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

Our knowledge about the photometric and structural properties of bulges in late-type galaxies (LTGs) is founded upon image decomposition into a Sersic model for the central luminosity excess of the bulge and an exponential model for the more extended 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 in the centers of LTGs. Extrapolating the fit to 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 (beneath the bulge). We refer to this effect as delta(io). Using predictions from evolutionary synthesis models and by applying to integral field spectroscopy data REMOVEYOUNG, a tool that allows the suppression of stellar populations younger than an adjustable age cutoff we estimate the delta(io) in the centers of massive SF-quenched LTGs to be up to similar to 2.5 (0.7) B (K) mag. The primary consequence of the neglect of delta(io) in bulge-disk decomposition studies is the oversubtraction of the disk underneath the bulge, leading to a systematic underestimation of the true luminosity of the latter. Secondary biases impact the structural characterization (e.g., Sersic exponent eta and effective radius) and color gradients of bulges, and might include the erroneous classification of LTGs with a moderately faint bulge as bulgeless disks. Framed in the picture of galaxy downsizing and inside-out SF quenching, delta(io) 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 the delta(io) effect will lead to a down-bending of the bulge versus supermassive black hole relation for galaxies below log(M/M-circle dot) similar to 10.7. A decreasing M-center dot/M ratio with decreasing M would help to consistently explain the scarcity and weakness of accretion-powered nuclear activity in low-mass spiral galaxies. Finally, it is pointed out that a well-detectable delta(io) (> 2 r mag) can emerge early on through inward migration of star-forming clumps from the disk in combination with a strong contrast of emission-line equivalent widths between the quenched protobulge and its star-forming periphery. Spatially resolved studies of delta(io) with the James Webb Space Telescope, the Extremely Large Telescope, and Euclid could therefore offer key insights into the chronology and physical drivers of SF-quenching in the early phase of galaxy assembly.