Indications of the invalidity of the exponentiality of the disk within bulges of spiral galaxies

TL;DR

This study challenges the assumption that galactic disks maintain an exponential profile into the bulge region, revealing that many disks exhibit a down-bending profile near the center, which impacts galaxy structural models.

Contribution

The paper introduces a spectrophotometric bulge-disk decomposition method and provides evidence that many disks do not follow a pure exponential profile inside the bulge radius.

Findings

Up to 30% of analyzed LTGs show unphysical bulge spectra assuming exponential disks.

Many disks exhibit a down-bending profile beneath the bulge.

Results suggest a need to revise galaxy structural decomposition models.

Abstract

(abridged) A fundamental subject in Extragalactic Astronomy concerns the formation and evolution of late-type galaxies (LTGs). The standard scenario comprises the early assembly of the bulge followed by disk accretion. However, recent observational evidence points to a joint formation and perpetual co-evolution of these structural components. Our current knowledge on the properties of bulge and disk is mostly founded on photometric decomposition studies, which sensitively depend on the adopted methodology and enclosed assumptions on the structure of LTGs. A critical assumption whose validity was never questioned is that galactic disks conserve their exponential nature up to the galactic center. This implies that bulge and disk co-exist without significant dynamical interaction and mass exchange over nearly the entire Hubble time. Our goal is to examine the validity of the standard assumption that galactic disks preserve their exponential intensity profile inside the bulge radius all the way to the galactic center. We developed a spectrophotometric bulge-disk decomposition technique that provides an estimation for the net spectrum of the bulge. A systematic application of our spectrophotometric bulge-disk decomposition tool to a representative sample of 135 local LTGs from the CALIFA Survey yields a significant fraction (up to ~30%) of unphysical net-bulge spectra when a purely exponential intensity profile is assumed for the disk. The obtained results suggest that, for a significant fraction of LTGs, the disk component shows a down-bending beneath the bulge. If proven to be true, such result will call for a substantial revision of structural decomposition studies for LTGs and have far-reaching implications in our understanding of the photometric properties of their bulges.