Spatially resolved electron density in the narrow line region of z < 0.02 radio AGNs

¹ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
e-mail: dkakkad@eso.org
² Ludwig-Maximilian-Universität, Professor-Huber-Platz 2, 80539 München, Germany
³ European Southern Observatory, Alonso de Cordova 3107, Vitacura, Casilla 19001, Santiago de Chile, Chile
⁴ Research School for Astronomy & Astrophysics, Australian National University, Cotter Road, Weston Creek, ACT 2611, Australia
⁵ ARC Center of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Australia
⁶ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse, 85748 Garching, Germany
⁷ National Centre for Radio Astrophysics, Tata Institute of Fundamental Research, S. P. Pune University Campus, Post Bag 3, Ganeshkhind, Pune 411007, India
⁸ Gemini Observatory, Northern Operations Center, 670 N. A’ohoku Pl., Hilo, HI 96720, USA
⁹ Max Planck Institute for Astronomy, Königstuhl, 17, 69117 Heidelberg, Germany

Received: 7 February 2018
Accepted: 6 June 2018

Abstract

Context. Although studying outflows in the host galaxies of active galactic nuclei (AGNs) have moved to the forefront of extragalactic astronomy in recent years, estimating the energy associated with these outflows has been a major challenge. Determining the energy associated with an outflow often involves an assumption of uniform density in the narrow line region (NLR), which spans a wide range in the literature, leading to large systematic uncertainties in energy estimation.

Aims. In this paper we present electron density maps for a sample of outflowing and non-outflowing Seyfert galaxies at z < 0.02 drawn from the Siding Spring Southern Seyfert Spectroscopic Snapshot Survey (S7) and try to understand the origin and values of the observed density structures to reduce the systematic uncertainties in outflow energy estimation.

Methods. We use the ratio of the [S II]λ6716,6731 emission lines to derive spatially resolved electron densities (≲50–2000 cm⁻³). Using optical Integral Field Unit observations from the Wide Field Spectrograph (WiFeS), we are able to measure densities across the central 2–5 kpc of the selected AGN host galaxies. We compare the density maps with the positions of the H II regions derived from the narrow Hα component, ionization maps from [O III] and spatially resolved BPT diagrams to infer the origin of the observed density structures. We also use the electron density maps to construct density profiles as a function of distance from the central AGN.

Results. We find a spatial correlation between the sites of high star formation and high electron density for targets without an active ionized outflow. The non-outflowing targets also show an exponential drop in the electron density as a function of distance from the centre, with a mean exponential index of ∼0.15. The correlation between the star forming sites and electron density ceases for targets with an outflow. The density within the outflowing medium is not uniform and shows both low- and high-density sites, most likely due to the presence of shocks and highly turbulent medium. We compare these results in the context of previous results obtained from fibre and slit spectra.