A spectral and quantitative comparison of FWHM between the model and sources. Top panel shows the normalized composite spectrum using the 13 UV-bright sources used in this work (black). The synthetic spectrum of a BPASS+VMS model is also shown in blue and assumes an upper mass cutoff of the IMF of M_up = 225 M_⊙ with a continuous star-formation over 10 Myr (using the VMS models of Martins & Palacios 2022). Spectral features associated with only VMS are marked in red while the spectral features produced by VMS and normal massive stars are marked in green. The bottom left panel shows a zoom-in at the He II line. BPASS models with and without VMS are also shown (blue and red, respectively). The bottom right panel illustrates the differences between the He II line widths (FWHM) measured in our sources (black) and model spectra (blue region).