Population synthesis from clustered star formation

In recent years, a series of papers (Kroupa & Weidner 2003, Weidner & Kroupa 2004, Weidner & Kroupa 2005 and Weidner & Kroupa 2006, WK06 from now on) have proposed that the stellar content of an entire galaxy may not be well described by the same initial mass function (IMF) that describes the distribution of stellar masses in the star clusters, where these stars form. The reason is that star clusters also form with a cluster mass function (CMF), which is a power law with a power law index of ~-2. If the lowest mass clusters are of masses smaller than the physical upper mass limit for stars they will be deficient in high mass stars. Therefore, if the stellar content of all clusters is added together, making up the Integrated Galactic Initial Mass Function (IGIMF), the distribution of stellar masses may be steeper at the high mass end, depending on the exact shape of the CMF.