Stochastic star formation and a (nearly) uniform stellar initial mass function
(16 votes over all institutions)
Because Halpha and FUV photons originate mainly in stars of different masses, the ratio of the integrated Halpha and FUV luminosities in a galaxy provides a sensitive probe of its stellar initial mass function (IMF). Recent observations indicate that this ratio is lower in dwarf galaxies than in brighter systems, suggesting a truncated and/or steeper IMF in small galaxies. However, at low star formation rates (SFRs), the Halpha to FUV ratio can vary due to stochastic sampling even for a universal IMF, a hypothesis that has, prior to this work, received limited investigation. Using SLUG, a fully stochastic code for synthetic photometry in star clusters and galaxies, we compare the Halpha and FUV luminosity in a sample of ~450 nearby galaxies with models drawn from a universal Salpeter IMF and a modified IMF, the integrated galactic initial mass function (IGIMF). Once random sampling is included, a Salpeter IMF reproduces the observed Halpha distribution at all FUV luminosities, particularly in models where stars are formed within clusters. This suggests that the observed luminosity is the result of the joint probability distribution function of the SFR, cluster mass function, and a universal IMF. While this idea is similar to the previously proposed IGIMF theory, the IGIMF underestimates the Halpha luminosity by more than an order of magnitude at the lowest SFRs, ruling out a truncation in the IMF. Future work will examine stochastic star formation in detail to study whether random sampling can explain other observations that suggest a varying IMF.
