The Early Chemical Evolution of Dwarf Irregular Galaxies

Abstract

Investigations have been performed to understand the age of dwarf irregular galaxies from the point of view of their presently low chemical abundances and the enhanced star-formation rate. While for a few objects, like e.g. I Zw 18, their assumed relative youth can be verified by models taking a double-starburst behavior into account, those dlrrs with old underlying stellar populations have passed the regime of observed N/O-0 values within almost 2 Gyrs after their formation even with gas loss by means of supernova-driven galactic winds. Therefore alternative processes like gas infall must be invoked.

Star Formation Regulation, Gas Cycles and the chemical evolution of dwarf irregular galaxies

“…2.1). If the hot gas is able to evaporate the cold clouds and due to its overpressure a galactic mass-loaded outflow occurs (Hensler et al 1999). Since these clouds contain elements from intermediate-mass stars of older stellar populations, e.g.…”

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

“…The tracks visible in Fig.2 (dots and crosses) are inevitably extended in 0 with a wider range for models of lowest effective yields what, on the other hand, can be translated to be the lowest masses. Since the tracks are mainly horizontal without loops as produced in models where primary 0 and secondary N are schematically disconnected (Hensler, Koppen, & Rieschick 1999), the scatter in NjO might be caused by different locations of the objects at the beginning of infall. The triangular region of observations (HEK and Fig.1) is produced because, roughly speaking, the fraction of infalling gas increases for galaxies of lower masses ).…”

“…The remaining 3/4 of produced SNII metals are carried away from the SF region by the expansion of hot superbubble gas. Nevertheless, this has incorporated evaporated clouds (Hensler et al 1999). Since the clouds also consist of elements from intermediatemass stars of an older population, i.e.…”