The role of metals from molecular clouds to galactic discs
Creators: name: Sharda, Piyush
Publisher: name: The Australian National University
Year: 2022

description: From the time the first generation of stars formed (redshift ~ 30) to the present-day, elements that were not produced by the Big Bang (hereafter, metals) have witnessed the assembly of structure in the Universe in great detail. Although metals only form in stars and stellar remnants, they are ubiquitously present everywhere -- from planetary cores to the intergalactic medium. However, we still do not understand how metals are effectively dispersed throughout the Universe, and what role do they play in shaping galaxies as we know today. In this thesis, we use a multi-scale approach to study the role of metals in galaxy evolution, from pc-sized molecular clouds to kpc-sized galactic discs. On smaller scales, we focus on understanding the physical processes that shape up the initial mass function (IMF, with a particular emphasis on metal-free and metal-poor environments) that directly set the integrated yield of metals in the first and early galaxies. On larger scales, we focus on the physics of gas-phase metal distribution in diverse galaxies both in the local and the high-z Universe. We develop a large suite of chemo-magnetohydrodynamic (chemo-MHD) simulations to study the formation of the first stars in metal-free environments. We find that initially weak magnetic fields exponentially grow in a short duration of time and suppress fragmentation of primordial molecular clouds, thereby preventing the formation of low mass first stars that would otherwise have lived to the present day. Magnetic fields grow via dynamo amplification during collapse, and later on in the accretion discs around the first stars. The overall impact of dynamically strong magnetic fields is that the mass distribution of the first stars shifts towards higher masses as compared to chemo-hydrodynamic simulations. An important consequence of our results is that the IMF of the first stars was likely top-heavy, as against the bottom-heavy IMF of the metal-rich Milky Way. We further create analytic models to explore the transition in the IMF as a function of the metallicity of the interstellar medium. We find that the IMF transitioned from top- to bottom-heavy in normal star-forming galaxies when the metallicity increased to 1/10000-1/100 times the Solar metallicity. Our models also provide an explanation for the more bottom-heavy IMF observed in the centres of massive ellipticals. Thanks to integral field unit spectroscopy, spatially-resolved gas-phase metallicities have now been measured for several thousand galaxies. In particular, the last decade has seen a surge in the measurements of metallicity gradients in galaxies, wherein galaxy centres are typically more metal enriched as compared to the outskirts. We develop a new, first-principles model to understand the physics behind these metallicity gradients. In contrast to existing models, our model incorporates all key modes of galactic metal transport (such as metal advection and diffusion), and allows for differential metal enrichment of galactic outflows. Further, the model is inherently linked to a galaxy evolution model, which ensures that metals are treated self-consistently with the gas in galactic discs and the number of free parameters in the model is restricted. We use our model to provide the first joint explanation for the mass-metallicity relation and the mass-metallicity gradient relation in local galaxies. We show that galaxies naturally transition from an advection-dominated to an accretion-dominated regime as they increase in mass. We also show that low mass galaxies preferentially loose more metals in galactic winds. Finally, we use the model to explore the complex relationship between metallicity gradients and galaxy kinematics, and use it to explain the trends observed in high-z galaxies. By studying the role of metals from pc to kpc scales, this research has thus set the scene to study the first galaxies and explore metal-poor environments of nearby dwarf galaxies.