In a Universe dominated by Cold Dark Matter (CDM), the growth of galaxies is expected to happen hierarchically: mergers of small systems eventually create a giant galaxy. This theory has been very successful at describing the evolution of Mpc-size structures (such as filaments and galaxy clusters) and it seems to work well for massive ellipticals. However, the stellar discs of massive spirals, such as our own Milky Way (MW), are fragile systems that may not survive a violent merger. Therefore, the question arises: how are discy giants built? This problem of forming MW-like galaxies has been largely discussed in simulations, but systematic observational constraints are still lacking. This proposed project (Coordinating BEARD - COBEARD) aims at witnessing the growth of MW-like galaxies through two main
assets: the unique dataset provided by the BEARD International Time Programme (ITP) and novel analysis tools. BEARD (Bulge Evolution And the Rise of Discs; PI: J. Méndez-Abreu) is an ITP awarded with 78 telescope nights at the Roque de los Muchachos observatory for observing a volume-limited sample of 66 MW-like galaxies, i.e., massive spirals with small bulges. The Milky Way is known to host a stellar bar as well, but the role that these structures play in galaxy growth within CDM (e.g. star formation enhancement versus quenching) is still unclear and needs to be specifically assessed. The BEARD sample conveniently includes 60% of barred galaxies, allowing to probe the effect of bars. BEARD provides a multi-nature dataset --very deep imaging (WFC@INT), long-slit spectroscopy (DOLORES@TNG), integral-field spectroscopy (MEGARA@GTC), narrow-band imaging (IO:O@LT)-, and ancillary integralfield spectra from MUSE@VLT are available for 9 BEARD galaxies (including 3 bar hosts). The BEARD collaboration gathers an international team of 25 experts in observations and simulations aiming at answering a variety of questions about MW-like galaxies. With this proposal, the applicants will have the support to lead and coordinate the exploitation of BEARD, thus guaranteeing the success of this ITP and recovering the investment made by the Spanish community in this programme. The BEARD webpage will be renewed and an interactive database will be created to provide the international community with the BEARD dataproducts, assuring its legacy character. Furthermore, the proposed project has two specific scientific objectives to be developed by the new team (the two PIs, one postdoctoral researcher, and one FPI predoctoral student): SO1) analyse the mass growth of massive spirals through a derivation of the episodes of star formation they have undergone (the so-called star formation history) and other diagnostics imprinted in their stellar populations (relative abundances of elements). This analysis will be performed not only on the galaxies as a whole, but also on the isolated stellar structures (bulges, discs, and bars) using our own-developed C2D spectrophotometric decomposition code; SO2) study the past merger history of MW-like galaxies by characterising low-surface brightness structures in the galaxy outskirts and the accreted mass fraction of their stellar haloes.