Emulating the Lyman-Alpha forest 1D power spectrum from cosmological simulations: new models and constraints from the eBOSS measurement

Abstract We present the Lyssa suite of high-resolution cosmological simulations of the Lyman- α forest designed for cosmological analyses. These 18 simulations have been run using the Nyx code with 4096 3 hydrodynamical cells in a 120 Mpc (∼ 81 Mpc/ h ) comoving box and individually provide sub-percent level convergence of the Lyman- α forest 1d flux power spectrum. We build a Gaussian process emulator for the Lyssa simulations in the lym1d likelihood framework to interpolate the power spectrum at arbitrary parameter values. We validate this emulator based on leave-one-out tests and based on the parameter constraints for simulations outside of the training set. We also perform comparisons with a previous emulator, showing a percent level accuracy and a good recovery of the expected cosmological parameters. Using this emulator we derive constraints on the linear matter power spectrum amplitude and slope parameters A Ly α and n Ly α . While the best-fit Planck ΛCDM model has A Ly α = 8.79 and n Ly α = -2.363, from DR14 eBOSS data we find that A Ly α < 7.6 (95% CI) and n Ly α = -2.369 ± 0.008. The low value of A Ly α , in tension with Planck, is driven by the correlation of this parameter with the mean transmission of the Lyman- α forest. This tension disappears when imposing a well-motivated external prior on this mean transmission, in which case we find A Ly α = 9.8 ± 1.1 in accordance with Planck.