Friday

The shape and universality of the initial mass function (IMF) is one of the most important questions in astronomy, being an integral part in both theoretical models and in deriving many key properties of galaxies. The most direct method to constrain the IMF, young stellar object counting, is impossible for cosmological distances and other traditional methods take advantage of UV, visible, and near- IR wavelengths making them inapplicable in the most extreme star forming galaxies due to the extreme dust extinction present in these systems. Many predictions of IMF variation occur within these systems, as such alternative methods to constrain the IMF are required, one promising method is via the chemical signatures left by massive stars. Due to their different production routes the 13C/18O isotope ratio is thought to be one such signature. By observing the rotational transitions of the isotopologues 13CO and C18O and thus the 13C/18O ratio we can probe the IMF free from the effects of dust. Here we present a new ALMA band 4 spectral scan containing the CO isotopologues in ‘9io9’, a strongly lensed sub-mm galaxy at z = 2.6. We find a global ratio an order of magnitude below that found in local spirals but in agreement with values in other SMGs and local ULIRGs. Using a state-of-the-art chemical evolution model, we probe the isotope ratio for a variety of IMFs and star formation histories finding that the low observed ratio in these extreme galaxies can be explained with a standard Milky Way IMF.