title: Galactic Archaeology using the stellar vertical structure of galactic discs
Creators: name: Garcia De La Cruz, J
Publisher: name: Liverpool John Moores University
Year: 2022

description: The vertical distribution of stars within galactic discs is intrinsically connected to the evolution of the host galaxy: from the creation of galactic components such as the thin and thick disc to many disc features, like flaring, age gradients, or warps. Therefore, by understanding how the stellar vertical structure shapes these features, and how it is affected by different evolutionary processes, we can recover information from the past of galaxies: we can perform galactic archaeology. In this thesis, I use simulated galaxies in their cosmological context to study the connection between different disc features and galactic evolutionary paths with the shape of the stellar vertical density structure within galactic discs. First, I analyse how the flaring of mono-age populations (MAPs) influences the flaring and the age structure of geometrically-defined thick discs. I also explore under which circumstances the geometric thin and thick discs are meaningfully distinct components, or are part of a single continuous structure as in the Milky Way. I find that flat thick discs are created when MAPs barely flare or have low surface density at the radius where they start flaring. When looking at the vertical distribution of MAPs, these galaxies show a continuous thin/thick structure. They also have radial age gradients and tend to have quiescent merger histories. Those characteristics are consistent with what is observed in the Milky Way. Flared thick discs, on the other hand, are created when the MAPs that flare have a high surface density at the radius where they start flaring. The thick discs' scale-heights can either be dominated by multiple MAPs or just a few, depending on the mass and scale-height distribution of the MAPs. In a large fraction of these galaxies, thin and thick discs are clearly distinct structures. Finally, flared thick discs have diverse radial age gradients and merger histories, with galaxies that are more massive or that have undergone massive mergers showing flatter age radial gradients in their thick disc. Second, I study the connection between disc warping and disc heating. I analyse the vertical stellar density structure within warped stellar discs, and monitor the evolution of the scale-heights of the MAPs and the geometrical thin and thick disc during the warp's lifetime. In addition, I compare the overall thickness and the vertical velocity dispersion in the disc before and after the warp. I find that for warps made of pre-existing stellar particles shifted off-plane, the scale-heights do not change within the disc's warped region: discs bend rigidly. For warps made of off-plane new stellar material (either born in-situ or accreted), the warped region of the disc is not well described by a double $\mathrm{sech^2}$ density profile. Yet, once the warp is gone, the thin and thick disc structure is recovered, with their scale-heights following the same trends as in the region that was never warped. Finally, I find that the overall thickness and vertical velocity dispersion do not increase during a warp, regardless of the warp's origin. This holds even for warps triggered by interactions with satellites, which cause disc heating but before the warp forms. These findings suggest that the vertical structure of galaxies does not hold any memory of past warps. I finish this thesis by proposing how the findings explained in this document can be applied to the study of the Milky Way's disc. On the one hand, exploring the stellar vertical structure of Milky Way's disc can constrain its evolutionary past. On the other hand, by comparing and supporting observations of the Milky Way's disc with samples of simulated galaxies, we can better understand the Milky Way in the context of its neighbouring disc galaxies.