2018MNRAS.480.1796S

In recent years, several radio-loud narrow-line Seyfert 1 galaxies (RL-NLS1s) possessing relativistic jets have come into attention with their detections in very large baseline array (VLBA) and in γ-ray observations. In this paper we attempt to understand the nature of radio jets in NLS1s by examining the kpc-scale radio properties of, hitherto, the largest sample of 11101 optically selected NLS1s. Using 1.4 GHz FIRST, 1.4 GHz NVSS, 327 MHz WENNS, and 150 MHz TGSS catalogues we find the radio-detection of merely ∼4.5 per cent (498/11101) NLS1s, with majority (407/498 ∼ 81.7 per cent) of them being RL-NLS1s. Our study yields the highest number of RL-NLS1s and it can only be a lower limit. We find that the most of our radio-detected NLS1s are compact (<30 kpc), exhibit both flat as well as steep radio spectra, and are distributed across a wide range of 1.4 GHz radio luminosities (10²²-10²⁷ W Hz^–1). At the high end of radio luminosity our NLS1s often tend to show blazar-like properties such as compact radio size, flat/inverted radio spectrum, radio variability, and polarization. The diagnostic plots based on the mid-IR colours suggest that the radio emission in NLS1s is mostly powered by active galactic nuclei (AGNs), while nuclear star formation may have a significant contribution in NLS1s of low radio luminosities. The radio luminosity versus radio-size plot infers that the radio jets in NLS1s are either in the early evolutionary phase or possibly remain confined within the nuclear region due to low-power or intermittent AGN activity.