Gas Accretion and Angular Momentum

TL;DR

This paper reviews the role of gas accretion in acquiring angular momentum in galaxies and their halos, highlighting recent evidence for cold flow accretion and proposing revisions to traditional models.

Contribution

It introduces a revised model of galaxy formation emphasizing cold flow accretion's impact on angular momentum acquisition, contrasting with the canonical shock-heating picture.

Findings

Cold flow accretion carries high angular momentum gas.

Recent simulations support the importance of anisotropic gas inflow.

Revised models better match observational data.

Abstract

In this chapter, we review the role of gas accretion to the acquisition of angular momentum, both in galaxies and in their gaseous halos. We begin by discussing angular momentum in dark matter halos, with a brief review of tidal torque theory and the importance of mergers, followed by a discussion of the canonical picture of galaxy formation within this framework, where halo gas is presumed to shock-heat to the virial temperature of the halo, following the same spin distribution as the dark matter halo before cooling to the center of the halo to form a galaxy there. In the context of recent observational evidence demonstrating the presence of high angular momentum gas in galaxy halos, we review recent cosmological hydrodynamic simulations that have begun to emphasize the role of "cold flow" accretion---anisotropic gas accretion along cosmic filaments that does not shock-heat before sinking to the central galaxy. We discuss the implications of these simulations, reviewing a number of recent developments in the literature, and suggest a revision to the canonical model as it relates to the expected angular momentum content of gaseous halos around galaxies.