NGC 4378's One-Armed Spiral Pattern: Leading or Trailing?

Observations and simulations of the galaxy NGC 4622 indicate that it has a true leading arm winding outward opposite the sense of disk spin. The arm is apparently a long-lived density wave leading out to a resonance ring. Simulations of a plunging retrograde passage of a small companion creates this pattern. A proper match to the observations does not result unless self gravity is unimportant in the disk. We now investigate whether or not NGC 4378, another single-armed disk galaxy, also has a leading density wave arm. We find via simulations that NGC 4378's pattern is not a leading density wave but is instead an impulsive trailing arm created by a grazing passage of a small companion. This impulsive arm has a short lifetime in the simulations. Nevertheless, like in NGC 4622, a nongravitating (halo dominated) disk is necessary to create the observed pattern in NGC 4378.