2022A&A...658A..74P

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 δ_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 δ_io in the centers of massive SF-quenched LTGs to be up to ∼2.5 (0.7) B (K) mag. The primary consequence of the neglect of δ_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 η 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, δ_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 δ_io effect will lead to a down-bending of the bulge versus supermassive black hole relation for galaxies below log(M*/M_☉) ∼ 10.7. A decreasing (dM/dt)/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 δ_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 δ_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.