Impact of the local conditions of temperature and ionization timescale on the energy at rest of the silicon line used to obtain the velocity in the LoS, V_z.

Left: Background map is the theoretical peak energy of the silicon line in the (kT, τ) space with contours in black. This is compared with the kT and τ local spectral measurements from Williams et al. (2017). Right: Assuming a LoS velocity of 4000 km s⁻¹, these panels show the associated velocity uncertainty if the energy of reference changes due temperature and ionization timescale variations.