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