astrobites | Daily Paper Summaries | 2020 Apr 11
Jason Hinkle wrote:
One widely-held theory of how galaxies grow and evolve throughout cosmic time is a process known as hierarchical growth. In this scenario, smaller galaxies merge with each other to form larger galaxies, which explains why the most massive galaxies we see live in the local universe. There are two major types of galaxies in the Universe: spirals and ellipticals, often called disk and spheroidal galaxies. For more details on galaxy evolution check out this astrobite.
Because galaxy mergers tidally distort pairs of galaxies, stars initially on ordered orbits within the galaxies tend to be disturbed and placed on randomized orbits. This means that the most massive galaxies, which have formed through multiple major mergers, tend to be spheroidal in nature. Despite this, roughly 10 percent of galaxies more massive than ~4 times the mass of the Milky Way have significant disks. Because the number of major mergers a galaxy experiences is highly dependent on its stellar mass, it is likely that all massive galaxies have gone through a similar number of large mergers throughout cosmic time. If this is true, then why do we still see massive galaxies with disks in the local universe?
In order to answer this question, the authors of this paper simulate the universe using the Horizon-AGN simulations. These simulations include both stellar and AGN feedback, which impart energy, momentum, and enriched material into the galaxy. With their simulation, the authors can trace the evolution of stars, black holes, and galaxy morphology in great detail over time. The merger histories of galaxies are traced from a redshift of z = 3 to z = 0.06 (corresponding to almost 11 billion years) in time steps of ~130 million years. ...
Why do extremely massive disc galaxies exist today? ~ Ryan A. Jackson et al