A Universal Stellar Initial Mass Function? A Critical Look at Variations

Abstract

Few topics in astronomy initiate such vigorous discussion as whether or not the initial mass function (IMF) of stars is universal, or instead sensitive to the initial conditions of star formation. The distinction is of critical importance: the IMF influences most of the observable properties of stellar populations and galaxies, and detecting variations in the IMF could provide deep insights into the process by which stars form. In this review, we take a critical look at the case for IMF variations, with a view towards whether other explanations are sufficient given the evidence. Studies of the field, local young clusters and associations, and old globular clusters suggest that the vast majority were drawn from a "universal" IMF: a power-law of Salpeter index (Γ = 1.35) above a few solar masses, and a log normal or shallower power-law (Γ ∼ 0 − 0.25) between a few tenths and a few solar masses (ignoring the effects of unresolved binaries). The shape and universality of the IMF at the stellar-substellar boundary is still under investigation and uncertainties remain large, but most observations are consistent with a IMF that declines (Γ < −0.5) well below the hydrogen burning limit. Observations of resolved stellar populations and the integrated properties of most galaxies are also consistent with a "universal IMF", suggesting no gross variations in the IMF over much of cosmic time. There are indications of "non-standard" IMFs in specific local and extragalactic environments, which clearly warrant further study. Nonetheless, there is no clear evidence that the IMF varies strongly and systematically as a function of initial conditions after the first few generations of stars. We close with suggestions for future work that might uncover more subtle IMF variations than those that could be discerned to date.

Formation of proto-clusters and star formation within clusters: apparent universality of the initial mass function?

“…There is a possible variation independent of the cluster dynamical evolution, however, as can be seen for example in Fig. 3 (left panel) of Bastian et al (2010) and in Fig. 2 of de Marci et al (2010), which show that at a given dynamical age, the value of m c typically varies over a factor of about 2.…”

“…Moraux et al 2007;Bastian et al 2010 the IMF parameters, in particular on the characteristic mass, or peak position, but as pointed out by de Marci et al (2010) dynamical evolution is a plausible explanation to account for the strongest variations. There is a possible variation independent of the cluster dynamical evolution, however, as can be seen for example in Fig.…”

“…It is worth stressing that a possible deficiency of brown dwarfs is observed in Taurus (e.g. Guieu et al 2006;Luhman et al 2009;Bastian et al 2010) while a possible excess (factor of about 2) may have been observed in σ Ori and Upper Sco (e.g. Caballero et al 2007).…”

Nonthermal particles and photons in starburst regions and superbubbles

“…A critical review of both the resolved and the integrated properties of stellar populations in galaxies by Bastian et al (2010) concluded that the IMF is consistent with the gross of the available data and suggested no noticeable variations of the IMF over much of cosmic time. The apparent universality of the stellar IMF is non trivial given the strong variance of the conditions in SF regions in the galaxies.…”

Dynamical masses of early-type galaxies at z ~ 2

“…Until recently it appeared sensible to assume the IMF is universal in different galaxies, and of Kroupa (2001) or Chabrier (2003) type. This assumption was motivated by the observed IMF universality in different environments within our own Galaxy (Kroupa 2002;Bastian et al 2010).…”