Galaxy discs are made of two components, the thin and the thick disc. Thick discs contain up to 50% of the baryonic mass of a galaxy. Because their stars are among the oldest in the Universe, thick discs are relics of the infancy of galaxies. Thus, understanding the origin of thick discs is key to unveil how galaxies themselves form, which in turn will constrain cosmological models. The formation mechanisms of thick discs are disputed. Are they the remains of the early violent galaxy formation processes or the effect of secular evolution over a cosmic time? Are their stars accreted in galaxy mergers or have they formed within their host galaxy? I have been leading pioneering studies of nearby thick discs and found that they are much more massive than previously thought and concluded that they most likely formed rapidly in the early stages of galaxy evolution. To definitely settle the issue of the thick disc origin I propose to extend previous work to the yet unexplored realms of spectroscopy and high redshift imaging:

1) Integral Field Unit (IFU) spectroscopy: Different thick disc formation scenarios predict distinct kinematic and stellar population signatures that are detectable through spectroscopy. Because of the faintness of thick discs, IFU observations with the largest telescopes are needed to study these signatures. The ideal instruments are MUSE at VLT and MEGARA at GTC. I will combine archival data with those obtained through new proposals to produce a survey of a few tens of nearby edge-on galaxies with different masses, morphological types, and environments. This will make it possible to detect the signatures of the origin of thick discs. It will also be possible to know whether different galaxy kinds have different thick disc formation mechanisms.

2) High-redshift imaging: In the past I have made photometric decompositions of nearby galaxies to study the thick disc properties. These studies are a zero redshift snapshot of the galaxy evolution. I plan to extend this work to high-redshift galaxies in order to study the evolution of the thick disc properties with cosmic time. Very deep high-resolution imaging is necessary. Therefore, I will use data from the public Hubble Space Telescope archive and forthcoming James Webb Space Telescope surveys. Understanding galaxy formation is key to constrain cosmological models which are intimately related to the nature of dark matter. Galaxy formation cannot be fully apprehended without understanding thick discs.

This project includes a postdoctoral researcher. I am also asking funding for an FPI PhD student. The IAC is an environment where tens of top-level scientists with a variety of skills (IFU, deep imaging, N-body simulations) work, which is ideal to support and provide training for  the postdoctoral researcher and the PhD student.