Spectral range around the intercombination O IV lines to estimate the electron density. The Si IV line is not part of this range and would peak around a Doppler shift of 0 km s⁻¹ . The orange and blue lines show the Doppler-shift range we used to estimate the total intensity of O IV 1400 Å and the total intensity of O IV 1401 Å, respectively. The black dashed horizontal lines show the continuum level.