  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.
