The negative BAO shift in the Lyα forest from cosmological simulations

We present the first measurement of the Lyα forest BAO shift parameter from cosmological simulations. In particular, we generate a suite of $1000$ accurate effective field-level bias based Lyα forest simulations of volume V=(1 \, h⁻¹ \, Gpc)³ at $z=2$, both in real and redshift space, calibrated upon two fixed-and-paired cosmological hydrodynamic simulations. To measure the BAO, we stack the three-dimensional power spectra of the $1000$ different realizations, compute the average, and use a model accounting for a proper smooth-peak component decomposition of the power spectrum, to fit it via an efficient Markov Chain Monte Carlo scheme estimating the covariance matrices directly from the simulations. We report the BAO shift parameters to be α=0.997+0.001-0.001 and α=0.990+0.003-0.003 in real and redshift space, respectively. We also measure the bias blya and the BAO broadening parameter Σₙₗ, finding blya=-0.1786+0.0001-0.0001 and Σₙₗ=3.87+0.20-0.20 in real space, and blya=-0.073+0.005-0.004 and Σₙₗ=6.55+0.23-0.22 in redshift space. Moreover, we measure the linear Kaiser factor βlya=0.68+0.12-0.09 from the isotropic redshift space fit. Overall, we find evidence for a negative shift of the BAO peak at the ~ 3σ and ~ 3.3σ level in real and redshift space, respectively. This result is qualitatively in agreement with measurements from other cosmological tracers in underdense regions, such as the cosmic void distribution. This work sets new important theoretical constraints, in the light of ongoing and upcoming Lyα forest spectroscopic surveys, such as DESI, PFS, and WEAVE-QSO.