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Galaxy size and angular momentum

Both the observed sizes and angular momentum contents of galaxies scale with mass, evolve with cosmic time, and depend of galaxy morphological type and star-formation rate. These two quantities are also intimately related to each other, and are governed by a large variety of physical processes.

Utilizing cosmological simulations such as Illustris and IllustrisTNG, I am working towards a better understanding of these issues in several aspects. Having demonstrated for the first time a good agreement between simulated and observed angular momentum scaling relations for a large galaxy population, I have explored the role that feedback plays in setting those relations. I found that the presence of strong galactic winds increases the specific angular momentum of galaxies of a given stellar mass by factors of between approximately three and ten, bringing some galaxies to the large values observed in real disk galaxies. These high angular momentum values, interestingly, are similar to those obtained from cosmological tidal torques on proto-galactic scales. An exploration of this similarity in simulated Milky Way-like galaxies by my student Daniel DeFelippis led to the conclusion that a multitude of factors, involving both angular momentum losses and gains in gas flows in and around galaxies, contribute to this result. This suggested a complex - rather than simple, as is often assumed - origin to this similarity.

A study I led on the size evolution of galaxies over cosmic time showed that the size evolution histories of simulated quenched galaxies differ substantially from those of star-forming galaxies in several respects. Most notably, the progenitors of present-day quenched galaxies are significantly smaller than those of present-day star-forming galaxies, both as a function of mass and of cosmic time - even back in the time when those progenitors were still star-forming themselves. These diverging evolution tracks hold clues to the mutual influence of galaxy size and star-formation activity on each other.