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

TL;DR

This study uses simulations to compare emission line nebulae around obscured and unobscured AGN, testing the unified model by analyzing their properties and proposing joint observations to better constrain the model.

Contribution

It provides the first detailed simulation-based comparison of nebulae around different AGN types, highlighting observational strategies to test the unified model.

Findings

Type-II nebulae are less symmetric and have broader emission lines.

Luminosity and size of nebulae are similar across AGN types when measured at the halo virial radius.

Joint Ly$ extalpha$ and radio observations can help reveal the AGN ionization cone.

Abstract

The CGM around unobscured AGN has received much attention in recent years. Comparatively, nebulae associated with obscured AGN are less studied. Here, we simulate the Ly$\alpha$, H$\alpha$, and HeII 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 Ly$\alpha$ radiative transfer code, RASCAS. Overall, we find that the Ly$\alpha$ nebulae around the unobscured AGN (type-I nebulae) and obscured AGN (type-II nebulae) do not exhibit significant differences in the luminosity, area, and HeII/Ly$\alpha$ 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 $R\geq 10$ 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 Ly$\alpha$ nebulae and radio jets can help to reveal the ionization cone to probe the unified model. Our calculations suggest that this method requires $\geq 75$ type-II Ly$\alpha$ nebulae with current instruments to reach a confidence level of $\geq 95\%$.