A New Statistical Analysis of the Morphology of Spiral Galaxies

TL;DR

This study classifies thousands of spiral galaxies from SDSS images, revealing the distribution of different spiral types and their structural characteristics, and compares these findings with the Milky Way's morphology.

Contribution

It provides a large-scale statistical analysis of spiral galaxy morphologies, including the distribution of arm types and their relation to bulge size, which was previously less comprehensively quantified.

Findings

Flocculent, multiple-arm, and grand-design galaxies constitute 38%, 59%, and 3% of the sample.

Larger bulges correlate with more multiple-arm and fewer flocculent galaxies.

Most galaxies with clear spiral arms have two inner arms, and barred spirals often have two inner and multiple outer arms.

Abstract

Morphology is the starting point for understanding galaxies. Elmegreen et al. classified spiral galaxies into flocculent, multiple-arm, and grand-design galaxies based on the regularity of their spiral arm structure. With the release of a vast number of clear spiral galaxy images from the Sloan Digital Sky Survey, we conducted a morphological classification of 5093 blue spiral galaxies. A statistical analysis of this sample shows that the fractions of flocculent, multiple-arm, and grand-design galaxies are 38 $\pm$ 1%, 59 $\pm$ 1%, and 3 $\pm$ 1%, respectively. Redshift has no obvious influence on this classification. However, as the bulge size becomes larger, the fraction of multiple-arm galaxies increases, while that of flocculent galaxies decreases. In addition, we performed a statistical analysis of 3958 galaxies with a clear spiral arm structure, finding 82% of these galaxies have two arms in their inner regions. We also found that the majority (74%) of the barred spiral galaxies exhibit the characteristics of two inner spiral arms and multiple outer spiral arms, and there is no barred spiral galaxy in this work with four continuous spiral arms from the inner to the outer regions. These results highlight that the spiral arm structure of the Milky Way, according to the current mainstream view of a four-arm galaxy with continuous arms extending from the inner to outer regions, is quite unique. However, our findings align with the spiral morphology of the Milky Way proposed by Xu et al., in which case our Galaxy can be considered typical.