be considered as the ratio of massive O and B stars to $\sim1$ [M$_\odot$]{} stars present in a galaxy.

The rest frame optical colours of a galaxy is tightly correlated with its [H$\alpha$]{} emission. The [H$\alpha$]{} flux probes the specific SFR (sSFR) of the shorter lived massive stars, while the optical colours probe the sSFR of the longer lived less massive stars. Therefore, in a smooth exponentially declining SFH, the optical colour of a galaxy will transit from bluer to redder colours with time due to the increased abundance of older less massive red stars. Similarly, with declining SFR the [H$\alpha$]{} flux will decrease and the continuum contribution of the older redder stars will increase, which will act to decrease the [H$\alpha$]{} EW in a similar SFH. The [H$\alpha$]{} EW and optical colours parameter space is degenerated in such a way that the slope of the function is equivalent to lowering the highest mass stars that are formed and/or increasing the fraction of intermediate-mass stars.

Multiple studies have investigated possibilities for IMF variation in galaxies using Balmer line flux in the context of probing SFHs [@Meurer2009; @Weisz2012; @Zeimann2014; @Guo2016; @Smit2016]. Modelling effects of IMF variation using [H$\alpha$]{} or [H$\beta$]{} to 