Ultra-Deep Hubble Space Telescope Imaging of the Small Magellanic Cloud: The Initial Mass Function of Stars with M <~ 1 Msun

TL;DR

This study uses ultra-deep HST imaging to measure the stellar initial mass function in the Small Magellanic Cloud, finding a single power-law slope shallower than Salpeter down to 0.37 solar masses, with implications for star formation theories.

Contribution

First direct measurement of the IMF in the SMC down to 0.37 Msun using ultra-deep HST imaging and advanced simulations, revealing a consistent single power-law slope.

Findings

IMF slope of -1.90 ± 0.15 in the 0.37-0.93 Msun range

IMF does not turn over to a shallower slope within this mass range

Results align with the observed luminosity function at higher masses

Abstract

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 Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) observations reveal this rich, co-spatial population behind the foreground globular cluster 47 Tuc, which we targeted for 121 HST orbits. The stellar main sequence of the SMC is measured in the F606W, F814W 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 4 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 (i.e., down to a ~75% completeness limit at F606W = 28.7), we demonstrate that the IMF is well represented by a single power-law form with slope \alpha = -1.90 (^{+0.15}_{-0.10}) (3 sigma error) (i.e., dN/dM \propto M^{\alpha}). This is shallower than the Salpeter slope of \alpha = -2.35, which agrees with the observed stellar luminosity function at higher masses. Our results indicate that the IMF does {\it not} turn over to a more shallow power-law form within this mass range. We discuss implications of this result for the theory of star formation, the inferred masses of galaxies, and the (lack of a) variation of the IMF with metallicity.