Scientific Rationale

The stellar initial mass function (IMF) is a fundamental parameter intimately related to the star formation history of a galaxy. It not only encodes the complex gas-phase astrophysical processes on galactic scales, but also dictates the fraction of gas converted to stars, the fraction of mass forever locked in sub-solar stars and the fraction returned to enrich the interstellar medium chemically. Despite over 60 years of history, however, the true nature of the IMF remains debatable. Therefore confirming whether the IMF varies or not, and constraining any variations between different environments, is imperative for the progress of galaxy evolution research.

Since the introduction of the “original” mass function by Salpeter in 1955, numerous efforts have been made to verify its true form. Much of the earlier observational work on the IMF focussed on resolved star counts as a direct and robust method led to the assumption that the IMF is invariant. The evidence challenging this assumption has, however, started to accumulate from work based on more recently developed indirect observational techniques of inferring or measuring the IMF (e.g. spectral synthesis, stellar dynamics, gravitational lensing). In particular, some observations and models suggest an excess of high mass stars in starbursting regions and high redshift galaxies, and a deficit of high mass stars in low-density regions, such as those in star-forming dwarf galaxies. Other studies suggest that massive early-type galaxies, at least, may have formed their stars according to a different IMF from that pertaining in the Milky Way disk.

The ongoing debate on the Universality of the IMF spans a range of astrophysical disciplines. Currently, many groups worldwide are working on exploiting new multi-IFU surveys such as SAMI, MaNGA, MUSE to probe IMF variations within galaxies, for dynamical modelling, and to reveal rare gravitational lensing systems yielding high-precision stellar mass measurements. The stellar population models (e.g. BPASS, FSPS, SLUG) are being continuously updated to take the effects of binary stars, Wolf-Rayet stars, and star formation histories into account. The improvements in numerical techniques and increased computing power have paved the way for simulation experiments (e.g. Eagle, Illustris, Fire) to be conducted in high-resolution, allowing us to isolate the effects on individual processes and compare with spatially resolved observations of galaxies.

As should be clear from above, progress in understanding the IMF demands input from many observational domains as well as theoretical modelling and advances in instrumentation. In July 2020, we aim to bring together in Durham the leading researchers from across this broad spectrum of expertise, for a week of discussion, exchanging ideas, and formulating new tests, especially in the context of new and upcoming facilities (e.g. ALMA, JWST, Taipan, Hector, 4-MOST surveys). The scope of the meeting is defined broadly, to include galactic and extragalactic studies, the high-mass and low-mass regimes, as well as the theoretical framework.