Mg II line properties in lobe-dominated quasars

TL;DR

This study examines Mg ii emission line properties in lobe-dominated quasars, revealing that Mg ii line width is not correlated with inclination, suggesting a non-thin disk emission region and potential for unbiased black hole mass estimates.

Contribution

It provides new insights into the geometry of Mg ii emission regions in lobe-dominated quasars and their independence from inclination effects, contrasting with previous findings in general quasars.

Findings

No correlation between Mg ii line width and inclination.

Mg ii equivalent width shows no strong anisotropy evidence.

High inclination quasars exhibit higher [O iii] equivalent widths.

Abstract

We investigate the relationships between Mg ii {\lambda}2798 emission line properties, as well as between these properties and inclination angle and Fe ii strength, in a lobe-dominated quasar sample. We find no correlation between Mg ii line width and inclination, unlike previous studies of the general quasar population. This suggests that the Mg ii emission region in these objects is not of a thin disk geometry, however the average equivalent width of the line negates a spherical alternative. A positive correlation between Mg ii equivalent width and inclination cannot be ruled out, meaning there is no strong evidence that Mg ii emission is anisotropic. Since thin disk emission would be highly directional, the geometric implications of these two findings are compatible. The lack of line width correlation with inclination may also indicate that Mg ii is useful for estimating black hole masses in lobe-dominated quasar samples, since it is unbiased by orientation. Some quasars in our sample have almost edge-on viewing angles and therefore cannot have a smooth toroidal obscurer co-planar with the accretion disk. Alternatives may be a distorted dusty disk or a clumpy obscurer. This could result from the sample selection bias towards high inclination objects, rather than intrinsic differences between lobe-dominated and typical quasars. Five objects have visible [O iii] allowing equivalent width calculation, revealing it to be higher than in typical quasars. Since these objects are of high inclination, this finding supports the positive correlation between [O iii] equivalent width and inclination found in a previous study.