Spatially resolved electron density in the Narrow Line Region of z<0.02 radio AGNs

Abstract

Although studying outflows in the host galaxies of AGN have become the forefront of extra-galactic astronomy in recent years, estimating the energy associated with these outflows have been a major challenge. Determination of the energy associated with an outflow often involves an assumption of uniform density in the NLR, which span a wide range in literature leading to large systematic uncertainties in energy estimation. 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 S7 survey and understand the origin and values of the observed density structures to reduce the systematic uncertainties in outflow energy estimation. We use the ratio of the [SII]6716,6731 emission lines to derive spatially resolved electron densities (<50-2000 cm$^{-3}$). Using optical IFU observations, 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 HII regions derived from the narrow H$\alpha$ component, ionization maps from [OIII], 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. 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 center, 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.