of the anisotropic correlation function, which is sensitive to the velocity of galaxies, is suppressed for ELGs, potentially due to the infalling motion of ELGs towards the centre of the hosting halos. The smaller amplitude leads to the underestimation of the linear growth rate and implies the velocity bias between ELGs and dark matter. When the analysis is limited to large scales $(\gtrsim 15 \, h^{-1} \, \mathrm{Mpc})$, the parameter bias vanishes. Next, we investigate the ELG-halo connection through the phase-space distribution of satellite ELGs within hosting halos and galactic conformity of star formation activity. The infalling motion is further confirmed by the phase-space distribution relative to the host halo, and this dynamics of ELGs challenges the assumption that the radial distribution of satellites follows that of dark matter.

Figures (12)

• Figure 1: The 2D anisotropic correlation functions of [Oii] ELGs and the corresponding control sample. The white pixels correspond to the no pair in the measurement.
• Figure 2: The isotropic correlation functions of [Oii] ELGs and the corresponding control sample.
• Figure 3: The monopole and quadrupole moments of correlation functions of [Oii] ELGs and the corresponding control sample.
• Figure 4: The parameter constraints with monopole and quadrupole moments of the anisotropic correlation function for [Oii] ELGs (left panels) and the control sample (right panels). The contours correspond to 1-$\sigma$ and 2-$\sigma$ levels. The minimum separation $s_\mathrm{min}$ is varied from $5 \, h^{-1} \, \mathrm{Mpc}$ to $15 \, h^{-1} \, \mathrm{Mpc}$.
• Figure 5: The marginalised posterior distributions with monopole and quadrupole moments of the anisotropic correlation function for [Oii] ELGs and the control sample. The minimum separation $s_\mathrm{min}$ is varied from $5 \, h^{-1} \, \mathrm{Mpc}$ to $20 \, h^{-1} \, \mathrm{Mpc}$. The thick (thin) grey bars correspond to $[15.87, 84.13]$ ($[2.28, 97.72]$) percentiles, and the white circles indicate the median values. The black dashed line corresponds to the true value of the linear growth rate $f$.