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

Abstract

Our knowledge about the photometric and structural properties of bulges in late-type galaxies (LTGs) is founded upon image decomposition into a Sérsic 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 δio. 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 δio 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 δio 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., Sérsic 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, δio 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 δio 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 ℳ∙/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 δio (> 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 δio 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.

A new method for age-dating the formation of bars in disc galaxies

“…Although galaxy discs are usually assumed as having exponential light profiles, recent studies demonstrate that some cases may follow a flat light profile (e.g. Zhu et al 2018;Breda et al 2020;Papaderos et al 2022). In order to assess how much such a decision may A8, page 12 of 17…”

“…Finally, to re-construct a data cube of the MD, we extrapolated the representative spectrum back to a range of radii, again taking an exponential light profile into account for the MD. Although the MD is often described with an exponential light profile, recent studies show that discs may not follow an exponential light profile all the way to the centre (e.g., Zhu et al 2018;Breda et al 2020;Papaderos et al 2022). In light of this, we tested the implications of assuming an exponential profile in Appendix B.2, by applying the same methodology to a flat light profile MD.…”

The effect of spiral arms on the Sérsic photometry of galaxies

“…This is usually done by describing the total surface brightness as the sum of a de Vaucouleurs profile (a Sérsic profile with n = 4) for the bulge and an exponential profile (n = 1) for the disk. Unfortunately, there are limitations with this approach, as recently pointed out by Papaderos et al (2022). The disk is generally associated with the younger component of the stellar population of a galaxy.…”

The SAMI Galaxy Survey: flipping of the spin–filament alignment correlates most strongly with growth of the bulge

“…However, bulge/disc decompositions do not respect the entire galaxy complexity. In particular, the photometric profiles show limitations and degeneracies (Head et al 2015;Fischer et al 2019;Barsanti et al 2021a;Papaderos et al 2022;Sonnenfeld 2022). The disc exponential component is dominated by the galaxy outskirts and it is not able to completely capture the central region (Méndez-Abreu et al 2019;Breda et al 2020).…”