The Dawes Review 8: Measuring the Stellar Initial Mass Function

Abstract

The birth of stars and the formation of galaxies are cornerstones of modern astrophysics. While much is known about how galaxies globally and their stars individually form and evolve, one fundamental property that affects both remains elusive. This is problematic because this key property, the birth mass distribution of stars, referred to as the stellar initial mass function (IMF), is a key tracer of the physics of star formation that underpins almost all of the unknowns in galaxy and stellar evolution. It is perhaps the greatest source of systematic uncertainty in star and galaxy evolution. A star's initial mass determines its luminosity, lifetime, and eventual return of material enriched by nuclear fusion back to the interstellar medium to be incorporated in later generations of stars. The distribution of stellar masses created in star formation events therefore determines the evolution of galaxies over cosmic time, and accurately measuring it is crucial. The past decade has seen a growing number and variety of methods for measuring or inferring the shape of the IMF, along with progressively more detailed simulations, paralleled by refinements in the way the concept of the IMF is applied or conceptualised on different physical scales. This range of approaches and evolving definitions of the quantity being measured has in turn led to conflicting conclusions regarding whether or not the IMF is universal. Here I review and compare the growing wealth of approaches to our understanding of this fundamental property that defines so much of astrophysics. I summarise the observational measurements from stellar analyses, extragalactic studies and cosmic constraints, and highlight the importance of considering potential IMF variations, reinforcing the need for measurements to quantify their scope and uncertainties carefully, in order for this field to progress. I present a new framework to aid the discussion of the IMF and promote clarity in the further development of this fundamental field.

The Star Formation History and Dynamics of the Ultra-diffuse Galaxy Dragonfly 44 in MOND and MOG

“…Based on the IGIMF theory, the gwIMF is top-light (deficit of massive stars) in low mass galaxies because they are expected to have a low SFR (Úbeda et al 2007;Lee et al 2009;Watts et al 2018;Yan et al 2017). The gwIMF is predicted to be more top-heavy in massive galaxies with a high SFR, as is observed (Hoversten & Glazebrook 2008;Lee et al 2009;Meurer et al 2009;Habergham et al 2010;Gunawardhana et al 2011;Zhang et al 2018;Hopkins 2018).…”

Chemical evolution of ultra-faint dwarf galaxies in the self-consistently calculated integrated galactic IMF theory

“…In addition, and as stressed by Kroupa et al (2013) and Hopkins (2018), the gwIMF of the entire galaxy appears to be different from the IMF in individual star-forming events (embedded clusters, i.e., a gravitationally driven collective process of transformation of the interstellar gaseous matter into stars in molecular-cloud overdensities on a spatial scale of about one parsec and within about one million years, see Yan et al 2017). This difference between the IMF and the gwIMF is evident in all types of observations, such as dwarf galaxies (Meurer et al 2009;Lee et al 2009;Watts et al 2018), SDSS star-forming galaxies (Hoversten & Glazebrook 2008;Gunawardhana et al 2011), starburst galaxies (Romano et al 2017;Zhang et al 2018), and massive elliptical galaxies (Matteucci 1994;van Dokkum & Conroy 2010;Martín-Navarro et al 2015;Parikh et al 2018).…”

The Role of Magnetic Fields in Setting the Star Formation Rate and the Initial Mass Function

“…There is clearly substantial disagreement on the low-mass end (M 1 M ) with the turnover mass (or characteristic mass) varying between ∼ 0.1 M and ∼ 0.4 M depending on the parameterization. This disagreement is a result of the challenges in observing low-mass stars, taking into account multiplicity, and converting from a luminosity function to a mass function Hopkins, 2018). For the high-mass tail (M (2011) (dash-dotted).…”