WISE colours and star-formation in the host galaxies of radio-loud narrow-line Seyfert 1

TL;DR

This study analyzes the mid-infrared properties of 42 radio-loud narrow-line Seyfert 1 galaxies using WISE data, revealing significant host-galaxy star formation and contributions from jets and dusty tori.

Contribution

It provides the largest mid-IR analysis of RL NLS1 galaxies, quantifying star formation rates and disentangling emission components from jets, host galaxies, and AGN structures.

Findings

Half of the sources show very red mid-IR colors indicating starburst activity.

Star formation rates range from 10 to 500 solar masses per year.

In about 10 objects, star formation may dominate radio emission.

Abstract

We investigate the mid-infrared properties of the largest (42 objects) sample of radio-loud narrow-line Seyfert 1 (RL NLS1) collected to date, using data from the Wide-field Infrared Survey Explorer (WISE). We analyse the mid-IR colours of these objects and compare them to what is expected from different combinations of AGN and galaxy templates. We find that, in general, the host-galaxy emission gives an importan contribution to the observed mid-IR flux in particular at the longest wavelengths (W3, at 12micron, and W4, at 22micron). In about half of the sources (22 objects) we observe a very red mid-IR colour (W4-W3>2.5) that can be explained only using a starburst galaxy template (M82). Using the 22micron luminosities, corrected for the AGN contribution, we have then estimated the star-formation rate for 20 of these "red" RL NLS1, finding values ranging from 10 to 500 Msun/y. For the RL NLS1 showing bluer colours, instead, we cannot exclude the presence of a star-forming host galaxy although, on average, we expect a lower star-formation rate. Studying the radio (1.4GHz) to mid-IR (22micron) flux ratios of the RL NLS1 in the sample we found that in ~10 objects the star-forming activity could represent the most important component also at radio frequencies, in addition (or in alternative) to the relativistic jet. We conclude that both the mid-IR and the radio emission of RL NLS1 are a mixture of different components, including the relativistic jet, the dusty torus and an intense star-forming activity.