Investigating kpc-scale radio emission properties of narrow-line Seyfert
  1 galaxies
  In recent years, several Radio-Loud Narrow-Line Seyfert 1 galaxies (RL-NLS1)
possessing relativistic jets have come into attention with their detections in
Very Large Baseline Array (VLBA) and in $\gamma$-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 $\sim$ 4.5 per
cent (498/11101) NLS1s, with majority (407/498 $\sim$ 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 (10$^{22}$ $-$
10$^{27}$ 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 AGN, 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.
