VV 655 and NGC 4418: Implications of an interaction for the evolution of
  a LIRG
  VV 655, a dwarf irregular galaxy with HI tidal debris, is a companion to the
lenticular luminous infrared galaxy (LIRG) NGC 4418. NGC 4418 stands out among
nearby LIRGs due to its dense central concentration of molecular gas and the
dusty, bi-polar structures along its minor axis suggestive of a wind driven by
a central starburst and possible nuclear activity. We seek to understand the
consequences of the ongoing minor interaction between VV 655 and NGC 4418 for
the evolution of the LIRG, including the origin of the gas supply responsible
for its unusual nuclear properties. We investigate the structural, kinematic,
and chemical properties of VV 655 and NGC 4418 by analyzing archival imaging
data and optical spectroscopic observations from the SDSS-III and new spectra
from SALT-RSS. We characterize their gas-phase metal abundances and spatially
resolved, ionized gas kinematics, and demonstrate that the gas-phase
metallicity in NGC 4418 significantly exceeds that in VV 655. No kinematic
disturbances in the ionized gas are observed along the minor axis of NGC 4418,
but we see evidence for ionized gas outflows from VV 655 that may increase the
cross-section for gas stripping in grazing collisions. A faint, asymmetric
outer arm is detected in NGC 4418 of the type normally associated with
galaxy-galaxy interactions. The simplest model suggests that the minor
interaction between VV 655 and NGC 4418 produced the unusual nuclear properties
of the LIRG via tidal torquing of the interstellar medium of NGC 4418 rather
than through a significant gas transfer event. In addition to inducing a
central concentration of gas in NGC 4418, this interaction also produced an
enhanced star formation rate and an outer tidal arm in the LIRG. The VV 655-NGC
4418 system offers an example of the potential for minor collisions to alter
the evolutionary pathways of giant galaxies.
