The Stellar Phase Density of the Local Universe and its Implications for Galaxy Evolution

TL;DR

This paper explores how stellar phase density constraints can inform galaxy evolution, demonstrating that some galaxy formation pathways are incompatible with collisionless processes and highlighting the potential of phase space analysis for understanding galaxy history.

Contribution

It introduces a method to use stellar phase density inequalities to constrain galaxy evolution pathways and applies it to local universe data, revealing limitations on collisionless formation scenarios.

Findings

Massive ellipticals cannot form from collisionless mergers of low-mass galaxies.

Including bulge components alleviates some evolutionary constraints.

Phase density inequalities can rule out certain galaxy formation pathways.

Abstract

This paper introduces the idea that the general mixing inequality obeyed by evolving stellar phase densities may place useful constraints on the possible history of the over-all galaxy population. We construct simple models for the full stellar phase space distributions of galaxies' disk and spheroidal components, and reproduce the well-known result that the maximum phase density of an elliptical galaxy is too high to be produced collisionlessly from a disk system, although we also show that the inclusion of a bulge component in the disk removes this evolutionary impediment. In order to draw more general conclusions about the evolution of the galaxy population, we use the Millennium Galaxy Catalogue to construct a model of the entire phase density distribution of stars in a representative sample of the local Universe. In such a composite population, we show that the mixing inequality rules out some evolutionary paths that are not prohibited by consideration of the maximum phase density alone, and thus show that the massive ellipticals in this population could not have formed purely from collisionless mergers of a low mass galaxy population like that found in the local Universe. Although the violation of the mixing inequality is in this case quite minor, and hence avoidable with a modest amount of non-collisionless star formation in the merger process, it does confirm the potential of this approach. The future measurement of stellar phase densities at higher redshift will allow this potential to be fully exploited, offering a new way to look at the possible pathways for galaxy evolution, and to learn about the environment of star formation through the way that this phase space becomes populated over time.