Portion of the spectrum around Lyα. Geocoronal Lyα airglow contaminates the spectrum significantly at −1283 km s⁻¹ relative to M87 (indicated with the dashed vertical line), and Galactic Lyα absorption also affects the spectrum considerably. We model the transmission through the Galactic disk with the blue dashed line, assuming a neutral Hydrogen column toward M87 of 1.4 × 10²⁰ cm⁻² (Danforth et al. 2016). We fit the Lyα from M87 with two Gaussians – one positive containing most of the line flux and a small negative one which models the self-absorption – and multiply both Gaussians by the Galactic transmission profile. The red solid line is the overall model and the red dotted line shows the model without Galactic absorption. The lower panel shows the residuals between the model and the data. There are several small-scale features in the Lyα which are not captured with our simple model. In order to estimate uncertainties in model parameters, we include the rms of these residuals as a systematic uncertainty term in the error budget, which changes the χ² from 182.8 to 39.1, while keeping the best-fit model parameters essentially unchanged.