JWST images reveal different geometries in spiral arms and dusty feathers

A spiral galaxy’s main arms are only part of its structure. Thin dusty features called feathers can project from those arms into the spaces between them. Measuring both requires a way to distinguish the galaxy’s overall pattern from the geometry of individual features.

In a preprint dated November 6, 2026, Stella Faria-Costa, Irapuan Rodrigues and Ivânio Puerari examine seven nearby galaxies using publicly released images from the PHANGS–JWST survey. Their central question is how the orientations of main arms and smaller structures differ—and how the answer changes with wavelength and measurement scale.

Measuring how open a spiral is

The study measures pitch angle: the angle between a structure’s direction and the tangent to a circle centered on the galaxy. A small absolute pitch angle describes a tightly wound feature, running nearly along that circle. A larger angle describes a more open feature, pointing farther outward. Positive and negative signs distinguish opposite winding directions, rather than stronger or weaker spirals.

Each galaxy was examined in eight infrared filters: four from JWST’s Near-Infrared Camera, NIRCam, and four from its Mid-Infrared Instrument, MIRI. These provide complementary views. The near-infrared images emphasize the large-scale stellar pattern, while the mid-infrared images make dusty structures more prominent. The distinction is not absolute: the filters can contain contributions from several kinds of emission.

The researchers first corrected the images for the galaxies’ inclinations. They then transformed them into a coordinate system using angle around the center and the logarithm of radius. In this representation, an ideal logarithmic spiral becomes a straight line, making its orientation easier to measure.

A two-dimensional Fourier analysis identified dominant spiral patterns. Reconstructing those patterns allowed the team to define separate arm and interarm regions. Within them, the researchers compared image features with directional filters inside moving circular windows, recording the orientation that gave the strongest match.

They repeated this procedure with three window diameters. These sizes are dimensionless quantities in the transformed coordinates, not fixed distances in parsecs. Larger windows emphasize extended, coherent structures; smaller windows are more sensitive to localized features.

Broad arms, more open feathers

NGC 1566 illustrates the contrast. Its main spiral arms were represented by a pitch angle of about 24 degrees. At the smallest window size, the researchers associated a component near 76 degrees with dusty feathers projecting from the arms. A separate component near 32 degrees followed a more extended interarm structure.

Other galaxies showed similar contrasts. NGC 628 had a representative main-arm angle near 17 degrees, with prominent interarm structures near 50 and 58 degrees. In NGC 4254, the main arms changed from roughly 24 degrees in their inner portions to 17 degrees farther out, while feathers reached about 58 degrees.

But the sample does not support one angle for all arms or all feathers. In NGC 4303, the inner arm geometry was much more open than the outer portions, and identified feathers had angles near 31 and 40 degrees. Regions around transitions between central bars and spiral arms showed particularly strong variations. In NGC 4535, the authors detected high-angle interarm structures but did not identify a feather within the analyzed region.

Geometry, not a formation verdict

The reported angles are approximate descriptions of recognizable structures. The team selected representative values by inspecting recurring peaks in correlation spectra alongside spatial maps. They did not fit those peaks parametrically or apply a formal statistical significance criterion. Sampling orientations in one-degree steps therefore should not be confused with establishing one-degree measurement accuracy.

Coverage also matters. Some images do not include the full extent of a galaxy’s spiral pattern, and the sample was selected for structures suitable for this analysis.

The work maps morphology; it does not establish which physical mechanism produced each feather or prove a particular theory of spiral formation. Its main result is that a galaxy’s geometry cannot always be reduced to one pitch angle. Combining infrared views and examining several scales reveals distinctions that a global measurement can obscure.