Apparent counter-rotation in the torus of NGC 1068: influence of an asymmetric wind

TL;DR

This paper models the apparent counter-rotation in NGC 1068's torus as caused by asymmetric radiation pressure wind, using simulations and ray-tracing to replicate observed velocity maps.

Contribution

It introduces a comprehensive simulation framework combining N-body, ray-tracing, and semi-analytical models to explain counter-rotation due to asymmetric winds.

Findings

Model reproduces observed counter-rotation in NGC 1068

Asymmetric wind influences cloud velocities in the torus

Simulation results align with ALMA observations

Abstract

The recent ALMA maps together with observations of H$_2$O maser emission seem to suggest the presence of a counter-rotation in the obscuring torus of NGC 1068. We propose to explain this phenomenon as due to the influence of a wind, considered as radiation pressure, and the effects of torus orientation. In order to test this idea: 1. we make $N$-body simulation of a clumpy torus taking into account mutual forces between particles (clouds); 2. we apply ray-tracing algorithm with the beams from the central engine to choose the clouds in the torus throat that can be under direct influence of the accretion disk emission; 3. we use semi-analytical model to simulate the influence of the asymmetrical radiation pressure (wind) forced on the clouds in the torus throat. An axis of such a wind is tilted with respect to the torus symmetry axis; 4. we orient the torus relative to an observer and again apply ray-tracing algorithm. In this step the beams go from an observer to the optically thick clouds that allows us to take into account the mutual obscuration of clouds; 5. after projecting on the picture plane, we impose a grid on the resulting cloud distribution and find the mean velocity of clouds in each cells to mimic the ALMA observational maps. By choosing the parameters corresponding to NGC 1068 we obtain the model velocity maps that emulate the effect of an apparent counter-rotation and can explain the discovery made by ALMA.