The Intrinsic Ellipticity of Spiral Disks

TL;DR

This study measures the intrinsic ellipticity distribution of spiral galaxy disks using combined kinematic and photometric data, revealing a mostly circular shape with some ellipticity affecting galaxy scaling relations.

Contribution

It provides the first direct measurement of intrinsic ellipticities for face-on spiral disks using combined velocity and imaging data.

Findings

Disks have a log-normal ellipticity distribution with mean 0.06.

Ellipticities range from 0 (circular) to 0.2.

Disk ellipticity contributes to Tully-Fisher scatter.

Abstract

We have measured the distribution of intrinsic ellipticities for a sample of 28 relatively face-on spiral disks. We combine H-alpha velocity fields and R and I-band images to determine differences between kinematic and photometric inclination and position angles, from which we estimate intrinsic ellipticities of galaxy disks. Our findings suggest disks have a log-normal distribution of ellipticities (mean epsilon =0.06) and span a range from epsilon= 0 (circular) to epsilon=0.2. We are also able to construct a tight Tully-Fisher relation for our face-on sample. We use this to assess the contribution of disk ellipticity on the observed Tully-Fisher scatter.