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 tohave "steep" luminosity functions with faint-end behavior roughlyphi(L)-propto- L^(-1.8). This form is exactly what is expected if theluminosities of these objects fade with time as L-propto-t^(-1.3), because theobjects spend more time at low luminosities than high, even if they form with awide range of initial masses. Models of young stellar populations show thisfading behavior when the star formation has occured in a single, short, recentburst. Steep luminosity functions therefore do not require steep mass functionsif the galaxies are powered by fading bursts. The local-galaxy H-alphaluminosity function---which is less steep than L^(-1.8)---is also well-fit bythis mechanism, because ionizing photon flux fades much more quickly thanbroad-band optical luminosity. An age-luminosity relation and awavelength-dependence of the luminosity function are both predicted. In thecontext of this mechanism, the slope of the luminosity function provides aconstraint on the stellar initial mass function in the bursts.Comment: 13 pages including one figure; accepted for publication in ApJ Letter