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Scientific Rationale

The stellar initial mass function (IMF) is a fundamental parameter that not only encodes the complex gas phase astrophysics of star formation, but is also crucial for interpreting the vast majority of observations made in extragalactic astronomy today. Therefore, it is imperative to constrain its functional form, and the manner in which it may vary between different environments.

Since the idea of the "original" mass function was introduced by Salpeter in 1955, considerable effort has been made to verify its form, and previous observations have led many to assume that it is invariant. In recent years, however, work challenging this assumption has been accumulating. In particular, some observations and models suggest a deficiency of high mass stars in low-density regions, such as those found in star-forming dwarf galaxies and the outer disks of spiral galaxies. Other observations suggest an excess of high mass stars in some regions, including starbursts and high redshift galaxies. High mass stars dominate the luminous and chemical output of galaxies, and are the basis of most star formation indicators. If such variations are proven real, they would have broad consequences for studies of the star formation histories and chemical evolution of galaxies, and theoretical work on star formation.

A general commonality of recent work providing possible evidence for IMF variations is that they are based on the light integrated over galaxies or regions of galaxies. Such observations only provide indirect constraints, since IMF variations will be degenerate with population age and star formation history. A range of other confounding parameters must also be considered given that stars are not being measured individually. On the other hand, there is evidence for consistency between observations and semi-analytic models which assume systematic deficiencies or excesses in high mass stars.

The presence of observations both for and against the universality of the IMF suggest that this is an appropriate time for a critical re-evaluation of the ensemble of accumulated observational evidence constraining the upper end of the IMF, and for garnering new insights into the physical parameters controlling high mass star and cluster formation. The goal of this meeting will be to synthesize a coherent picture of the IMF slope and characteristic mass within individual galaxies and across the galaxy population. Further implications of potential IMF variations and further observational tests must also be formulated, particularly in the context of the new capabilities enabled by the recent refurbishment of HST, the next generation of extremely large telescopes, or capabilities not yet planned that must be developed to finally provide unambiguous constraints on this fundamental astrophysical issue.