CSST cosmological emulator III: Hybrid lagrangian bias expansion emulation of galaxy clustering

Abstract

Galaxy clustering is an important probe in the upcoming China Space Station Telescope (CSST) survey to understand the structure growth and reveal the nature of the dark sector. However, it is a long-term challenge to model this biased tracer and connect the observable to the underlying physics. In this work, we present a hybrid Lagrangian bias expansion emulator, combining the Lagrangian bias expansion and the accurate dynamical evolution from N-body simulation, to predict the power spectrum of the biased tracer in real space. We employ the Kun simulation suite to construct the emulator, emulating across the space of 8 cosmological parameters including dynamic dark energy w₀, w_a, and total neutrino mass Σ m_v. The sample variance due to the finite simulation box is further reduced using the Zel’dovich variance control, and it enables the precise measurement of the Lagrangian basis spectra up to the quadratic order. The emulation of basis spectra realizes 1% level accuracy, covering wavelength k ⩽ 1 Mpc⁻¹ h and redshift 0 ⩽ z ⩽ 3 up to the quadratic order field. To validate the emulator, we perform a joint fit to the halo auto power spectrum and the halo-matter cross power spectrum measured from 46 independent simulations. Depending on the choice of counterterm, the joint fit is unbiased up to k_max ≃ 0.7 Mpc⁻¹ h within 1–2 percent accuracy, for all the redshift and halo mass samples. As part of the CSST cosmological emulator series, this emulator is expected to provide accurate theoretical predictions for the galaxy power spectrum in upcoming CSST survey.