The Small Magellanic Cloud through the lens of the James Webb Space Telescope: Binaries and the mass function in the galaxy’s outskirts

¹ Dipartimento di Fisica e Astronomia “Galileo Galilei”, Univ. di Padova, Vicolo dell’Osservatorio 3, Padova 35122, Italy
² Istituto Nazionale di Astrofisica – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, Padova 35122, Italy
³ Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia
⁴ South-Western Institute for Astronomy Research, Yunnan University, Kunming 650500, PR China

^★ Corresponding author: mariavittoria.legnardi@unipd.it

Received: 3 July 2025
Accepted: 8 September 2025

Abstract

The stellar initial mass function (IMF) and the fraction of binary systems are fundamental ingredients that govern the formation and evolution of galaxies. Whether the IMF is universal or varies with environment remains one of the central open questions in astrophysics. Dwarf galaxies such as the Small Magellanic Cloud (SMC), with their low metallicity and diffuse star-forming regions, offer critical laboratories to address this issue. In this work, we exploit ultra-deep photometry from the James Webb Space Telescope to investigate the stellar populations in the field of the SMC. Using the m_F322W2 versus m_F115W – m_F322W2 color-magnitude diagram (CMD), we derive the luminosity function and measure the fraction of unresolved binary systems. We find a binary fraction of f_bin^q>0.6 = 0.14 ± 0.01, consistent with results from synthetic CMDs incorporating the metallicity distribution of the SMC. Additionally, the measured binary fraction in the SMC field is consistent with those observed in Galactic open clusters and Milky Way field stars of similar ages and masses, suggesting similar binary formation and evolutionary processes across these low-density environments. By combining the luminosity function with the best-fit isochrone, we derive the mass function (MF) down to 0.22 M_⊙, the lowest mass limit reached for the SMC to date. The resulting MF follows a power-law with a slope of α = −1.99 ± 0.08. This value is shallower than the canonical Salpeter slope of α = −2.35, providing new evidence for IMF variations in low-metallicity and low-density environments.