The low-mass IMF: a review and an illustration of future prospects with JWST

Abstract

The Initial Mass Function (IMF) represents the end product of the process of star formation. Understanding whether the IMF varies with the environment is paramount for interpreting the cosmic history of baryonic matter. Theories predict that the IMF shape and in particular its turnover mass, may depend on several factors, including the parental cloud chemical composition and its kinematics. Multiple recent lines of empirical evidence suggest in fact that the IMF may vary with galaxy mass, velocity dispersion or metallicity. This evidence ranges from unresolved studies of distant elliptical galaxies to nearby resolved populations. In particular, very low-metallicity systems, such as Local Group ultra-faint dwarfs (UFDs), provide clues towards understanding the conditions in which stars formed in the early Universe, before the epoch of re-ionization. In this contribution I provide a brief review of our theoretical understanding of the IMF, as well as the most recent results from giant ellipticals and from UFDs studies that use HST data. With HST it is possible to measure IMF variations with galaxy metallicity, with the most metal-poor UFDs having a more bottom-light IMF, i.e., fewer low-mass stars, relative to the Milky Way. The significance of these findings is however limited by the sensitivity of HST which prevents a high-significance detection of the IMF turnover outside the Milky Way. JWST, on the other end, can probe well below the IMF turnover mass, thus allowing us to reach a robust conclusion on IMF variations using direct star counts. I conclude this contribution showing preliminary results from our team based on JWST observations of the UFD Boötes I.