Morphology of AGN emission-line regions in SDSS-IV MaNGA survey

Abstract

Extended narrow-line regions (NLRs) around active galactic nuclei (AGN) are shaped by the distribution of gas in the host galaxy and by the geometry of the circumnuclear obscuration, and thus they can be used to test the AGN unification model. In this work, we quantify the morphologies of the NLRs in 308 nearby AGN (z = 0 - 0.14, L-bol similar to 10(42.4 -) (44.1) erg s(-1)) from the MaNGA survey. Based on the NLR maps, we find that a large fraction (81 per cent) of these AGN have bi-conical NLR morphology. The distribution of their measured opening angles suggests that the intrinsic opening angles of the ionization cones have a mean value of 85 degrees-98 degrees with a finite spread of 39 degrees-44 degrees (1 sigma). Our inferred opening angle distribution implies a number ratio of type I to type II AGN of 1:1.6-2.3, consistent with other measurements of the type I/type II ratio at low AGN luminosities. Combining these measurements with the WISE photometry data, we find that redder mid-IR colour (lower effective temperature of dust) corresponds to stronger and narrower photoionized bicones. This relation is in agreement with the unification model that suggests that the bi-conical NLRs are shaped by a toroidal dusty structure within a few pc from the AGN. Furthermore, we find a significant alignment between theminor axis of host galaxy discs and AGN ionization cones. Together, these findings suggest that obscuration on both circumnuclear (similar to pc) and galactic (similar to kpc) scales is important in shaping and orienting the AGN NLRs.