2020ApJ...900...64S

Active galaxies form a clear pattern in the optical plane showing the correlation between the FWHM of the Hβ line and the ratio of the equivalent width (EW) of the optical Fe II emission and the broad EW(Hβ). This pattern is frequently referred to as the quasar main sequence. In this paper, we study the UV plane showing the FWHM of Mg II line against the ratio of the EW of UV Fe II emission to the broad EW(Mg II). We show that the UV plane trends are different, with the underlying strong correlation between the FWHM(Mg II) and the EW(Mg II). This correlation is entirely driven by the choice of the continuum used to measure the EW(Mg II). If instead of the observationally determined continuum, we use a theoretically motivated power law extrapolated from the wide wavelength range, the behavior of the FWHM versus EW for Mg II becomes similar to the behavior for Hβ. Such a similarity is expected since both the lines belong to the low-ionization group of emission lines and come from a similar region. We discuss the behavior of the lines in the context of the broad line region model based on the presence of dust in the accretion disk atmosphere.