Inside-out star formation quenching and the need for a revision of bulge-disk decomposition concepts for spiral galaxies

Our knowledge about the photometric properties of bulges in late-type galaxies (LTGs) is founded upon image decomposition into a S\'ersic model for central luminosity excess bulge and an exponential underlying disk. We argue that standard practice adopting disk all way to its center inadequate because it implicitly neglects fact star formation (SF) quenching (SFQ) centers LTGs. Extrapolating fit observable star-forming zone (outside bulge) inwardly overestimates true surface brightness SF-quenched zone. refer this effect as Dio. The primary consequence neglect Dio bulge-disk studies oversubtraction underneath bulge, leading systematic underestimation latter. Framed picture galaxy downsizing inside-out SFQ, expected differentially impact across redshift stellar mass M*, thus complex biases scatter slope various scaling relations. conjecture correction will lead downbending vs. super-massive black hole (SMBH) relation below log(M*/Msolar)~10.7. A decreasing M(SMBH)/M* ratio with M* would help consistently explain scarcity weakness accretion-powered nuclear activity low-mass spiral galaxies. well detectable (~2 r mag) can emerge early on through inward migration SF clumps from combination strong contrast emission-line equivalent widths between quenched proto-bulge periphery. Spatially resolved JWST, ELT, Euclid could therefore offer key insights chronology physical drivers SFQ phase assembly. (abridged)