The diverse evolutionary paths of simulated high-z massive, compact galaxies to z=0

TL;DR

This study uses cosmological simulations to trace the evolution of high-redshift compact galaxies, revealing diverse evolutionary paths, significant size growth driven by mergers, and the complex relationship between progenitors and descendants.

Contribution

It provides a detailed analysis of the evolutionary trajectories of simulated massive compact galaxies from z=2 to z=0, highlighting the diversity and mechanisms of size and mass growth.

Findings

Only about 10% of compact galaxies remain compact at z=0.

Majority grow in size mainly through ex-situ mass acquisition.

Evolutionary paths are highly diverse, with various merger histories.

Abstract

Massive quiescent galaxies have much smaller physical sizes at high redshift than today. The strong evolution of galaxy size may be caused by progenitor bias, major and minor mergers, adiabatic expansion, and/or renewed star formation, but it is difficult to test these theories observationally. Herein, we select a sample of 35 massive, compact galaxies ($M_* = 1-3 \times 10^{11}$ M$_\odot$, $M_*/R^{1.5} > 10^{10.5}$ M$_\odot$/kpc$^{1.5}$) at $z=2$ in the cosmological hydrodynamical simulation Illustris and trace them forward to $z=0$ to uncover their evolution and identify their descendants. By $z=0$, the original factor of 3 difference in stellar mass spreads to a factor of 20. The dark matter halo masses similarly spread from a factor of 5 to 40. The galaxies' evolutionary paths are diverse: about half acquire an ex-situ envelope and are the core of a more massive descendant, a third survive undisturbed and gain very little mass, 15% are consumed in a merger with a more massive galaxy, and a small remainder are thoroughly mixed by major mergers. The galaxies grow in size as well as mass, and only $\sim$10% remain compact by $z=0$. The majority of the size growth is driven by the acquisition of ex-situ mass. The most massive galaxies at $z=0$ are the most likely to have compact progenitors, but this trend possesses significant dispersion which precludes a direct linkage to compact galaxies at $z=2$. The compact galaxies' merger rates are influenced by their $z=2$ environments, so that isolated or satellite compact galaxies (which are protected from mergers) are the most likely to survive to the present day.