Stellar angular momentum can be controlled from cosmological initial conditions

TL;DR

This study demonstrates that the angular momentum of galaxies can be manipulated from early cosmological conditions, affecting their evolution, structure, and observable properties, through high-resolution numerical simulations.

Contribution

It introduces a method to control galaxy angular momentum from initial conditions and explores its impact on galaxy evolution and morphology.

Findings

Altering early-universe angular momentum affects merger timing and orbital parameters.

Changing stellar angular momentum influences galaxy size, kinematic ratios, and bulge fraction.

Controlling angular momentum helps understand galaxy scaling relations and intrinsic alignments.

Abstract

The angular momentum of galaxies controls the kinematics of their stars, which in turn drives observable quantities such as the apparent radius, the bulge fraction, and the alignment with other nearby structures. To show how angular momentum of galaxies is determined, we build high (${35}\,\mathrm{pc}$) resolution numerical experiments in which we increase or decrease the angular momentum of the Lagrangian patches in the early universe. We simulate three galaxies over their histories from $z=200$ to $z=2$, each with five different choices for the angular momentum (fifteen simulations in total). Our results show that altering early-universe angular momentum changes the timing and orbital parameters of mergers, which in turn changes the total stellar angular momentum within a galaxy's virial radius in a predictable manner. Of our three galaxies, one has no large satellite at $z=2$; in this case, the specific angular momentum is concentrated in the central galaxy. We modify its stellar angular momentum over $0.7\,\mathrm{dex}$ (from $61$ to $320\,\mathrm{kpc.km.s^{-1}}$) and show that this causes its effective radius to grow by $40\,\%$, its $v/\sigma$ parameter to grow by a factor $\times 2.6$ and its bulge fraction to decrease from $0.72$ to $0.57$. The ability to control angular momentum will allow future studies to probe the causal origin of scaling relations between galaxy mass, angular momentum and morphology, and to better understand the origin of galactic intrinsic alignments.