Quasar main sequence in the UV plane

TL;DR

This study explores the UV plane of active galaxies, revealing that after correcting for continuum curvature, the FWHM and EW of MgII behave similarly to Hβ, supporting models involving dust in the accretion disk atmosphere.

Contribution

It demonstrates that the UV plane trends differ from optical ones and shows how continuum correction aligns MgII behavior with Hβ, providing insights into the Broad Line Region structure.

Findings

FWHM(MgII) correlates with EW(MgII) due to continuum choice.

Correcting for UV continuum curvature aligns MgII behavior with Hβ.

Supports the dust-influenced Broad Line Region model.

Abstract

Active galaxies form a clear pattern in the optical plane showing the correlation between the Full Width Half Maximum (FWHM) of the H$\beta$ line and the ratio of the Equivalent Width (EW) of the optical FeII emission and the EW(H$\beta$). This pattern is frequently refereed to as the quasar main sequence. In this paper we study the UV plane showing FWHM of the MgII line against the ratio of EW of UV FeII emission to EW(MgII). We show that the UV plane trends are different, with the underlying strong correlation between FWHM(MgII) and EW(MgII). This correlation is entirely driven by the choice of the continuum used to measure EW(MgII). If we correct for the strong curvature in the UV continuum related to the character of emission expected from accretion disk, the behavior of the FWHM vs. EW for MgII becomes similar to the behaviour of H$\beta$. Such a similarity is expected since both lines belong to the Low Ionization group of emission lines and come from a similar region. Such a similarity and weak coupling between FWHM and EW of the corresponding line is then also consistent with the Broad Line Region model based on the presence of dust in the accretion disk atmosphere.