Mutual information between galaxy properties and the initial predisposition

Abstract

The immense diversity of the galaxy population in the universe is believed to stem from their disparate merging and star formation histories, and multi-scale influences of diverse environments. No single causal factor of the initial state is known to explain how the galaxies formed and evolved to end up possessing such various traits as they have at the present epoch. However, several observational studies have revealed that the key physical properties of the observed galaxies in the local universe appeared to have a much simpler, lower-dimensional correlation structure than expected, the origin of which remains unexplained. Speculating that the emergence of such a simple correlation structure of the galaxy properties must be triggered by nature rather than by nurture, we explore if the present galaxy properties may be correlated with the initial precondition for protogalaxy angular momentum, $\tau$, and test it against the data from the IllustrisTNG300-1 hydrodynamic simulation. Employing Shannon's information theory, we discover that $\tau$ shares a significantly large amount of mutual information with each of the four basic traits of the TNG galaxies at $z=0$: the spin parameters, formation epochs, stellar-to-total mass ratios, and fraction of kinetic energy in ordered rotation. These basic traits except for the stellar-to-total mass ratios are found to contain even a larger amount of MI about $\tau$ than about the total masses and environments for the case of giant galaxies with $11.5\le \log[M_{\rm t}/(h^{-1}\,M_{\odot})]<13$. Our results imply that the initial condition of the universe must be more impactful on the galaxy evolution than conventionally thought.