Results from the atmospheric retrieval of WASP-121 b. The left corner plot shows the posteriors of the retrieved parameters, with the marginalised 1D posteriors on the diagonal and the 2D posteriors on the off-diagonals. The contours on the 2D posteriors represent the 1, 2, and 3 σ uncertainties. The first four parameters are the base ten logarithm of volume mixing ratios of the four detected species: H₂O, OH, Fe, and Mg. The 1D posteriors for the retrieved abundances show relatively broad distributions. However, 2D posteriors between abundances depict a strong positive correlation, indicating that the abundance ratios are better constrained. logH⁻ and loge⁻ are the volume mixing ratio of the H⁻ radical and of electrons, respectively, used as broadband opacities. Only a weak upper bound is obtained on logH⁻, while the posterior abundance of loge⁻ spans the full prior. The grey lines on the K_p and V_sys parameters are the expected orbital velocities of the planet (Bourrier et al. 2020; Brown et al. 2018). Both orbital parameters are consistent with expectations. ΔV_sys,H2O is the additional V_sys shift added to H₂O. The posterior is constrained to a value above 0, indicating that the retrieval also finds the shift seen in the cross-correlation maps (Fig. 4). The top right plot is the retrieved vertical temperature profile, with the shaded areas representing the 1 and 2 σ uncertainties. The profile has a temperature inversion, which is expected for UHJs.