The Lick AGN Monitoring Project: Velocity-Delay Maps from the Maximum-Entropy Method for Arp 151

TL;DR

This study uses reverberation mapping and the maximum-entropy method to create velocity-delay maps of emission lines in Arp 151, revealing complex gas dynamics and geometry in the broad-line region.

Contribution

It introduces detailed velocity-delay maps derived from spectrophotometric data using the maximum-entropy method, providing new insights into the geometry and kinematics of the broad-line region in Arp 151.

Findings

Detection of H I response within 0-15 days confined within the virial envelope.

Radial stratification observed in Balmer-line delays, increasing from Hgamma to Halpha.

Asymmetries in line response suggest warped-disk geometry and potential disk warping due to radiation pressure.

Abstract

We present velocity-delay maps for optical H I, He I, and He II recombination lines in Arp 151, recovered by fitting a reverberation model to spectrophotometric monitoring data using the maximum-entropy method. H I response is detected over the range 0-15 days, with the response confined within the virial envelope. The Balmer-line maps have similar morphologies but exhibit radial stratification, with progressively longer delays for Hgamma to Hbeta to Halpha. The He I and He II response is confined within 1-2 days. There is a deficit of prompt response in the Balmer-line cores but strong prompt response in the red wings. Comparison with simple models identifies two classes that reproduce these features: freefalling gas, and a half-illuminated disk with a hotspot at small radius on the receding lune. Symmetrically illuminated models with gas orbiting in an inclined disk or an isotropic distribution of randomly inclined circular orbits can reproduce the virial structure but not the observed asymmetry. Radial outflows are also largely ruled out by the observed asymmetry. A warped-disk geometry provides a physically plausible mechanism for the asymmetric illumination and hotspot features. Simple estimates show that a disk in the broad-line region of Arp 151 could be unstable to warping induced by radiation pressure. Our results demonstrate the potential power of detailed modeling combined with monitoring campaigns at higher cadence to characterize the gas kinematics and physical processes that give rise to the broad emission lines in active galactic nuclei.