Initial stellar mass function: new theoretical results explain its variability

A new theoretical development of the variable Initial Mass Function (IMF) explains recent observations that do not agree with the previous model of a universal IMF.  The result has been obtained by an international team led by Fabio Fontanot of the INAF - Osservatorio astronomico di Trieste and published on the Monthly Notices of the Royal Astronomical Society 

 

The initial stellar mass function (IMF) represents a fundamental concept for theoretical models of galaxy formation and evolution in the standard cosmological model. The IMF is defined as the number of
stars formed per stellar mass bin in a given star formation episode. Given the key role played by stellar mass in stellar evolution, the IMF defines the relative abundance of different spectral types, the amount of baryonic matter locked in long-lived low-mass stars and the fraction of stars bound to explode as supernovae (both Type I and II) and feedback energy and metals into the interstellar medium.

Until recently, the consistency of IMF measurement in the local neighbourhood (whose star formation regions are sufficiently close to be resolved into individual stars) favoured a scenario with a universal shape for IMF. Indeed, new observational facilities (like Integral Field Units and/or high-resolution spectroscopy) put this
scenario into question, by mapping in great detail the dynamical and spectroscopic properties of local galaxies. The results are in significant tension with the expectations of a universal IMF framework, thus suggesting an alternative variable IMF approach. It is worth stressing that these results are mainly based on the analysis of
composite stellar populations, i.e. based on the combination of single stellar populations, each of them possibly characterized by a different IMF.

The definition of consistent theoretical framework to study the time/redshift evolution of the IMF in a given galaxy, as a function of its physical properties, is thus a key requirement to better understand the growth of baryonic structures in a cosmological framework. Fabio Fontanot (INAF-Osservatorio astronomico di Trieste) recently led an international team to a better characterisation of the variable IMF. They propose a new set of solution to predict the mean IMF shape of a galaxy starting from its instantaneous star formation rate and its cosmic ray density field, which has been published on the Monthly Notices of the Royal Astronomical Society
(https://arxiv.org/abs/1807.01319). These solutions rely both on analytic arguments and numerical simulations to describe the physical properties of the star forming regions (i.e. molecular clouds). In detail, they predict an evolution both at the high-mass end (responsible for setting the fraction of supernovae, the stellar
feedback and chemical enrichment), and at the low-mass end (setting the total dynamical mass of the galaxy). This scenario can thus reproduce at the same time a series of different constraints, that are
difficult to reconcile assuming that only one of the two ends is evolving with respect to the typical Milky Way IMF. The next step in this analysis requires the self-consistent implementation of this variable IMF scenario in theoretical models of galaxy formation, with the aim of studying in detail the full implications for galaxy
evolution.

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IMF shape variations in the Fontanot et al. framework. Different panels refer to different cosmic ray energy densities (U_CR, normalized to the Milky Way value U_MW). Colored lines correspond to different star formation rates (SFR) as labeled in the caption. In all panels, the thin solid line shows the typical Milky Way IMF.

 

IMF shape variations in the Fontanot et al. framework. Different panels refer to different cosmic ray energy densities (U_CR, normalized to the Milky Way value U_MW). Colored lines correspond to different star formation rates (SFR) as labeled in the caption. In all panels, the thin solid line shows the typical Milky Way IMF.