Unique Tracks Drive the Scatter of the Spatially-Resolved Star Formation Main Sequence

TL;DR

This study investigates the causes of scatter in the spatially resolved star formation main sequence (SFMS) in nearby galaxies, revealing the influence of galaxy structure, star formation rate estimates, and a critical stellar mass density.

Contribution

It identifies three distinct galaxy tracks in the SFMS based on stellar mass density, highlighting the role of internal processes and mass accretion in galaxy evolution.

Findings

Three galaxy tracks delineated by stellar mass density.

SFR estimates vary with timescales and physics, affecting SFMS position.

Morphology does not significantly influence SFMS scatter.

Abstract

The scatter of the spatially resolved star formation main sequence (SFMS) is investigated in order to reveal signatures about the processes of galaxy formation and evolution. We have assembled a sample of 355 nearby galaxies with spatially resolved H{\alpha} and mid-infrared fluxes from the Survey for Ionized Neutral Gas in Galaxies and the Wide-field Infrared Survey Explorer, respectively. We examine the impact of various star formation rate (SFR) and stellar mass transformations on the SFMS. Ranging from 10^6 to 10^11.5 M_sun and derived from color to mass-to-light ratio methods for mid-infrared bands, the stellar masses are internally consistent within their range of applicability and inherent systematic errors; a constant mass-to-light ratio also yields representative stellar masses. The various SFR estimates show intrinsic differences and produce noticeable vertical shifts in the SFMS, depending on the timescales and physics encompassed by the corresponding tracer. SFR estimates appear to break down on physical scales below 500 pc. We also examine the various sources of scatter in the spatially resolved SFMS and find morphology does not play a significant role. We identify three unique tracks across the SFMS by individual galaxies, delineated by a critical stellar mass density of log (\Sigma_M*)~7.5. Below this scale, the SFMS shows no clear trend and is likely driven by local, stochastic internal processes. Above this scale, all spatially resolved galaxies have comparable SFMS slopes but exhibit two different behaviors, resulting likely from the rate of mass accretion at the center of the galaxy.