Angular momentum and morphological sequence of massive galaxies through Dark Sage

TL;DR

This study uses the Dark Sage model to explore how galaxy morphology correlates with angular momentum and halo properties, revealing that halo mass, not angular momentum, primarily influences galaxy structure at high masses.

Contribution

It demonstrates the complex relationship between galaxy morphology, stellar and total disk angular momentum, and dark matter halo properties using a semi-analytic model.

Findings

Disk galaxies have higher stellar disk angular momentum at fixed stellar mass.

Bulge-dominated galaxies have higher total disk angular momentum due to extended cold gas disks.

Halo mass, not angular momentum, is the main factor determining galaxy morphology at high stellar masses.

Abstract

We study the present-day connection between galaxy morphology and angular momentum using the {\sc Dark Sage} semi-analytic model of galaxy formation. For galaxies between $ 10^{11}-10^{12} \mathrm{M}_{\odot}$ in stellar mass, the model successfully predicts the observed trend whereby galaxies with more prominent disks exhibit higher {\em stellar} disk specific angular momentum ($j_{\rm stellar, disk}$) at fixed stellar mass. However, when we include the gas in the disk, bulge-dominated galaxies have the highest {\em total} disk specific angular momentum ($j_{\rm total, disk}$). We attribute this to a large contribution from an extended disk of cold gas in typical bulge-dominated galaxies. We find the relationship between $j_{\rm dark matter}$ and morphology to be quite complex. Surprisingly, in this stellar mass range, not only do bulge-dominated galaxies tend to live in halos with higher $j_{\rm dark matter}$ than disk-dominated galaxies, but intermediate galaxies (those with roughly equal fractions of bulge and disk mass) have the lowest $j_{\rm dark matter}$ of all. Yet, when controlling for halo mass, rather than stellar mass, the relationship between $j_{\rm dark matter}$ and morphology vanishes. Based on these results, halo mass rather than angular momentum is the main driver of the predicted morphology sequence at high masses. In fact, in our stellar mass range, disk-dominated galaxies live in dark matter halos that are roughly 1/10th the mass of their bulge-dominated counterparts.