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Massive stars as gravitationally lensed transients -- Insights on the high-mass initial mass function

Sung Kei Li

TL;DR

The paper discusses using the detection rate of transient lensed stars in strongly lensed galaxies as a probe of the high-mass end of the stellar IMF beyond the local universe. By comparing observed transient counts to predictions under different IMF slopes, notably the Salpeter value $\alpha=2.3$ versus top-heavy $\alpha=1$, and marginalizing over lens models and star-formation histories, it assesses IMF evolution with redshift. Analyses of the Spock arc support a Salpeter-like IMF at $z\approx1$, Warhol is consistent with the local IMF while disfavouring a strong top-heaviness, and the HST Flashlights sample shows rates between these predictions, suggesting possible IMF evolution at $z>1$. Overall, the approach demonstrates a path to constrain IMF universality through lensed-star transients, with future JWST observations expected to yield larger samples and map IMF evolution across cosmic time.

Abstract

A robust stellar initial mass function (IMF) is crucial in any studies related to star formation. However, the direct measurement of the stellar IMF is confined to the local universe, limited by the resolving power of telescopes. Recently, a new method for accessing the stellar IMF beyond the local universe has been developed. The observed detection rate of transient lensed stars -- individual, massive, thus luminous stars in strongly lensed galaxies that are temporarily detectable upon stellar microlensing -- can serve as a probe to break the IMF-star formation history degeneracy in studies utilizing spectral energy distribution fitting, hence providing a window to look at the IMF at a subsample of gravitationally lensed galaxies. In this proceeding, I summarize the contributed talk given at IAUS402 entitled the same as this contribution and highlight some key results, which currently show no evidence for a top-heavy IMF in $z \approx 1$ galaxies.

Massive stars as gravitationally lensed transients -- Insights on the high-mass initial mass function

TL;DR

The paper discusses using the detection rate of transient lensed stars in strongly lensed galaxies as a probe of the high-mass end of the stellar IMF beyond the local universe. By comparing observed transient counts to predictions under different IMF slopes, notably the Salpeter value versus top-heavy , and marginalizing over lens models and star-formation histories, it assesses IMF evolution with redshift. Analyses of the Spock arc support a Salpeter-like IMF at , Warhol is consistent with the local IMF while disfavouring a strong top-heaviness, and the HST Flashlights sample shows rates between these predictions, suggesting possible IMF evolution at . Overall, the approach demonstrates a path to constrain IMF universality through lensed-star transients, with future JWST observations expected to yield larger samples and map IMF evolution across cosmic time.

Abstract

A robust stellar initial mass function (IMF) is crucial in any studies related to star formation. However, the direct measurement of the stellar IMF is confined to the local universe, limited by the resolving power of telescopes. Recently, a new method for accessing the stellar IMF beyond the local universe has been developed. The observed detection rate of transient lensed stars -- individual, massive, thus luminous stars in strongly lensed galaxies that are temporarily detectable upon stellar microlensing -- can serve as a probe to break the IMF-star formation history degeneracy in studies utilizing spectral energy distribution fitting, hence providing a window to look at the IMF at a subsample of gravitationally lensed galaxies. In this proceeding, I summarize the contributed talk given at IAUS402 entitled the same as this contribution and highlight some key results, which currently show no evidence for a top-heavy IMF in galaxies.
Paper Structure (6 sections, 2 figures)

This paper contains 6 sections, 2 figures.

Table of Contents

  1. Introduction
  2. Lensed stars' insight on the IMF
  3. "Spock"
  4. "Warhol"
  5. HST "Flashlights" Sample
  6. Conclusion

Figures (2)

  • Figure 1: Observed transient detection rate in the Spock arc (gray data points), compared with predicted detection rate when adopting a Salpeter IMF (blue, $\alpha = 2.3$), and a top-heavy IMF ($\alpha = 1$, red) as predicted by Li_2025_IMF. Filters leftward of the vertical dashed line are HST filters, whereas those on the right are JWST filters.
  • Figure 2: Observed transient detection rate (gray) versus transient detection rate predicted with a Salpeter IMF (blue) in the Warhol arc, numbers adopted from Palencia_2025_microlensing.