Peaked sources and narrow-line Seyfert 1s: a love story

TL;DR

This paper reviews the connection between peaked radio sources and narrow-line Seyfert 1 galaxies, highlighting their shared features, potential unification, and recent discoveries impacting their classification and understanding.

Contribution

It provides a comprehensive overview of the orientation-based unification of PS and NLS1s and discusses how new findings challenge traditional classification methods.

Findings

Shared characteristics between PS and NLS1s.

Potential for some gamma-ray emitting NLS1s to be hidden among PS.

Absorbed radio jets in NLS1s may alter classification metrics.

Abstract

The first similarities between peaked sources (PS) and narrow-line Seyfert 1 (NLS1) galaxies were noticed already twenty years ago. Nowadays, it is known that several sources can share both classifications, and that part of the parent population of $\gamma$-ray emitting NLS1s could be hiding among PS. In this brief review, we describe how and why this orientation-based unification was developed. We also show how the recent discovery of absorbed radio jets in NLS1s, basically invisible at frequencies below 10 GHz, could impact our knowledge of PS and, in particular, render the widely used radio-loudness parameter obsolete.