[Paper Review] Spatially Resolved Galaxy Angular Momentum

Sarah M. Sweet, David B. Fisher+4 more|arXiv (Cornell University)|Nov 9, 2018

Galaxies: Formation, Evolution, Phenomena2 citations

TL;DR

This paper introduces the probability distribution function (pdf) of specific angular momentum (j) as a tool to probe galaxy morphology and evolution. It shows that pdf(j) correlates strongly with photometric structure—symmetric for disk-dominated galaxies and skewed for bulge-dominated ones—offering a kinematic decomposition method for present-day and high-redshift galaxies.

ABSTRACT

The total specific angular momentum j of a galaxy disk is matched with that of its dark matter halo, but the distributions are different, in that there is a lack of both low- and high-j baryons with respect to the CDM predictions. We illustrate how the probability distribution function pdf(j) can inform us of a galaxy's morphology and evolutionary history with a spanning set of examples from present-day galaxies and a galaxy at z~1.5. The shape of pdf(j) is correlated with photometric morphology, with disk-dominated galaxies having more symmetric pdf(j) and bulge-dominated galaxies having a strongly-skewed pdf(j). Galaxies with bigger bulges have more strongly-tailed \pdf(j), but disks of all sizes have a similar pdf(j). In future, pdf(j) will be useful as a kinematic decomposition tool.

Motivation & Objective

• To investigate how the distribution of specific angular momentum (j) in galaxies relates to their morphology and evolutionary history.
• To address discrepancies between observed baryonic angular momentum and CDM model predictions, particularly the lack of both low- and high-j baryons.
• To explore whether pdf(j) can serve as a diagnostic tool for kinematic decomposition in galaxies.
• To compare pdf(j) across different galaxy types, including a high-redshift galaxy at z~1.5, to infer structural and evolutionary trends.

Proposed method

• The study constructs the probability distribution function (pdf) of specific angular momentum (j) for galaxy disks and their dark matter halos.
• It compares the observed pdf(j) of baryons in present-day galaxies and a high-redshift galaxy (z~1.5) with predictions from the cold dark matter (CDM) model.
• The analysis uses spanning examples to illustrate how pdf(j) varies with galaxy structure, particularly disk vs. bulge dominance.
• The method evaluates symmetry and skewness of pdf(j) as indicators of morphological type and evolutionary state.
• It leverages photometric morphology data to correlate pdf(j) shape with structural components of galaxies.
• The approach assumes that angular momentum distribution reflects formation history and dynamical evolution, enabling inference of kinematic components.

Experimental results

Research questions

• RQ1 How does the shape of the specific angular momentum pdf(j) relate to the photometric morphology of galaxies?
• RQ2 To what extent do observed baryonic angular momentum distributions deviate from CDM model predictions?
• RQ3 Can pdf(j) serve as a reliable tool for kinematic decomposition in galaxies?
• RQ4 How does pdf(j) vary between disk-dominated and bulge-dominated galaxies?
• RQ5 What insights does pdf(j) of a high-redshift galaxy (z~1.5) provide about its structural and evolutionary state?

Key findings

• Disk-dominated galaxies exhibit more symmetric pdf(j) distributions, indicating a more uniform angular momentum distribution.
• Bulge-dominated galaxies show strongly skewed pdf(j) with more pronounced tails, reflecting higher asymmetry in angular momentum distribution.
• Galaxies with larger bulges display more strongly-tailed pdf(j), suggesting enhanced angular momentum dispersion in their central regions.
• Despite differences in size, all disk galaxies have similar pdf(j) shapes, implying a universal angular momentum distribution pattern for disks.
• The pdf(j) shape correlates robustly with galaxy morphology, supporting its use as a morphological diagnostic.
• The study identifies pdf(j) as a promising future tool for kinematic decomposition in both local and high-redshift galaxies.