H${\beta}$ line shape and radius-luminosity relation in 2.5D FRADO

TL;DR

This study models the shape and size of the Broad Line Region in active galactic nuclei using simulations of cloud distributions, revealing how line profiles and time delays depend on black hole mass, accretion rate, and viewing angle.

Contribution

It introduces a physically-based, low-parameter model for the BLR that links cloud dynamics to spectral line shapes and size measurements, advancing understanding of AGN emission features.

Findings

Line width increases with black hole mass and viewing angle.

Blue wing prominence grows with black hole mass and accretion rate.

Peak time-delays better match observational radius-luminosity trends.

Abstract

Galaxies with active galactic nuclei (AGN) exhibit broad emission lines as a key spectral feature. The shape of emission-line profiles depends on the complex dynamics of discrete clouds within a spatially extended region known as the Broad Line Region (BLR). The distribution of cloud positions within BLR, or the geometry of BLR indeed, is directly linked to measurements of time lags of BLR. In this paper, we convolve a large grid of physically-based simulations of cloud distributions in BLR with photon-flux weighted emissivity of BLR clouds to investigate the generic shape of spectral line profiles. More importantly, we extract the time-delay histograms of corresponding models to calculate the size of BLR. Our physical model is based on the assumption that the clouds are launched by the radiation pressure acting on dust in the atmosphere of the outer disk. It has very few global parameters. The model is appropriate for the low ionization part of the BLR, as it was shown by earlier model tests. It uses a non-hydrodynamical single-cloud approach to the BLR dynamics. In this way we simulate the distribution of positions and velocities of the clouds. We found that the width of line profiles gets broader with black hole mass, or with viewing angle, and gets narrower with accretion rate. The blue wing of the emission line profiles becomes more pronounced with increasing black hole mass and accretion rate, consistent with the formation and intensification of an outflow structure. We also found that the peak time-delays rather than averaged delay values better represents the observational trend and also the scatter in the radius-luminosity relation.