  Holes and clumps in the interstellar gas of dwarf irregular galaxies are
gravitational scattering centers that heat field stars and change their radial
and vertical distributions. Because the gas structures are extended and each
stellar scattering is relatively weak, the stellar orbits remain nearly
circular and the net effect accumulates slowly over time. We calculate the
radial profile of scattered stars with an idealized model and find that it
approaches an equilibrium shape that is exponential, similar to the observed
shapes of galaxy discs. Our models treat only scattering and have no bars or
spiral arms, so the results apply mostly to dwarf irregular galaxies where
there are no other obvious scattering processes. Stellar scattering by gaseous
perturbations slows down when the stellar population gets thicker than the gas
layer. An accreting galaxy with a growing thin gas layer can form multiple
stellar exponential profiles from the inside-out, preserving the remnants of
each Gyr interval in a sequence of ever-lengthening and thinning stellar
subdiscs.
