Ultra-Deep HST Imaging of the SMC: The IMF at M < 1 Msun

We present a new measurement of the stellar Initial Mass Function (IMF) based on ultra-deep, high-resolution photometry of >5,000 stars in the outskirts of the Small Magellanic Cloud (SMC) galaxy. The stellar main sequence of the SMC is measured in the color-magnitude diagram (CMD) down to ~30th magnitude, and is cleanly separated from the foreground star cluster population using proper motions. We simulate the SMC population by extracting stellar masses (single and unresolved binaries) from specific IMFs, and converting those masses to luminosities in our bandpasses. The corresponding photometry for these simulated stars is drawn directly from a rich cloud of million artificial stars, thereby accounting for the real photometric scatter and completeness of the data. Over a continuous and well populated mass range of M = 0.37-0.93 Msun, we demonstrate that the IMF is well represented by a single power-law form with slope α = -1.90 ([+0.15] [-0.10]) (3 σ error) (i.e., dN/dM propto M^(α), Salpeter α = -2.35). Our results indicate that the IMF does not turn over to a more shallow power law form within this mass range.