…The calculations that do exist demonstrate time and again that the IMF is invariant: There exists no statistically meaningful evidence for a variation of the IMF from metal-poor to metal-rich populations. This means that currently existing star-formation theory fails to describe the stellar outcome. Indirect evidence, based on chemical evolution calculations, however indicate that extreme star-bursts that assembled bulges and elliptical galaxies may have had a top-heavy IMF…
It would have seemed that when two mutually more or less exclusive theories of the origin of stellar masses make the same basic prediction concerning the variation of the average stellar mass with physical conditions, that this expected variation would be very robust and born out in observational data.
Alas, the observational data on the IMF are resilient - they do not yield what we desire to see
. The stellar IMF is invariant and can be best described by a two-part power-law form. This holds true for metallicities
ranging from those of globular clusters to super-solar values near the Galactic centre, and for densities less than about 106 stars/pc3. Even the maximal stellar mass of about 150M⊙ seems to be independent of metallicity for Z>∼ 0.008.
Given the quite total failure to account for the resilience of the stellar IMF towards changes, it is clear that this IMF-conservatism poses some rather severe challenges on liberal star formation theory.