The Small Magellanic Cloud through the lens of the James Webb Space Telescope : binaries and mass function within the galaxy outskirts

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 $m_{\rm F322W2}$ versus $m_{\rm F115W}-m_{\rm F322W2}$ color-magnitude diagram (CMD), we derive the luminosity function and measure the fraction of unresolved binary systems. We find a binary fraction of $f_{\rm bin}^{q>0.6}=0.14\pm0.01$, 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 the mass function (MF) down to $0.22\,M_{\odot}$, the lowest mass limit reached for the SMC to date. The resulting MF follows a power-law with a slope of $\alpha=-1.99\pm0.08$. This value is shallower than the canonical Salpeter slope of $\alpha=-2.35$, providing new evidence for IMF variations in low-metallicity and low-density environments.