The H alpha Galaxy Survey. VIII. Close companions and interactions, and
  the definition of starbursts
  (Shortened) We consider the massive star formation properties, radial
profiles, and atomic gas masses of those galaxies in our H alpha Galaxy Survey,
a representative sample of the local Universe of 327 disk galaxies, that have
close companion galaxies, in comparison with a matched control sample of
galaxies without companions. We find that the presence of a close companion
raises the star formation rate by a factor of just under two, while increasing
hardly at all the equivalent width of the H alpha emission. This means that
although statistically galaxies with close companions form stars at a higher
rate, they do this over extended periods of time, and not as bursts. We find no
significant increase in the central concentration of the star formation as a
result of the presence of a close companion. The fraction of truly interacting
or merging galaxies is very small in the local Universe, at around 2%, and
possibly 4% of bright galaxies. Most of these interacting galaxies currently
have unremarkable star formation properties. We also study the properties of
the Survey galaxies with the most extreme values for star formation indicators
such as rate, equivalent width, star formation rate per area, and gas depletion
timescale. We find that each of these indicators favors a different subset of
galaxies, and use this information to discuss critically the possible
definitions of the term starburst to describe galaxies with enhanced star
formation activity. We conclude that no one starburst definition can be devised
which is objective and generally discriminant. Unless one restricts the use of
the term "starburst" to a very small number of galaxies, the term will continue
to be used for a heterogeneous and wide-ranging collection of objects with no
physical basis for their classification as starburst.
