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

We investigate the mid-infrared (mid-IR) 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. 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 important contribution to the observed mid-IR flux in particular at the longest wavelengths (W3, at 12 μm, and W4, at 22 μm). 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 22 μm luminosities, corrected for the AGN contribution, we have then estimated the star formation rate (SFR) for 20 of these 'red' RL NLS1, finding values ranging from 10 to 500M⊙ yr-1. For the RLNLS1 showing bluer colours, instead, we cannot exclude the presence of a star-forming (SF) host galaxy although, on average, we expect a lower SFR. Studying the radio (1.4 GHz) to mid-IR (22 μm) flux ratios of the RL NLS1 in the sample, we found that in ~10 objects the SF 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 SF activity.
Part of this work was supported by the COST Action MP0905 ‘Black Holes in a Violent Universe’ and by the European Commission Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 267251 Astronomy Fellowships in Italy (AstroFIt). The authors acknowledge financial support from the Italian Ministry of Education, Universities and Research (PRIN2010-2011, grant no. 2010NHBSBE). Support from the Italian Space Agency is acknowledged by LB (contract ASI INAF NuSTAR I/037/12/0). SM acknowledges funding from the Spanish Ministry of Economy and Competitiveness under grant AYA2012-31447, which is partly funded by the FEDER programme. SM acknowledges financial support from the ARCHES project (7th Framework of the European Union, no. 313146).