Balmer line shifts in quasars

TL;DR

This paper reviews the phenomenology of Balmer lines in type 1 quasars, discussing their physical interpretation, peculiar profiles, and the need for further dynamical modeling to understand their origins.

Contribution

It provides a comprehensive review of Balmer line features in quasars, highlighting the complexity and the need for multi-line analysis along the 4D Eigenvector 1 sequence.

Findings

Peculiar Balmer line profiles are still not fully understood.

Multiple dynamical models can explain the observed line shifts.

Inter-line comparison may advance understanding of quasar physics.

Abstract

We offer a broad review of Balmer line phenomenology in type 1 active galactic nuclei, briefly sum- marising luminosity and radio loudness effects, and discussing interpretation in terms of nebular physics along the 4D eigenvector 1 sequence of quasars. We stress that relatively rare, peculiar Balmer line profiles (i.e., with large shifts with respect to the rest frame or double and multiple peaked) that start attracted attentions since the 1970s are still passable of multiple dynamical interpretation. More mainstream objects are still not fully understood as well, since competing dynamical models and geometries are possible. Further progress may come from inter-line comparison across the 4D Eigenvector 1 sequence.