4 Young Massive Star Clusters in Normal Galaxies

Young star clusters with masses similar to those of classical old globular clusters are observed not only in starbursts, mergers or otherwise disturbed galaxies, but also in normal spiral galaxies. Some young clusters with masses as high as ∼ 10 6 M ⊙ have been found in the disks of isolated spirals. Dynamical mass estimates are available for a few of these clusters and are consistent with Kroupa-type IMFs. The luminosity (and possibly mass-) functions of young clusters are usually well approximated by power-laws. Thus, massive clusters at the tail of the distribution are naturally rare, but appear to be present whenever clusters form in large numbers. While bound star clusters may generally form with a higher efficiency in environments of high star formation rate, many of the apparent differences between clusters in starbursts and " normal " galaxies might be simply due to sampling effects. It is a human habit to characterise those things with which we are most familiar as normal. Although large spiral galaxies are not the most common type of galaxy in the Universe, we happen to live within one and many astronomers would probably tend to characterise the Milky Way as a fairly normal galaxy. Thus, at least for the purpose of this paper, " normal " galaxies mostly refer to non-interacting star forming disk galaxies. Our location within the Milky Way gives us a unique perspective from which we can study many of its properties in great detail, and it naturally provides a benchmark for comparison with other galaxies. Nevertheless, we should question whether it is justified to apply results obtained from studies of our own Galaxy to other galaxies which may appear superficially similar to it. In the context of this workshop, it is of particular relevance to ask how similar the cluster system in the Milky Way is to those in other galaxies. An increasing amount of observational evidence is pointing to the conclusion that many spirals host " young massive clusters " (YMCs) or " super star clusters " similar to those observed in large numbers in starburst galaxies. The definition of a YMC is rather vague and varies from one author to another, but the term generally seems to refer to young clusters that are more massive than the most massive open clusters in the Milky Way. However, giving a meaningful definition of a massive cluster may eventually be as …