The disc-like host galaxies of radio-loud Narrow-Line Seyfert 1s

TL;DR

This study reveals that radio-loud Narrow-Line Seyfert 1 galaxies predominantly reside in disc galaxies with bars and signs of mergers, challenging the idea that relativistic jets only originate from elliptical hosts.

Contribution

It provides the first detailed morphological analysis of RL-NLSy1 host galaxies, showing their disc nature and features associated with jet production, which is a novel insight into AGN host galaxy types.

Findings

72% of hosts are disc galaxies.

Over half of the hosts show bars promoting gas inflows.

75% of gamma-NLSy1 hosts show signs of minor mergers.

Abstract

Until recently, relativistic jets were ubiquitously found to be launched from giant elliptical galaxies. However, the detection by the Fermi-LAT of gamma-ray emission from radio-loud narrow-line Seyfert 1 (RL-NLSy1) galaxies raised doubts on this relation. Here, we morphologically characterize a sample of 29 RL-NLSy1s (including 12 gamma-ray emitters, gamma-NLSy1s) in order to find clues on the conditions needed by AGN to produce relativistic jets. We use deep near-infrared images from the Nordic Optical Telescope and the ESO VLT to analyze the surface brightness distribution of the galaxies in the sample. We detected 72% of the hosts (24% classified as gamma-NLSy1s). Although we cannot rule out that some RL-NLSy1s are hosted by dispersion supported systems, our findings strongly indicate that RL-NLSy1s hosts are preferentially disc galaxies. 52% of the resolved hosts (77% non-gamma-emitters and 20% gamma-emitters) show bars with morphological properties (long and weak) consistent with models that promote gas inflows, which might trigger nuclear activity. The extremely red bulges of the gamma-NLSy1s, and features that suggest minor mergers in 75% of their hosts might hint to the necessary conditions for gamma-rays to be produced. Among the features that suggest mergers in our sample, we find 6 galaxies that show offset stellar bulges with respect to their AGN. When we plot the nuclear versus the bulge magnitude, RL-NLSy1s locate in the low-luminosity end of flat-spectrum radio quasars (FSRQs), suggesting a similar accretion mode between these two AGN types.