The sub-galactic and nuclear main sequences for local star-forming galaxies

We describe a sub-galactic main sequence (SGMS) relating star formation rate (SFR) surface density (Sigma(SFR)) and stellar mass density (Sigma(*)) for distinct regions within star-forming galaxies, including their nuclei. We use a sample of 246 nearby star-forming galaxies from the 'Star Formation Reference Survey and demonstrate that the SGMS holds down to similar to 1 kpc scales with a slope of alpha = 0.91 and a dispersion of 0.31 dex, similar to the well-known main sequence (MS) measured for globally integrated SFRs and stellar masses. The SGMS slope depends on galaxy morphology, with late-type galaxies (Sc-Irr) having alpha = 0.97 and early-type spirals (Sa-Sbc) having alpha = 0.81. The SGMS constructed from subregions of individual galaxies has on average the same characteristics as the composite SGMS from all galaxies. The SGMS for galaxy nuclei shows a dispersion similar to that seen for other subregions. Sampling a limited range of SFR-M-* space may produce either sublinearity or superlinearity of the SGMS slope. For nearly all galaxies, both SFR and stellar mass peak in the nucleus, indicating that circumnuclear clusters are among the most actively star-forming regions in the galaxy and the most massive. The nuclear SFR also correlates with total galaxy mass, forming a distinct sequence from the standard MS of star formation. The nuclear MS will be useful for studying bulge growth and for characterizing feedback processes connecting AGN and star formation.