Spatially resolved circumnuclear coronal $[{\rm Fe\,VII}]\,\lambda6087$ emission in nearby Seyfert galaxies

Abstract

Coronal lines are forbidden emission lines with a ionisation potential $\chi\gtrsim100\,{\rm eV}$. They are linked to energetic phenomena triggered by AGNs in the circumnuclear medium. We present the first high-angular-resolution integral-field analysis of the $[{\rm Fe\,VII}]\,\lambda6087$ coronal line in a sample of four nearby low-inclination Seyfert galaxies (three of Type 1 and one of Type 2). The data were obtained with the adaptive-optics-assisted mode of MUSE, and have angular resolutions of $0.06-0.18\,{\rm arcsec}$, allowing us to probe regions down to a few tens of parsecs in size. In three of the objects, we find a resolved coronal emission in a relatively compact configuration ($\lesssim200\,{\rm pc}$ in radius). The coronal emission is smooth and symmetric with respect to the centre of the galaxy, except for one object where an off-nucleus clump of emission is detected. Through the use of spectroastrometry we find that the $[{\rm Fe\,VII}]$ outflow of the Type 2 AGN host has a redshifted and a blueshifted component whose centroids are separated by $\sim20\,{\rm pc}$. We interpret this as evidence that some of the coronal emission comes from the inner part of a biconic outflow, also seen in low-ionisation lines. Similar $[{\rm Fe\,VII}]$ properties are found in two of the Type 1 AGN hosts, but with a much smaller separation between the centroids of the lobes of the outflow ($<7\,{\rm pc}$). This could be due to the foreshortening of the axis of the bicone in Type 1 objects. We also studied the spectrum of the unresolved nuclear source and found that in three out of four galaxies a fraction of at least $\sim60\%$ of the $[\textrm{Fe VII}]$ emission has kinematics similar to those of $[{\rm O\,III}]$. We conclude that part of the coronal emission within the inner few tens of parsecs is co-spatial and shares kinematics with the outflows as traced by lower-ionisation lines.