Testing the AGN unified model with simulated emission lines from the circumgalactic medium (CGM)

The circumgalactic medium around unobscured active galactic nucleus (AGN) has received much attention in recent years. Comparatively, nebulae associated with obscured AGN are less studied. Here, we simulate the Lyα, Hα, and He II nebulae around the two types of AGN at z = 2 − 3 with ten massive systems from the FIRE simulations based on the unified model to show their differences and to test if they can be used to constrain the AGN model. We post-process the data with the CLOUDY and the Lyα radiative transfer code, RASCAS. Overall, we find that the Lyα nebulae around the unobscured AGN (type-I nebulae) and obscured AGN (type-II nebulae) do not exhibit significant differences in the luminosity, area, and He II/Lyα when the simulated cutout is set to the halo virial radius. Whereas, the type-II nebulae exhibit less symmetric morphologies, flatter surface brightness profiles, and larger emission line widths (at R ≥ 10 kpc) than those of the type-I nebulae. These nebulae properties exhibit complicated correlations with the AGN, indicating that nebulae observations can be applied to constrain the AGN engine. However, independent observations on nebulae in the mentioned emissions are insufficient to test the unified model as a priori in observations is not possible to know the direction and opening angle of the ionization cone. We prompt that the joint observations of Lyα nebulae and radio jets can help to reveal the ionization cone to probe the unified model. Our calculations suggest that this method requires ≥ 75 type-II Lyα nebulae with current instruments to reach a confidence level of ≥ 95 per cent.