Starbursts in isolated galaxies

Abstract

Context. Starbursts and substantial variations in the star formation histories are a common phenomenon in galaxies. Although predominantly found in interacting galaxies, they also occur in isolated galaxies. Aims. 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, i.e. a spontaneous and an induced star formation mode, the initial mass function (IMF), the stellar feedback and the interstellar medium (ISM) model on the galactic star formation history are investigated. We especially focus on dynamically driven starbursts induced by stellar feedback. Methods. We apply a one-zone model for a star-gas system coupled by both 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, such as star formation activity or the thermal state of the ISM. Results. While the influence of the dynamics on the total star formation rate is strong, especially with nonlinear stellar birth functions, 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, in regimes beyond 10 4 K. Conclusions. Bursts of star formation occur during transitory phases, when dynamical equilibrium is established. Then they are quasiperiodic 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.

Starbursts in Isolated Galaxies: the influence of the ISM model

“…We use an extended version of the analytical models of KTH95 that allows us to deal with the dynamical evolution of a galaxy in a simple way (for details cf. to Theis & Köppen (2009)). Here we compare the evolution of dwarf galaxies for two different ISM models: in the diffuse ISM model energy is dissipated by radiation, whereas in the clumpy ISM model energy is lost via inelastic cloud collisions.…”