Evolution of Cold Streams and Emergence of the Hubble Sequence

TL;DR

This paper proposes a physical framework linking the evolution of cold gas streams in galaxies to the emergence of the Hubble sequence, supported by high-resolution cosmological simulations and consistent with observations.

Contribution

It introduces a novel analysis of cold stream properties over cosmic time, explaining galaxy morphological evolution and the formation of the Hubble sequence.

Findings

Number of streams decreases with redshift

Inflow rate per stream decreases with redshift

Early-type galaxies lack cold gas streams at low redshift

Abstract

A new physical framework for the emergence of the Hubble sequence is outlined, based on novel analyses performed to quantify the evolution of cold streams of a large sample of galaxies from a state-of-the-art ultra-high resolution, large-scale adaptive mesh-refinement hydrodynamic simulation in a fully cosmological setting. It is found that the following three key physical variables of galactic cold inflows crossing the virial sphere substantially decrease with decreasing redshift: the number of streams N_{90} that make up 90% of concurrent inflow mass flux, average inflow rate per stream dot M_{90} and mean (mass flux weighted) gas density in the streams n_{gas}. Another key variable, the stream dimensionless angular momentum parameter lambda, instead is found to increase with decreasing redshift. Assimilating these trends and others leads naturally to a physically coherent scenario for the emergence of the Hubble sequence, including the following expectations: (1) the predominance of a mixture of disproportionately small irregular and complex disk galaxies at z>2 when most galaxies have multiple concurrent streams, (2) the beginning of the appearance of flocculent spirals at z~1-2 when the number of concurrent streams are about 2-3, (3) the grand-design spiral galaxies appear at z<1 when galaxies with only one major cold stream significantly emerge. These expected general trends are in good accord with observations. Early type galaxies are those that have entered a perennial state of zero cold gas stream, with their abundance increasing with decreasing redshift.