Spiral galaxies appear simple at first glance, but astronomers have been unable to explain their structure for many decades. One of the biggest questions is why some systems have two arms while others have more. Now an answer is beginning to emerge. It appears that the decisive factor is how densely matter is concentrated in the galactic core.
The Problem of Winding Spiral Arms
The inner parts of a galactic disk complete a full rotation in less time than the outer parts. If the arms consisted of a permanent set of stars and gas, they would wind increasingly tightly over several hundred million years until the spiral disappeared.
Because this does not happen, the density-wave theory was developed in the 1960s. According to this theory, an arm is a region where matter becomes concentrated; stars and gas pass through it and continue moving, while the pattern itself retains its shape. The model explains many observed properties, but not the most important one. It is still unknown why the spiral pattern persists for so long, even though physical processes should gradually cause it to fade.
What the Statistics Showed
A team led by Beverly Smith of East Tennessee State University analyzed a catalog from the first quick data release of the Euclid space telescope, launched in 2023.
The galaxy shapes were classified using the Zoobot program, and the results were published in the peer-reviewed journal The Astrophysical Journal. The full text is available as a preprint on arXiv. The analysis included more than 380,000 galaxies whose light took between four and eight billion years to reach us.
Between 60% and 70% of the spirals in the sample have two arms. Another 15% to 20% have three, while one-armed systems are rare and account for about 1%. Approximately one-fifth of the sample was excluded from the count because the number of arms in those objects could not be determined, Universe Today notes.
Two-armed spirals were, on average, less massive and had lower star-formation rates. However, among systems of equal mass, the difference disappeared, indicating that the intensity of star formation is determined by mass rather than by the number of arms.
The Role of a Massive Core
The main conclusion concerns the inner part of a galaxy, the dense concentration of stars known as the bulge. Systems with a massive core, including the Milky Way, usually have two prominent arms. In galaxies where this region is less massive, the spiral branches into a greater number of arms.
The explanation lies in how the speed of matter changes with distance from the center. In galaxies with a large bulge, the speed decreases noticeably in the outer layers of the disk, producing strong shear between neighboring regions. Such conditions stabilize a two-armed structure, in agreement with the predictions of computer models.
The distribution of mass in a disk system determines both the form of the spiral pattern and the contribution of dark matter. Therefore, counting the arms may become an indirect way of estimating how much dark matter is concentrated in a galaxy’s core.
“The number of spiral arms turned out to be more than just an interesting detail. It reflects what is happening deep within the galaxy itself,” Beverly Smith said in a statement from East Tennessee State University.
Limits of the Sample
The data analyzed cover less than half of one percent of the sky planned for Euclid’s full survey. The first quick release was compiled from observations of approximately 63 square degrees, while the complete survey is expected to cover 14,000 square degrees. The proportions identified in the study will therefore be tested using a dataset hundreds of times larger.
The authors themselves caution that galaxies with perfectly defined two- or three-arm structures are almost nonexistent in the Universe. Real spirals form a continuous range of shapes, with branches and short spurs, so the established classification describes their structure only approximately.
