Starbursts in isolated galaxies

I. The influence of stellar birth function and IMF

¹ Institute of Astronomy, University of Vienna, Türkenschanzstr. 17, 1180 Vienna, Austria e-mail: theis@astro.univie.ac.at
² Institut für Theoretische Physik und Astrophysik, Universität Kiel, 24098 Kiel, Germany
³ Observatoire Astronomique de Strasbourg, 11 rue de l'Université, 67000 Strasbourg, France e-mail: koppen@astro.u-strasbg.fr
⁴ International Space University, Parc d'Innovation, 1, rue Jean-Dominique Cassini, 67400 Illkirch-Graffenstaden, France

Received: 26 May 2008
Accepted: 22 April 2009

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 10⁴ K.

Conclusions. 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.