Morphological diversity of spiral galaxies originating in the cold gas inflow from cosmic webs

TL;DR

This paper proposes a unified model explaining the morphological diversity of spiral galaxies based on cold gas inflow from cosmic webs, linking accretion processes to galaxy structure variations across different masses.

Contribution

It introduces a comprehensive scenario connecting cold and hot gas accretion to the formation of galaxy components, explaining observed mass-dependent morphological features.

Findings

More massive galaxies have lower thick disc mass fractions.

Higher bulge mass fractions are observed in more massive galaxies.

Thick discs are older and poorer in iron, consistent with the model.

Abstract

Spiral galaxies comprise three major structural components; thin discs, thick discs, and central bulges. Relative dominance of these components is known to correlate with the total mass of the galaxy, and produces a remarkable morphological variety of spiral galaxies. Although there are many formation scenarios regarding individual components, no unified theory exists which explains this systematic variation. The cold-flow hypothesis predicts that galaxies grow by accretion of cold gas from cosmic webs (cold accretion) when their mass is below a certain threshold, whereas in the high-mass regime the gas that entered the dark matter halo is first heated by shock waves to high temperatures and then accretes to the forming galaxy as it cools emitting radiation (cooling flow). This hypothesis also predicts that massive galaxies at high redshifts have a hybrid accretion structure in which filaments of cold inflowing gas penetrate surrounding hot gas. In the case of Milky Way, the previous study suggested that the cold accretion created its thick disc in early times and the cooling flow formed the thin disc in later epochs. Here we report that extending this idea to galaxies with various masses and associating the hybrid accretion with the formation of bulges reproduces the observed mass-dependent structures of spiral galaxies: namely, more massive galaxies have lower thick disc mass fractions and higher bulge mass fractions. The proposed scenario predicts that thick discs are older in age and poorer in iron than thin discs, the trend observed in the Milky Way (MW) and other spiral galaxies.