We explore whether we can constrain the shape of the INITIAL mass distribution of the star cluster population in M82s ~1 Gyr-old post-starburst region B, in which the present-day cluster mass function (CMF) is closely approximated by a log-normal distribution. We conclude that the M82 B initial CMF must have had a mean mass very close to that of the equilibrium CMF of Vesperini (1998). Consequently, if the presently observed M82 B CMF has remained approximately constant since its formation, as predicted, then the INITIAL CMF must have been characterized by a mean mass that was only slightly larger than the present mean mass. From our detailed analysis of the expected evolution of CMFs, we conclude that our observations of the M82 B CMF are inconsistent with a scenario in which the 1 Gyr-old cluster population originated from an initial power-law mass distribution. Our conclusion is supported by arguments related to the initial density in M82 B, which would have been unphysically high if the present cluster population were the remains of an initial power-law distribution.
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