Unravelling the origin of extended radio emission in narrow-line Seyfert 1 galaxies with the JVLA

Abstract

Narrow-line Seyfert 1 (NLS1) galaxies are believed to be active galactic nuclei (AGN) in the early stages of their evolution. Some dozens of them have been found to host relativistic jets, whilst the majority of NLS1s has not even been detected in radio, emphasising the heterogeneity of the class in this band. In this paper, our aim is to determine the predominant source of radio emission in a sample of 44 NLS1s, selected based on their extended kpc-scale radio morphologies at 5.2 GHz. We accomplish this by analysing their spatially resolved radio spectral index maps, centred at 5.2 GHz, as the spectral index carries information about the production mechanisms of the emission. In addition, we utilise several diagnostics based on mid-infrared emission to estimate the star formation activity of their host galaxies. These data are complemented by archival data to draw a more complete picture of each source. We find an extraordinary diversity among our sample. Approximately equal fractions (∼10-12 sources) of our sources can be identified as AGN-dominated, composite, and host-dominated. Among the AGN-dominated sources are a few NLS1s with very extended jets, reaching distances of tens of kpc from the nucleus. One of these, J0814+5609, hosts the most extended jets found in an NLS1 so far. We also identify five NLS1s that could be classified as compact steep-spectrum sources. In addition, one source shows a possible kpc-scale relic reaching well outside the host galaxy and restarted nuclear activity, and one could belong to the NLS1 subclass hosting relativistic jets that seems to be absorbed at lower radio frequencies (< 10 GHz). We further conclude that due to the variety seen in NLS1s simple proxies, such as the star formation diagnostics also employed in this paper, and the radio loudness parameter, are not ideal tools for characterising NLS1s. We emphasise the necessity of examining NLS1s as individuals, instead of making assumptions based on their classification. When these issues are properly taken into account, NLS1s offer an exceptional environment to study the interplay of the host galaxy and several AGN-related phenomena, such as jets and outflows.

Radio spectra of narrow-line Seyfert 1 galaxies observed with ATCA and VLASS

“…Radio observations of RL NLS1s with the Very Long Baseline Array (VLBA) show that they exhibit pc-scale jets, compact morphologies, flat or inverted spectra, high brightness temperatures, and a significant degree of polarization (Abdo et al 2009a,b;Gu et al 2015;Lister et al 2018). Unlike FSRQs which show these jet features on kpc scales, NLS1s rarely harbor extended radio emission and only exhibit an unresolved core on kpc scales (Berton et al 2018;Chen et al 2020;Järvelä et al 2021b). Radio monitoring campaigns also reveal the presence of fast and intense variability which is a common feature of relativistic jets (Foschini et al 2012;Angelakis et al 2015;Fuhrmann et al 2016;Lähteenmäki et al 2017Lähteenmäki et al , 2018.…”