The Scatter Dependence of the Spatially-Resolved Star Formation Main Sequence Relation

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Hall, Christine
galaxies: spiral , galaxies: star formation , galaxies: stellar content , galaxies: fundamental parameters , galaxies: photometry – surveys
The scatter of the spatially-resolved star formation main sequence (SFMS) is investigated in order to reveal signatures about processes of galaxy formation and evolution. We have assembled a sample of 355 nearby galaxies with spatially-resolved Hα 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 observed SFMS. Ranging from 10^6 to 10^11.5 M⊙ and derived from colour to mass-to-light ratio methods for mid-infrared bands, the stellar masses are internally consistent within their range of applicability, given the systematic errors inherent to such transformations; a constant mass-to-light ratio (M∗/L_3.4μm = 0.5) 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 star formation tracer. SFR estimates appear to break down on physical scales below 500 pc. We examine the various sources of scatter in the spatially- resolved SFMS and find that morphology plays little or no role. We identify three unique tracks across the spatially-resolved SFMS by individual galaxies, demarcated by a critical stellar mass of log( M∗/ M⊙)∼8.5. Below this scale, the spatially-resolved SFMS shows no clear trend and is likely driven by local, stochastic internal processes. Above this scale, galaxies have comparable spatially-resolved SFMS slopes but exhibit two different behaviours, likely resulting from the mass accretion rate and/or quenching at the center of the galaxy. The effect of a bulge or AGN on these types must be considered. Additionally, we contrast these observational results with the study of 58 galaxy simulations by the NIHAO (Numerical Investigation of a Hundred Astrophysical Objects) project. We uncover the three unique tracks also in simulations, though they are not as prominent. While the local simulated SFMS matches observations fairly well, some of our more detailed comparisons point to deficiencies in the underlying physics that regulate the evolution of NIHAO galaxies.
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