Keyi Ding, Mario Gennaro, Roberto J. Avila, Massimo Ricotti, Rachael L. Beaton, Martha L. Boyer, Thomas M. Brown, Annalisa Calamida, Santi Cassisi, Vedant Chandra, Roger E. Cohen, Matteo Correnti, Denija Crnojević, Kareem El-Badry, Marla Geha, Puragra Guhathakurta, Nitya Kallivayalil, Evan N. Kirby, Kristen. B. W. McQuinn, Alessandro Savino, Cheyanne Shariat, Joshua D. Simon, Daniel R. Weisz

The dependence of the stellar initial mass function (IMF) on star-formation environment, particularly at low metallicities and high redshifts, remains poorly constrained. Ultra-faint dwarf galaxies (UFDs) are local fossils of high-redshift galaxies hosting old, metal-poor populations, and their resolved stellar populations provide unique pathways to constrain the sub-solar IMF. We investigate the low-mass IMF in the Boötes I (Boo I) UFD with JWST/NIRCam, leveraging its capability to resolve over 10,000 stars reaching $\lesssim0.15 M_{\odot}$, obtaining one of the largest, deepest resolved stellar samples for UFDs. We explore three different functional forms of the IMF with machine learning and statistical techniques, combining forward modeling of synthetic color-magnitude diagrams with simulation-based inference. We find that a single power-law IMF fails to reproduce the observed luminosity function and also deviates from the canonical Salpeter IMF. Our best-fit broken power-law and lognormal IMF parameters are consistent with the Milky Way within 68% confidence level, providing evidence that star formation at metallicities as low as [Fe/H]$\approx-2.4$ follows a similar IMF as in the Milky Way. By treating Boo I as a local relic analogous to a high-redshift galaxy with a stellar mass of $\lesssim10^5 M_{\odot}$ at $z\gtrsim6$, our results provide evidence for the universality of the IMF across both local and high-redshift environments.