Parent population of flat-spectrum radio-loud narrow-line Seyfert 1 galaxies

TL;DR

This study investigates the parent population of flat-spectrum radio-loud NLS1 galaxies by analyzing candidate sources and their properties to understand their relation to beamed NLS1s and the nature of their parent population.

Contribution

The paper tests hypotheses about the parent sources of flat-spectrum radio-loud NLS1s by analyzing optical spectra and statistical tests, providing insights into their possible identities.

Findings

Steep-spectrum radio-loud NLS1s may be the parent population at larger angles.

Disk-hosted radio-galaxies with low black hole mass and high Eddington ratio are potential parent sources.

Radio-quiet NLS1s' role in the parent population remains uncertain.

Abstract

Flat-spectrum radio-loud Narrow-Line Seyfert 1 galaxies (NLS1s) are a recently discovered class of $\gamma$-ray emitting Active Galactic Nuclei (AGN), that exhibit some blazar-like properties which are explained with the presence of a relativistic jet viewed at small angles. When blazars are observed at larger angles they appear as radio-galaxies, and we expect to observe an analogue parent population for beamed NLS1s. However, the number of known NLS1s with the jet viewed at large angles is not enough. Therefore, we tried to understand the origin of this deficit. Current hypotheses about the nature of parent sources are steep-spectrum radio-loud NLS1s, radio-quiet NLS1s and disk-hosted radio-galaxies. To test these hypotheses we built three samples of candidate sources plus a control sample, and calculated their black hole mass and Eddington ratio using their optical spectra. We then performed a Kolmogorov-Smirnov statistical test to investigate the compatibility of our different samples with a beamed population. Our results indicate that, when the inclination angle increases, a beamed source appears as a steep-spectrum radio-loud NLS1, or possibly even as a disk-hosted radio-galaxy with low black hole mass and high Eddington ratio. Further investigations, involving larger complete samples and observations at radio frequency, are needed to understand the incidence of disk-hosted radio-galaxies in the parent population, and to assess whether radio-quiet NLS1s can play a role, as well.