The effect of clouds in a galactic wind on the evolution of gas-rich dwarf galaxies

¹ Institut für Astronomie der Universität Wien, Türkenschanzstrasse 17, 1180 Wien, Austria e-mail: hensler@astro.univie.ac.at
² INAF – Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34131 Trieste, Italy e-mail: recchi@oats.inaf.it

Received: 3 July 2007
Accepted: 9 October 2007

Abstract

Context.Gas-rich dwarf galaxies are probably the closest counterparts to primeval objects we can find in the local Universe, therefore it is interesting to study their evolution in different astrophysical contexts.

Aims.We study the effects of interstellar clouds on the dynamical and chemical evolution of gas-rich dwarf galaxies. In particular, we focus on two model galaxies similar to IZw18 and NGC 1569 in comparison to models in which a smooth initial distribution of gas is assumed.

Methods.We use a 2D hydrodynamical code coupled with a series of routines able to trace the chemical products of SNeII, SNeIa and intermediate-mass stars. Clouds are simulated by adding overdense regions in the computational grid, whose locations are chosen randomly and whose density profiles match observed ones. We consider both cloud complexes put at the beginning of the simulation and a mechanism for continuous cloud formation. The clouds are inherently dynamically coupled to the diffuse gas, and they experience heat conduction from a hot surrounding gas.

Results.Due to dynamical processes and thermal evaporation, the clouds survive only a few tens of Myr. Due to the additional cooling agent, the internal energy of cloudy models is typically reduced by 20-40% compared with models of diffuse gas alone. The clouds delay the development of large-scale outflows by mass loading, therefore helping to retain a larger amount of gas inside the galaxy. However, especially in models with continuous creation of infalling clouds, their bullet effect can pierce the expanding supershell and create holes through which the superbubble can vent freshly produced metals. Moreover, assuming a pristine chemical composition for the clouds, their interaction with the superbubble dilutes the gas, reducing the metallicity. The resulting final metallicity is therefore generally lower (by ~0.2-0.4 dex) than the one attained by diffuse models.