# The Evolution of Sizes and Specific Angular Momenta in Hierarchical Models of Galaxy Formation and Evolution

## TL;DR

This study uses the GAEA semi-analytic model to analyze the evolution of galaxy sizes and angular momenta up to redshift 2, comparing predictions with observations and exploring the effects of various physical processes.

## Contribution

It extends previous work to higher redshifts, providing insights into galaxy size evolution, the impact of dissipation, and the role of mergers and feedback in shaping galaxy properties.

## Key findings

- Model agrees with observed size-mass relations up to z~2.
- Bulge-dominated sizes are underestimated due to disk instability modeling.
- Massive galaxies are more affected by dissipation, leading to smaller sizes than observed.

## Abstract

We extend our previous work focused at $z\sim0$, studying the redshift evolution of galaxy dynamical properties using the state-of-the-art semi-analytic model GAEA: we show that the predicted size-mass relation for disky/star forming and quiescent galaxies is in good agreement with observational estimates, up to $z\sim2$. Bulge dominated galaxies have sizes that are offset low with respect to observational estimates, mainly due to our implementation of disk instability at high redshift. At large masses, both quiescent and bulge dominated galaxies have sizes smaller than observed. We interpret this as a consequence of our most massive galaxies having larger gas masses than observed, and therefore being more affected by dissipation. We argue that a proper treatment of quasar driven winds is needed to alleviate this problem. Our model compact galaxies have number densities in agreement with observational estimates and they form most of their stars in small and low angular momentum high-$z$ halos. GAEA predicts that a significant fraction of compact galaxies forming at high-$z$ is bound to merge with larger structures at lower redshifts: therefore they are not the progenitors of normal-size passive galaxies at $z=0$. Our model also predicts a stellar-halo size relation that is in good agreement with observational estimates. The ratio between stellar size and halo size is proportional to the halo spin and does not depend on stellar mass but for the most massive galaxies, where AGN feedback leads to a significant decrease of the retention factor (from about 80 per cent to 20 per cent).