Characteristics of Spiral Arms in Late-type Galaxies

TL;DR

This study analyzes the structure of spiral arms in four late-type galaxies by measuring H II regions, revealing variable pitch angles, increasing arm widths with radius, and segmental arm features consistent with N-body simulation predictions.

Contribution

It provides detailed measurements of spiral arm properties in multiple galaxies, highlighting segmental structures and trends in arm width and pitch angle, with comparisons to simulation results.

Findings

Spiral pitch angles vary along arms but show no systematic radial trend.

Arm widths generally increase with galactocentric distance, except in outer regions.

Spiral arms are composed of ~5 kpc segments with kinks and abrupt changes, matching simulation predictions.

Abstract

We have measured the positions of large numbers of H II regions in four nearly face-on, late-type, spiral galaxies: NGC 628 (M 74), NGC 1232, NGC 3184 and NGC 5194 (M 51). Fitting log-periodic spiral models to segments of each arm yields local estimates of spiral pitch angle and arm width. While pitch angles vary considerably along individual arms, among arms within a galaxy, and among galaxies, we find no systematic trend with galactocentric distance. We estimate the widths of the arm segments from the scatter in the distances of the H II regions from the spiral model. All major arms in these galaxies show spiral arm width increasing with distance from the galactic center, similar to the trend seen in the Milky Way. However, in the outer-most parts of the galaxies, where massive star formation declines, some arms reverse this trend and narrow. We find that spiral arms often appear to be composed of segments of ~5 kpc length, which join to form kinks and abrupt changes in pitch angle and arm width; these characteristics are consistent with properties seen in the large N-body simulations of D'Onghia, Vogelsberger and Hernquist (2013) and others.