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

TL;DR

This paper introduces a sub-galactic main sequence (SGMS) linking star formation rate surface density and stellar mass density within galaxies, demonstrating its consistency across scales and its dependence on galaxy morphology, with implications for understanding galaxy evolution.

Contribution

The study presents the first detailed analysis of the sub-galactic main sequence across different galaxy regions, including nuclei, and explores its dependence on morphology and implications for galaxy growth.

Findings

SGMS holds down to ~1 kpc scales with a slope of 0.91.

Late-type galaxies have a steeper SGMS slope (0.97) than early-types (0.81).

Nuclear regions show a distinct SFR-mass sequence, correlating with total galaxy mass.

Abstract

We describe a sub-galactic main sequence (SGMS) relating star formation rate surface density ($\Sigma_{\textrm{SFR}}$) and stellar mass density ($\Sigma_{\star}$) 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 $ \sim $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 star formation rates (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 sub-regions 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 sub-regions. Sampling a limited range of SFR$-$M$_{\star} $ space may produce either sub-linearity or super-linearity 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 main sequence will be useful for studying bulge growth and for characterizing feedback processes connecting AGN and star formation.