Space Telescope and Optical Reverberation Mapping Project. IX.
  Velocity-Delay Maps for Broad Emission Lines in NGC 5548
  We report velocity-delay maps for prominent broad emission lines, Ly_alpha,
CIV, HeII and H_beta, in the spectrum of NGC5548. The emission-line responses
inhabit the interior of a virial envelope. The velocity-delay maps reveal
stratified ionization structure. The HeII response inside 5-10 light-days has a
broad single-peaked velocity profile. The Ly_alpha, CIV, and H_beta responses
peak inside 10 light-days, extend outside 20 light-days, and exhibit a velocity
profile with two peaks separated by 5000 km/s in the 10 to 20 light-day delay
range. The velocity-delay maps show that the M-shaped lag vs velocity structure
found in previous cross-correlation analysis is the signature of a Keplerian
disk with a well-defined outer edge at R=20 light-days. The outer wings of the
M arise from the virial envelope, and the U-shaped interior of the M is the
lower half of an ellipse in the velocity-delay plane. The far-side response is
weaker than that from the near side, so that we see clearly the lower half, but
only faintly the upper half, of the velocity--delay ellipse. The delay
tau=(R/c)(1-sin(i))=5 light-days at line center is from the near edge of the
inclined ring, giving the inclination i=45 deg. A black hole mass of M=7x10^7
Msun is consistent with the velocity-delay structure. A barber-pole pattern
with stripes moving from red to blue across the CIV and possibly Ly_alpha line
profiles suggests the presence of azimuthal structure rotating around the far
side of the broad-line region and may be the signature of precession or orbital
motion of structures in the inner disk. Further HST observations of NGC 5548
over a multi-year timespan but with a cadence of perhaps 10 days rather than 1
day could help to clarify the nature of this new AGN phenomenon.
