Truncated Initial Mass Function in Starburst Galaxies

Observations of nearby star-forming regions suggest that the initial mass function is deficient in low-mass stars when the ambient star formation rate is high. We use new models of stellar population synthesis to investigate how a truncation of the lower end of the initial mass function affects the photometric evolution of starburst galaxies. A major difference with respect to the case of a standard initial mass function is that the spectra of the galaxies become unusually red (V - K >~ 3.5) when the turnoff mass reaches the lower mass cutoff. This red phase can last for more than 1o^9^ yr if the lower mass cutoff is below 2 M_sun_. The amplitude of the reddening increases with the fraction of the galaxy mass involved in the starburst but cannot be used to determine uniquely the lower cutoff of the initial mass function. Such unusually red poststarburst galaxies can be distinguished from galaxies reddened by dust or with abnormally high metallicities by their unusually strong 4000 A break and the presence of stellar absorption features typical of late- type giants.