Kinematic Evidence for Halo Substructure in Spiral Galaxies

TL;DR

This study measures the velocities of planetary nebulae in the outskirts of two spiral galaxies, revealing evidence of halo substructure and disk flaring consistent with hierarchical galaxy formation models.

Contribution

It provides the first kinematic evidence supporting halo substructure and disk flaring in spiral galaxies' outer regions, validating models of galaxy formation involving halo interactions.

Findings

Vertical velocity dispersion decreases exponentially in inner regions.

Outer regions show a plateau in velocity dispersion around 20 km/s.

Outer disk flaring is observed, especially in M94.

Abstract

We present the results of a kinematic study of planetary nebulae in the extreme outskirts of two spiral galaxies, M83 (NGC 5236) and M94 (NGC 4736). We find that in the inner regions of the galaxies, the vertical velocity dispersion (sigma_z) falls off exponentially with the light, as expected for a constant mass-to-light ratio, constant thickness disk. However, starting at four optical scale lengths, sigma_z asymptotes out at roughly 20 km/s. Our analysis finds evidence for significant flaring in the outer regions as well, especially in M94. These observations are in excellent agreement with predictions derived from models of disk heating by halo substructure, and demonstrate how kinematic surveys in the outer disks of spirals can be used to test hierarchical models of galaxy formation.