Zi-Hua Ho

Galaxy quenching, the cessation of star formation, can proceed in spatially distinct ways, commonly described as inside-out or outside-in. However, the inferred quenching pattern depends strongly on how quenched or quenching regions are defined observationally. We utilize a sample of ∼10,000 galaxies from the Mapping Nearby Galaxies at Apache Point Observatory DR17 survey to systematically compare four widely used diagnostics of star formation suppression—the specific star formation rate (sSFR), the 4000 Å break ( D _n 4000), poststarburst (PSB), and low-ionization nuclear emission-line region (LI(N)ER) emission—to examine how tracer choice influences the inferred spatial quenching pattern. Using the nonparametric method developed by L. Lin et al., we classify galaxies into inside-out and outside-in quenching modes based on the location on the plane of the fraction of the quenched area ( F _q ) and the concentration of the quenched area ( C _q ). We find that sSFR criterion yields comparable proportions of galaxies classified as inside-out and outside-in, while the D _n 4000 and LI(N)ER diagnostics strongly favor inside-out patterns. Because PSB traces a distinct transitional phase, PSB-selected spaxels occupy a different region of the F _q – C _q plane. Across most diagnostics, the fraction of galaxies classified as inside-out increases with stellar mass, while outside-in patterns are more common in lower-mass systems, especially among satellites. In contrast, the dependence of quenching mode on halo mass is weaker and less consistent across diagnostics. These differences show that the tracers probe complementary stages and timescales of star formation suppression, and together provide a more complete view of spatially resolved quenching.