Building disc structure and galaxy properties through angular momentum: The DARK SAGE semi-analytic model

TL;DR

The DARK SAGE semi-analytic model simulates galaxy evolution by tracking disc properties in angular momentum space, accurately reproducing observed mass--specific angular momentum relations and highlighting the role of instabilities in galaxy development.

Contribution

This work introduces DARK SAGE, a detailed semi-analytic model that predicts galaxy disc structures and their angular momentum evolution, emphasizing the importance of disc instabilities.

Findings

The model reproduces the observed mass--specific angular momentum relation.

Disc instabilities are crucial for regulating galaxy spin and mass.

Without instabilities, high-mass discs would have significantly lower angular momentum.

Abstract

We present the new semi-analytic model of galaxy evolution, DARK SAGE, a heavily modified version of the publicly available SAGE code. The model is designed for detailed evolution of galactic discs. We evolve discs in a series of annuli with fixed specific angular momentum, which allows us to make predictions for the radial and angular-momentum structure of galaxies. Most physical processes, including all channels of star formation and associated feedback, are performed in these annuli. We present the surface density profiles of our model spiral galaxies, both as a function of radius and specific angular momentum, and find the discs naturally build a pseduobulge-like component. Our main results are focussed on predictions relating to the integrated mass--specific angular momentum relation of stellar discs. The model produces a distinct sequence between these properties in remarkable agreement with recent observational literature. We investigate the impact Toomre disc instabilities have on shaping this sequence and find they are crucial for regulating both the mass and spin of discs. Without instabilities, high-mass discs would be systematically deficient in specific angular momentum by a factor of ~2.5, with increased scatter. Instabilities also appear to drive the direction in which the mass--spin sequence of spiral galaxy discs evolves. With them, we find galaxies of fixed mass have higher specific angular momentum at later epochs.