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