The completed SDSS-IV extended Baryon Oscillation Spectroscopic Survey: BAO and RSD measurements from the anisotropic power spectrum of the quasar sample between redshift 0.8 and 2.2

We measure the clustering of quasars of the final data release (DR16) of eBOSS. The sample contains $343\, 708$ quasars between redshifts 0.8 ≤ z ≤ 2.2 over $4699\, \mathrm{deg}^2$. We calculate the Legendre multipoles (0,2,4) of the anisotropic power spectrum and perform a BAO and a Full-Shape (FS) analysis at the effective redshift z_eff = 1.480. The errors include systematic errors that amount to 1/3 of the statistical error. The systematic errors comprise a modelling part studied using a blind N-body mock challenge and observational effects studied with approximate mocks to account for various types of redshift smearing and fibre collisions. For the BAO analysis, we measure the transverse comoving distance D_M(z_eff)/r_drag = 30.60 ± 0.90 and the Hubble distance D_H(z_eff)/r_drag = 13.34 ± 0.60. This agrees with the configuration space analysis, and the consensus yields: D_M(z_eff)/r_drag = 30.69 ± 0.80 and D_H(z_eff)/r_drag = 13.26 ± 0.55. In the FS analysis, we fit the power spectrum using a model based on Regularised Perturbation Theory, which includes redshift space distortions and the Alcock-Paczynski effect. The results are D_M(z_eff)/r_drag = 30.68 ± 0.90 and D_H(z_eff)/r_drag = 13.52 ± 0.51 and we constrain the linear growth rate of structure f(z_eff)σ₈(z_eff) = 0.476 ± 0.047. Our results agree with the configuration space analysis. The consensus analysis of the eBOSS quasar sample yields: D_M(z_eff)/r_drag = 30.21 ± 0.79, D_H(z_eff)/r_drag = 3.23 ± 0.47, and f(z_eff)σ₈(z_eff) = 0.462 ± 0.045 and is consistent with a flat ΛCDM cosmological model using Planck results.