The star formation main sequence: The dependence of specific star formation rate and its dispersion on galaxy stellar mass

The dispersion of the star formation main sequence (SFMS) reflects the diversity of star formation histories (SFHs) and variation in star formation rates (SFRs) in star-forming galaxies (SFGs) with similar stellar masses (M*). We examine the dispersion of the local SFMS using a complete sample of Sloan Digital Sky Survey galaxies at $0.01<z<0.03$ with $log(M\ast /M_{\odot })>$ 8.8. The SFRs are estimated from Hα in combination with 22 μm observation from the Wide-field Infrared Survey Explorer. We measure the dispersion of specific SFRs (sSFRs) as a function of M*. We confirm that the dispersion increases with M* from 0.37 ± 0.01 dex at $log(M\ast /M_{\odot })<9.6$ to 0.51 ± 0.02 dex at $log(M\ast /M_{\odot })>$ 10.2. Despite star formation being mostly associated with disks, the dispersion of disk sSFR still increases with M*. We conclude that the presence of bulges/bars is likely responsible for the large dispersion of sSFR in massive SFGs, while low-mass SFGs are mostly disk dominated and thus have a small dispersion. Our results suggest that star formation on galactic scales is dramatically affected by central dense structures through both enhancing and/or quenching processes, while lower-mass SFGs tend to have less bursty SFHs. However, the dispersion of the sSFR becomes significantly smaller and remains constant when only disk-dominated SFGs are counted. This finding implies that the impact of stochastic stellar feedback on star formation is likely to follow the same pattern in all disk galaxies, showing no correlation with halo potential.