THE FADING OF YOUNG STELLAR POPULATIONS AND THE LUMINOSITY FUNCTIONS OF DWARF IRREGULAR, AND STARBURST GALAXIES

Dwarf, irregular, and infrared-luminous starburst galaxies are all kno wn to have ''steep'' luminosity functions with faint-end behavior roug hly phi(L) proportional to L-1.8. This form is exactly what is expecte d if the luminosities of these objects fade with time as L proportiona l to t(-1.3), because the objects spend more time at low luminosities than high, even if they form with a wide range of initial masses. Mode ls of young stellar populations show this fading behavior when the sta r formation has occurred in a single, short, recent burst. Steep lumin osity functions therefore do not require steep mass functions if the g alaxies are powered by fading bursts. The local galaxy H alpha luminos ity function-which is less steep than L-1.8-is also well fitted by thi s mechanism, because ionizing photon flux fades much more quickly than broadband optical luminosity. An age-luminosity relation and a wavele ngth dependence of the luminosity function are both predicted. In the context of this mechanism, the slope of the luminosity function provid es a constraint on the stellar initial mass function in the bursts.