Massive Compact Galaxies: A Case Study of the Diversity of Evolutionary Pathways in Cosmological Simulations

Abstract

Observations of the high-redshift Universe have revealed a population of remarkably dense massive quiescent galaxies at z~2. These objects have already formed 10^11 Msun of stars but have radii of only 1-2 kpc, approximately five times smaller than a typical quiescent galaxy at the same mass today. Their discovery has prompted a number of questions about galaxy formation in the early Universe: How do these galaxies attain such high stellar densities? Where are their (presumably compact, star-forming) progenitors? And what have those galaxies become in the local Universe? In this Thesis, I undertake a detailed study of the formation, evolution, and properties of the massive compact galaxy population in the cosmological hydrodynamical simulation Illustris. I identify an analogous population at z = 2 and trace them back in time to uncover their formation mechanisms, and forward to their present-day descendants. In either direction, I find a variety of available evolutionary paths. Massive compact galaxies may form in the simulation through an intense burst of centrally-concentrated star formation, or simply by assembling their mass at very early times. Likewise, their descendants take a variety of forms. Most commonly, the compact galaxies from z = 2 lie at the hearts of today's most massive galaxies, having accreted an envelope of ex-situ stars in the intervening 10 Gyr. However, they may also be consumed or disrupted in mergers, or they may survive unscathed to the present day. The diversity of evolutionary pathways that are taken by simulated galaxy populations presents a generic problem for current observational methods of predicting the properties of progenitor or descendant galaxy populations. I conclude by presenting a modification to such methods which is probabilistic in nature, taking the variety of potential pathways into account.