ZFire: Using Hα equivalent widths to investigate the in situ initial mass function at z ∼ 2

We use the ZFIRE (http://zfire.swinburne.edu.au) survey to investigate the high-mass slope of the initial mass function (IMF) for a mass-complete ( log10(M∗/M⊙)∼9.3 ) sample of 102 star-forming galaxies at z ∼ 2 using their Hα equivalent widths (Hα EWs) and rest-frame optical colours. We compare dust-corrected Hα EW distributions with predictions of star formation histories (SFHs) from pegase.2 and starburst 99 synthetic stellar population models. We find an excess of high Hα EW galaxies that are up to 0.3–0.5 dex above the model-predicted Salpeter IMF locus and the Hα EW distribution is much broader (10–500 Å) than can easily be explained by a simple monotonic SFH with a standard Salpeter-slope IMF. Though this discrepancy is somewhat alleviated when it is assumed that there is no relative attenuation difference between stars and nebular lines, the result is robust against observational biases, and no single IMF (i.e. non-Salpeter slope) can reproduce the data. We show using both spectral stacking and Monte Carlo simulations that starbursts cannot explain the EW distribution. We investigate other physical mechanisms including models with variations in stellar rotation, binary star evolution, metallicity and the IMF upper-mass cut-off. IMF variations and/or highly rotating extreme metal-poor stars (Z ∼ 0.1 Z⊙) with binary interactions are the most plausible explanations for our data. If the IMF varies, then the highest Hα EWs would require very shallow slopes (Γ > −1.0) with no one slope able to reproduce the data. Thus, the IMF would have to vary stochastically. We conclude that the stellar populations at z ≳ 2 show distinct differences from local populations and there is no simple physical model to explain the large variation in Hα EWs at z ∼ 2.