Discovering AGN-driven winds through their infrared emission - I. General method and wind location

Large-scale outflows of different gas phases are ubiquitous in the host galaxies of active galactic nuclei (AGNs). Despite their many differences, they share a common property - they all contain dust. The dust is carried with the outflow, heated by the AGN, and emits at infrared wavelengths. This paper shows that the infrared emission of this dust can be used to detect AGN outflows and derive their properties. We use a sample of ∼4000 type II AGNs and compare the infrared properties of systems that show a spectroscopic signature of ionized gas outflows to those of systems that do not. We detect an additional mid-infrared emission component in galaxies with spectroscopically discovered winds, and attribute it to the dust in the outflow. This new component offers novel constraints on the outflow properties, such as its mean location and covering factor.We measure the location of the outflow for ∼1700 systems, with the distribution showing a prominent peak around r ∼ 200 pc, a tail that extends to large distances (∼1 kpc), and no objects with a location smaller than 10 pc. The covering factor of the wind shows a wide distribution, which is centred around 0.1, with 24 per cent (8 per cent) of the winds showing covering factors larger than 0.2 (0.5). The dust emission is not sensitive to various systematics affecting optically selected outflows, and can be used to estimate the mass outflow rate in thousands of galaxies with only 1D spectra.