Galaxy And Mass Assembly (GAMA): The sSFR-M* relation part I - \(\sigma_{\mathrm{sSFR}}\)-M* as a function of sample, SFR indicator and environment

Abstract

Recently a number of studies have proposed that the dispersion along the star formation rate - stellar mass relation (\(\sigma_{\mathrm{sSFR}}\)-M\(_{*}\)) is indicative of variations in star-formation history (SFH) 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 feed-back respectively. However, measuring \(\sigma_{\mathrm{sSFR}}\) and the shape of the \(\sigma_{\mathrm{sSFR}}\)-M\(_{*}\) relation is problematic and can vary dramatically depending on the sample selected, chosen separation of passive/star-forming systems, and method of deriving star-formation rates (\(i.e.\) H\(\alpha\) emission vs spectral energy distribution fitting). As such, any astrophysical conclusions drawn from measurements of \(\sigma_{\mathrm{sSFR}}\) must consider these dependencies. Here we use the Galaxy And Mass Assembly survey to explore how \(\sigma_{\mathrm{sSFR}}\) varies with SFR indicator for a variety of selections for disc-like `main sequence' star-forming galaxies including colour, star-formation rate, visual morphology, bulge-to-total mass ratio, S\'{e}rsic 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 \(\sigma_{\mathrm{sSFR}}\)-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 populations at intermediate stellar masses.