The disc-like host galaxies of radio-loud narrow-line Seyfert 1s

Until recently, relativistic jets were ubiquitously found to be launched from giant elliptical galaxies. However, the detection by the Fermi-LAT of γ -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 γ -emitters, γ -NLSy1s) in order to find clues on the conditions needed by active galactic nuclei (AGNs) to produce relativistic jets. We use deep near-infrared images from the Nordic Optical Telescope and the ESO VLT to analyse the surface brightness distribution of the galaxies in the sample. We detected 72 per cent of the hosts (24 per cent classified as γ -NLSy1s). Although we cannot rule out that some RL-NLSy1s are hosted by dispersion-supported systems, our findings strongly indicate that RL-NLSy1 hosts are preferentially disc galaxies. 52 per cent of the resolved hosts (77 per cent non-γ -emitters and 20 per cent γ -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 γ -NLSy1s, and features that suggest minor mergers in 75 per cent of their hosts, might hint to the necessary conditions for γ -rays to be produced. Among the features that suggest mergers in our sample, we find six galaxies that show offset stellar bulges with respect to their AGNs. When we plot the nuclear versus the bulge magnitude, RL-NLSy1s locate in the low-luminosity end of flat spectrum radio quasars, suggesting a similar accretion mode between these two AGN types.