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

Abstract

We use the ZFIRE a survey to investigate the high mass slope of the initial mass function (IMF) for a mass-complete (log 10 (M * /M ) ∼ 9.3) sample of 102 star-forming galaxies at z ∼ 2 using their Hα equivalent widths (Hα-EW) and rest frame optical colours. We compare dust-corrected Hα-EW distributions with predictions of star-formation histories (SFH) from PEGASE.2 and Starburst99 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 (10-500Å) 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 cutoff. IMF variations and/or highly rotating extreme metal poor stars (Z∼ 0.1Z ) 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.

Calibrated, cosmological hydrodynamical simulations with variable IMFs III: spatially resolved properties and evolution

“…As for the ionizing fluxes, this relation does not evolve with redshift for the simulated galaxies, but shifts to higher Σ SFR,Salp , making the average high-mass slope shallower in high-z star-forming galaxies. These slopes are shallower than those found for the z ≈ 2 observations by Nanayakkara et al (2017), for which high-mass IMF slopes of ≈ −2 may be needed to explain their Hα EWs. Zhang et al (2018) recently concluded that high-mass IMF slopes as shallow as −2.1 would be required to explain the 13 CO/C 18 O ratios observed in z ≈ 2 − 3 submillimetre galaxies, which is also steeper than predicted by HiM-50.…”

“…Nanayakkara et al 2017;Zhang et al 2018). In particular, Nanayakkara et al (2017) find that the enhanced Hα equivalent widths (EW) of z ≈ 2 galaxies could be explained with an IMF slope shallower than (under our IMF definition) −2.0. To compare with their data, we compute the ratio of ionizing flux to the SDSS r-band flux, fion/fr, for our simulated galaxies, where fion is the flux of photons with λ < 912 • A, and is a proxy for the Hα flux (Shivaei et al 2018).…”

“…That present-day high-mass ETGs formed the majority of their stars at high redshift leads to questions concerning how the IMF may have evolved over time. Indeed, observations of strongly star-forming galaxies at high redshift conclude that the high-mass slope of the IMF may need to be shallower to account for their Hα equivalent widths (Nanayakkara et al 2017) or abundance ratios (Zhang et al 2018). On the other hand, observations of present-day highmass ETGs, which are the descendants of high-redshift starbursts, are typically found to have bottom-heavy stellar populations, with their stellar spectra indicating an excess of dwarf stars relative to a Milky Way-like IMF (e.g.…”

Testing Feedback-regulated Star Formation in Gas-rich, Turbulent Disk Galaxies

“…Also we note in closing that more exotic possibilities such as a top-heavy IMF in high Σ SFR environment (e.g. Nanayakkara et al 2017) could reduce SFR and thus flatten out the redshift evolution of t dep ; more work to investigate this possibility could be informative. Finally, as stated above, maps of gas in turbulent disk galaxies will be crucial to determine how pressure may be impacting the properties of massive star forming clusters (as described in Elmegreen & Efremov 1997).…”

The Dawes Review 8: Measuring the Stellar Initial Mass Function

“…They found no signature corresponding to the significant bursts of star formation that would be seen if stochastic SFHs were the dominant effect. Subsequently, Nanayakkara et al (2017), too, demonstrated quantitatively that stochastic star formation histories could not explain their measurements, in a sample at much higher redshift. Gunawardhana et al (2011) further demonstrate that the result is robust to the implementation of dust correction and choice of population synthesis model.…”