The Intrinsic Flattening of Galaxy Disks

TL;DR

This study measures the intrinsic flattening of galaxy disks using infrared imaging of edge-on galaxies, finding it to be mostly constant with some dependence on galaxy type, which informs galaxy structure and formation models.

Contribution

It provides a dust-extinction free, consistent measurement of galaxy disk flattening and explores its correlations with various galactic parameters, highlighting its dependence on Hubble type.

Findings

Intrinsic flattening is roughly constant at 0.124.

Flattening varies with galaxy Hubble type, being greater in earlier types.

No significant correlation with other galactic parameters was found.

Abstract

Highly inclined (edge-on) disk galaxies offer the unique perspective to constrain their intrinsic flattening, $c/a$, where $c$ and $a$ are respectively the vertical and long radial axes of the disk measured at suitable stellar densities. The ratio $c/a$ is a necessary quantity in the assessment of galaxy inclinations, three-dimensional structural reconstructions, total masses, as well as a constraint to galaxy formation models. 3.6 micron maps of 133 edge-on spiral galaxies from the Spitzer Survey of Stellar Structure in Galaxies (S4G) and its early-type galaxy extension are used to revisit the assessment of $c/a$ free from dust extinction and away from the influence of a stellar bulge. We present a simple definition of $c/a$ and explore trends with other galactic physical parameters: total stellar mass, concentration index, total HI mass, mass of the central mass concentration, circular velocity, model-dependent scales, as well as Hubble type. Other than a dependence on early/late Hubble types, and a related trend with light concentration, no other parameters were found to correlate with the intrinsic flattening of spiral galaxies. The latter is mostly constant with $\langle c/a \rangle$ = 0.124 $\pm$ 0.001 (stat) $\pm$ 0.033 (intrinsic/systematic) and greater for earlier types.