Mid-Infrared Properties of Narrow-Line Seyfert 1 Galaxies Detected by LoTSS DR2

Abstract

Narrow-line Seyfert 1 galaxies (NLS1s), a subclass of active galactic nuclei (AGNs) at early stage of accretion process, are also found to host relativistic jets. However, currently known jetted NLS1s are rare. The majority of NLS1s are undetected at radio band. The radio detection rate of NLS1s raises with the LOFAR Two-metre Sky Survey (LoTSS), which gives a good opportunity to find more jetted NLS1s. The better sensitivity brings another question of whether the radio emission of NLS1s with low radio luminosity originates from jet activity. In order to clarify the origin of radio emission for NLS1s, and search for more jetted NLS1s, we explore the mid-infrared properties of LoTSS detected NLS1s by comparing them with known jetted AGNs and star forming galaxies (SFGs), which locate above and on the well studied radio/far-infrared correlation, respectively. The majority of NLS1s show mid-infrared (MIR) excess compared with SFGs. Their radio emission shows significant correlation with MIR emission. In MIR color-color diagram, NLS1s are overlapped with flat spectrum radio quasars, but well separated from SFGs and optically selected radio galaxies. The flux ratio between radio and MIR emission of these NLS1s is also similar with a radio quiet quasar with weak jet. These results imply substantial contributions from AGN activities for both radio and MIR emission of NLS1s. A small fraction of NLS1s with relatively higher radio luminosity locate in similar region with blazars in radio-MIR diagram, which suggests that the radio emission of these NLS1s is dominated by jet. We obtain a sample of jetted NLS1 candidates through their radio excess in radio-MIR diagram.