Andreas Rieschick

Gas Mixing, Gas Cycles and the Chemical Evolution of Dwarf Irregular Galaxies

Dwarf galaxies are ideal laboratories to study influential effects on galaxy evolution. In particular, their gas-rich variant with very active star formation, starbursting dwarf irregulars, shows chemical and structural signatures that lead unambiguously to the conclusion that they are standing in a vital contact with their surroundings. Gas infall cannot only trigger star formation but also allows for a reduction of the metal content. on the other hand, active star formation ignites numerous supernovae type II which accumulate and can produce a galactic wind. This again depletes the metals pushing them into a gas mixing cycle with different timescales, locally of about 10 Myrs, but an galactic scales of at least 1 Gyr. This paper illuminates the different processes like gas infall and outflow and their effects on the chemical evolution, the star formation, and the gas mixing in dwarf irregular galaxies.

Chemodynamical gas flow cycles and their influence on the chemical evolution of dwarf irregular galaxies

Here we investigate an exemplary chemodynamical evolutionary simulation of a dwarf irregular galaxy. By means of this model we demonstrate the existence of three gas mixing cycles: 1) An inner local cycle mixing the metals produced in stars locally, and 2) an outer galactic cycle on which hot gas is driven out of the galaxy by multiple supernovae type II and mixes on a short timescale with the available cold gas. 3) Only a small fraction of the metals leaves the galactic gravitational field and follows the global cycle with the intergalactic matter. The large-scale mixing results in a temporary depletion of supernova ejected metals. We will discuss this delayed recycling and its influence on the chemical evolution, especially on the nitrogen over oxygen ratio which is increased temporarily. The results presented here are also relevant for less sophisticated analytical approaches and chemical evolutionary models of galaxies which have to parameterize the metal loss through outflow.

Chemodynamical Mixing Cycles in Dwarf Galaxies

Observations of dwarf irregular galaxies show no significant metal abundance gradients throughout the galaxies at a generally low abundance between 1/40 and 1/3 Z/Z ⊙. Additionally, in several starburst dwarf galaxies (SBDGs) large HI reservoirs envelope the luminous galactic body (e.g. in NGC 4449 and I Zw 18) and obtrude that the starburst is fuelled by enhanced gas infall. Current SBDGs are also characterized by superwinds or by large expanding X-ray plumes. These chemical peculiarities can be explained by chemodynamical evolutionary simulations of dwarf galaxies that show large-scale mixing of gas phases by outflow and mixing of metal-enriched and expelled supernova (SN type II) gas with almost pristine or slightly metal-enriched infalling clouds from the gas envelope.