The constrainability of galaxy positions in the $M_{\ast }$ -- SFR plane from SED fitting

Using a sample of 6,029 $z<0.06$ galaxies from the GAMA survey, we characterise the spread of galaxy property inferences on the M${}_{\ast }$ – SFR plane via Spectral Energy Distribution (SED) fitting. We analyse 24 different model configurations using the stellar population library code $\text{textsc}ProGeny$, and the SED-fitting code $\text{textsc}ProSpect$, and quantify how modelling assumptions affect inferred stellar masses, star formation rates, and the resulting classification of galaxies. We find that the reproducibility of inferred galaxy positions depends strongly on location within the $M_{\ast }$ – SFR plane, with uncertainties increasing dramatically below ${\log }_{10}(\mathrm{s}\mathrm{S}\mathrm{F}\mathrm{R}\mathrm{/}{\mathrm{y}\mathrm{r}}^{-1})\sim -11$. This boundary closely tracks the transition between robustly and poorly constrained SFR estimates, regardless of whether SFRs are derived from SED fitting or H${}_{\alpha }$ measurements. We find that star-bursting galaxies cannot be robustly isolated from SED fitting alone, while the separation of star-forming and passive galaxies is most consistently conducted by a cut in sSFR, rather than a dex cut relative to either a curved or linear SFS (Star Forming Sequence). We further show that transitioning (or Green Valley) selections using SED fitting are highly unstable, with only $\sim$25% of the sample being repeatedly selected, on average.