Stellar Initial Mass Function Dynamics in Galactic Systems

The stellar initial mass function (IMF) describes the distribution of stellar masses at birth and underpins our understanding of galaxy formation and evolution. Its shape controls the rates of supernova feedback, chemical enrichment and radiative output across cosmic time. Whereas the classical view posits a universal form, accumulating evidence points to variations driven by environmental factors such as gas temperature, density and metallicity. Dynamical processes—including protostellar core coalescence, regulated sampling of stellar masses and temperature‐dependent fragmentation—can alter the high‐mass and low‐mass ends of the IMF. Observational studies of resolved star clusters in nearby galaxies and statistical surveys of distant systems now reveal subtle but systematic departures from the canonical power‐law slopes. These advances refine models of star formation feedback and the interpretation of galaxy luminosities, colours and chemical abundances from the local neighbourhood to the epoch of reionisation.

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One comprehensive analysis of panchromatic photometry has introduced a temperature‐dependent IMF framework, revealing that most galaxies exhibit a continuum of mass functions that are bottom‐lighter than the Milky Way. Incorporating gas temperature into spectral energy distribution models yields lower inferred stellar masses and star formation rates by factors of up to three or more, and uncovers systematic variation of the IMF along the star‐forming main sequence and at quiescence.

High‐density environments have also been shown to influence the high‐mass slope of the IMF through protostellar core coalescence. Observational correlations between the power‐law slope and stellar surface density are reproduced by models in which close packing of cores enhances merging efficiency, leading to shallower high‐mass slopes in compact regions. This process provides a physical explanation for non‐universal IMFs in different galactic and extragalactic settings.

Resolved‐star studies in the Local Group, notably of young clusters in the disc of M33, have measured the high‐mass IMF with high precision and found a mean slope somewhat steeper than canonical values. Despite varying star formation rates and galactocentric radii, the IMF appears remarkably uniform across these clusters, suggesting that local processes imprint only mild variations on an otherwise near‐universal form in similar galactic environments.

Stellar Initial Mass Function Dynamics in Galactic Systems publication trend

The graph below shows the total number of articles in stellar initial mass function dynamics in galactic systems across all publications each year (not limited to Nature Index journals).

Technical terms

Stellar initial mass function (IMF): The distribution of stellar masses at the moment of their formation.

Power‐law slope (Γ): The exponent describing how the number of stars declines with increasing mass at the high‐mass end of the IMF.

Gas temperature (T_gas): The thermal state of molecular clouds that influences fragmentation and the resulting mass distribution of stars.

Protostellar coalescence: The merger of dense protostellar cores in crowded environments, leading to more massive stars.

Stellar surface density (σ*): The mass of stars per unit area in a system, reflecting the local density of star formation.

References

1. Implications of a Temperature-dependent Initial Mass Function. I. Photometric Template Fitting. The Astrophysical Journal (2022).
2. Implications of a Temperature-dependent Initial Mass Function. II. An Updated View of the Star-forming Main Sequence. The Astrophysical Journal (2022).
3. Variation of the High-mass Slope of the Stellar Initial Mass Function: Theory Meets Observations. The Astrophysical Journal (2023).
4. The Panchromatic Hubble Andromeda Treasury: Triangulum Extended Region (PHATTER). VI. The High-mass Stellar Initial Mass Function of M33. The Astronomical Journal (2024).