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We present velocity-delay maps for optical H I, He I, and He II recombinationlines in Arp 151, recovered by fitting a reverberation model tospectrophotometric monitoring data using the maximum-entropy method. H Iresponse is detected over the range 0-15 days, with the response confinedwithin the virial envelope. The Balmer-line maps have similar morphologies butexhibit radial stratification, with progressively longer delays for Hgamma toHbeta to Halpha. The He I and He II response is confined within 1-2 days. Thereis a deficit of prompt response in the Balmer-line cores but strong promptresponse in the red wings. Comparison with simple models identifies two classesthat reproduce these features: freefalling gas, and a half-illuminated diskwith a hotspot at small radius on the receding lune. Symmetrically illuminatedmodels with gas orbiting in an inclined disk or an isotropic distribution ofrandomly inclined circular orbits can reproduce the virial structure but notthe observed asymmetry. Radial outflows are also largely ruled out by theobserved asymmetry. A warped-disk geometry provides a physically plausiblemechanism for the asymmetric illumination and hotspot features. Simpleestimates show that a disk in the broad-line region of Arp 151 could beunstable to warping induced by radiation pressure. Our results demonstrate thepotential power of detailed modeling combined with monitoring campaigns athigher cadence to characterize the gas kinematics and physical processes thatgive rise to the broad emission lines in active galactic nuclei.

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