The Star-Formation Main Sequence: The Dependence of Specific Star Formation Rate and Its Dispersion on Galaxy Stellar Mass

Abstract: The dispersion of the star-formation main sequence (SFMS) reflects the diversity of star formation histories and variation in star formation rates (SFRs) in star-forming galaxies (SFGs) with similar stellar masses ($M^\ast$). We examine the dispersion of local SFMS using a complete sample of Sloan Digital Sky Survey galaxies at 0.01$<z<$0.03 with $\log(M^\ast/M_\odot)>$8.8. The SFRs are estimated from H$\alpha$ in combination with 22$\mu m$ observation from WISE. The catalog of bulge+disk decomposition from Simard et al. (2011) is available for the sample galaxies. We measure the dispersion of specific SFR (SSFR) as a function of $M^*$. We confirm that the dispersion increases with $M^*$ from 0.37$\pm0.01$dex at $\log(M^\ast/M_\odot)<$9.6 to 0.51$\pm0.02$dex at $\log(M^\ast/M_\odot)>$10.2. Despite star formation is 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 with 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 star formation histories. However, the dispersion of 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.