Starbursts in isolated galaxies. I. The influence of stellar birth
  function and IMF
  Starbursts and substantial variations in the star formation histories are a
common phenomenon in galaxies. We study the stability properties of isolated
star-forming dwarf galaxies with the aim of identifying starburst modes. The
impact of the stellar birth function, the initial mass function (IMF), the
stellar feedback and the interstellar medium (ISM) model are investigated. We
apply a one-zone model for a star-gas system coupled by mass and energy
transfer. Additionally, we extend the network for active dynamical evolution.
This allows for a coupling between the dynamical state of the galaxy and its
internal properties. While the influence of the dynamics on the total star
formation rate is strong, the coupling of the internal properties (gas
temperature) on the dynamics is rather limited, because radiative cooling keeps
the gas temperature well below the virial temperature. Because of short cooling
and feedback timescales, the star formation rate is close to the equilibrium
star formation rates. Quasi-periodic starbursts occur, because star formation
follows the variations in the gas density induced by decaying virial
oscillations. This behaviour is quite insensitive to the nature and the details
of the stellar birth description, viz. whether spontaneous or induced star
formation is considered or the IMF is varied. A second type of burst is found
as an instability operating when the cooling may drop at very low densities
with increasing temperature. Bursts of star formation occur during transitory
phases, when dynamical equilibrium is established. Then they are quasi-periodic
on the dynamical timescale. Because of short heating and cooling timescales,
the star formation rate follows the equilibrium star formation rate
corresponding to the actual gas density.
