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 known to have ''steep'' luminosity functions with faint-end behavior roughly phi(L) proportional to L-1.8. This form is exactly what is expected if the luminosities of these objects fade with time as L proportional 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. Models of young stellar populations show this fading behavior when the star formation has occurred in a single, short, recent burst. Steep luminosity functions therefore do not require steep mass functions if the galaxies are powered by fading bursts. The local galaxy H alpha luminosity function-which is less steep than L-1.8-is also well fitted by this mechanism, because ionizing photon flux fades much more quickly than broadband optical luminosity. An age-luminosity relation and a wavelength dependence of the luminosity function are both predicted. In the context of this mechanism, the slope of the luminosity function provides a constraint on the stellar initial mass function in the bursts.