GALYKOS · Galactic ArchaeoLoGy based on K-band and Optical Spectroscopy

The standard paradigm of the build-up of spiral galaxies recognises a two-phase scenario that comprises a quasi-monolithic formation of the bulge early-on, followed by the gradual assembly of the disk. However, recent observational evidence strongly suggests joint formation and perpetual co-evolution of both structures. Elucidating this process in its full complexity is of key astrophysical relevance and only possible through accurate characterisation of the main stellar components (bulge, disk and bar). These are primarily studied via photometric decomposition techniques, conventionally by extrapolating the exponential profile of the disk up to the galactic centre. Though hardly ever questioned, this is a strong assumption with broad and far-reaching implications, which could effectively be violated, particularly if the disk evolves in tandem with the bulge. Our latest work, Breda et al. (2020b), provided, for the first time, indirect observational evidence for a central down-bending of the disk, reinforcing theoretical predictions. This project, GALYKOS, will pioneer an unprecedented exploration of the central region of spiral galaxies through the combination of two techniques, i.e., spatially resolved spectral synthesis and kinematical Schwarzschild orbital decomposition of optical and K-band integral field spectroscopy data. This innovative methodology will permit to quantitatively constrain the true structure of the disk within the galactic centre and explore its dependence on the physical properties of spirals, revolutionizing photometric decomposition studies. Apart from opening new routes for the exploration of the assembly history of galaxies, this action will prompt for conceptual changes in our understanding of galaxy formation and evolution. Finally, it will rectify well-established galaxy scaling relations such as super-massive black hole vs. bulge and yield crucial insight into the nature of bulgeless galaxies, a remaining challenge for the ΛCDM model.