The curious activity in the nucleus of NGC 4151: jet interaction causing variability?

Abstract

A key characteristic of many active galactic nuclei (AGN) is their variability, but its origin is poorly understood, especially in the radio domain. Williams et al. (2017) reported a ∼50 per cent increase in peak flux density of the AGN in the Seyfert galaxy NGC 4151 at 1.5 GHz with the e-MERLIN array. We present new high resolution e-MERLIN observations at 5 GHz and compare these to archival MERLIN observations to investigate the reported variability. Our new observations allow us to probe the nuclear region at a factor three times higher-resolution than the previous e-MERLIN study. We separate the core component, C4, into three separate components: C4W, C4E and X. The AGN is thought to reside in component C4W, but this component has remained constant between epochs within uncertainties. However, we find that the Eastern-most component, C4E, has increased in peak flux density from 19.35±1.10 to 37.09±1.86 mJy/beam, representing a 8.2σ increase on the MERLIN observations. We attribute this peak flux density increase to continued interaction between the jet and the emission line region (ELR), observed for the first time in a low-luminosity AGN such as NGC 4151. We identify discrete resolved components at 5 GHz along the jet axis, which we interpret as areas of jet-ELR interaction.© 2020 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society ; We thank the anonymous reviewer for their comments and revisions, which greatly improved the quality of this manuscript. We acknowledge funding from the Mayflower Scholarship from the University of Southampton afforded to DW to complete this work. This work was supported by the Oxford Centre for Astrophysical Surveys, which is funded through generous support from the Hintze Family Charitable Foundation. The research leading to these results has received funding from the European Union's Horizon 2020 Programme under the AHEAD project (grant agreement no. 654215). This publication has also received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement no. 730562 [RadioNet]. IMcH thanks the Royal Society for the award of a Royal Society Leverhulme Trust Senior Research Fellowship. RDB and IMcH also acknowledge the support of STFC under grant [ST/M001326/1]. FP acknowledges support from grant PRIN-INAF SKA-CTA 2016. GB acknowledges financial support under the INTEGRAL ASI-INAF agreement 2013-025-R1 and under the INTEGRAL ASI-INAF agreement 2019-35-HH.0. JHK acknowledges financial support from the European Union's Horizon 2020 research and innovation programme under Marie Skłodowska-Curie grant agreement no. 721463 to the SUNDIAL ITN network, and from the Spanish Ministry of Science, Innovation and Universities (MCIU) and the European Regional Development Fund (FEDER) under the grant with reference AYA2016-76219-P, from IAC project P/300724, financed by the Ministry of Science, Innovation and Universities, through the State Budget and by the Canary Islands Department of Economy, Knowledge and Employment, through the Regional Budget of the Autonomous Community, and from the Fundacion BBVA under ´ its 2017 programme of assistance to scientific research groups, for the project 'Using machine-learning techniques to drag galaxies from the noise in deep imaging'. DMF wishes to acknowledge funding from an STFC Q10 consolidated grant [ST/M001334/1]. EB and JW acknowledge support from the UK's Science and Technology Facilities Council grant numbers [ST/M503514/1] and [ST/M001008/1], respectively. JM acknowledges financial support from the State Agency for Research of the Spanish MCIU through the 'Center of Excellence Severo Ochoa' award to the Instituto de Astrof´ısica de Andaluc´ıa (SEV-2017-0709) and from the grant RTI2018-096228-B-C31 (MICIU/FEDER, EU). CGM acknowledges financial support from STFC. We also acknowledge Jodrell Bank Centre for Astrophysics, which is funded by the STFC. eMERLIN and formerly, MERLIN, is a National Facility operated by the University of Manchester at Jodrell Bank Observatory on behalf of STFC. MP acknowledges the support from the Royal Society Newton International Fellowship. ; Peer reviewed