Galaxy And Mass Assembly (GAMA): the sSFR-M* relation part I - σ_sSFR-M* as a function of sample, SFR indicator, and environment

Recently, a number of studies have proposed that the dispersion along the star formation rate (SFR) – stellar mass relation (σ_sSFR–M*) – is indicative of variations in star formation history driven by feedback processes. They found a ‘U’-shaped dispersion and attribute the increased scatter at low and high stellar masses to stellar and active galactic nuclei feedback, respectively. However, measuring σ_sSFR and the shape of the σ_sSFR–M* relation is problematic and can vary dramatically depending on the sample selected, chosen separation of passive/star-forming systems, and method of deriving SFRs (i.e. H α emission versus spectral energy distribution fitting). As such, any astrophysical conclusions drawn from measurements of σ_sSFR must consider these dependencies. Here, we use the Galaxy And Mass Assembly survey to explore how σ_sSFR varies with SFR indicator for a variety of selections for disc-like ‘main-sequence’ star-forming galaxies including colour, SFR, visual morphology, bulge-to-total mass ratio, Sérsic index, and mixture modelling. We find that irrespective of sample selection and/or SFR indicator, the dispersion along the sSFR–M* relation does follow a ‘U’-shaped distribution. This suggests that the shape is physical and not an artefact of sample selection or method. We then compare the σ_sSFR–M* relation to state-of-the-art hydrodynamical and semi-analytic models and find good agreement with our observed results. Finally, we find that for group satellites this ‘U’-shaped distribution is not observed due to additional high scatter population at intermediate stellar masses.