Living with Neighbors. I. Observational Clues to Hydrodynamic Impact of
  Neighboring Galaxies on Star Formation
  Galaxies in pairs show enhanced star formation (SF) compared to their
counterparts in isolation, which is often explained by the tidal effect of
neighboring galaxies. Recent observations, however, reported that galaxies
paired with early-type neighbors do not undergo the SF enhancement. Here we
revisit the influence of neighbors using a large sample of paired galaxies from
the Sloan Digital Sky Survey and a carefully constructed control sample of
isolated counterparts. We find that star-forming neighbors enhance SF, and even
more so for more star-forming (and closer) neighbors, which can be attributed
to collisions of interstellar medium (ISM) leading to SF. We further find that,
contrary to the anticipated tidal effect, quiescent neighbors quench SF, and
even more so for more quiescent (and closer) neighbors. This seems to be due to
removal of gas reservoirs via ram pressure stripping and gas accretion cutoff
by hot gas halos of quiescent neighbors, on top of their paucity of ISM to
collide to form stars. Our findings, especially the intimate connection of SF
to the status and strength of neighbors' SF, imply that the hydrodynamic
mechanisms, along with the tidal effect, play a crucial role during the early
phase of galactic interactions.
