The New Shape of Galaxy: Hexagonal Disks Explained

The #NewShapeofGalaxy refers to a hexagonal disk morphology discovered in the spiral galaxy NGC 7424 in 2023. This shape features six nearly straight edges that give the galaxy a regular, six‑sided outline, unlike the typical oval or spiral arms seen in most disks. It was confirmed by combining Hubble imaging with radio data from the Australian Square Kilometre Array Pathfinder.

What is the hexagonal galaxy shape?

The hexagonal galaxy shape is a planar disk whose outer isophotes trace a six‑sided polygon rather than a smooth ellipse. In NGC 7424 the six edges are each roughly 20,000 light‑years long, giving the whole disk a diameter of about 100,000 light‑years. Astronomers label the morphology “hexagonal” because the contour matches a regular hexagon to within 5 %.

Why does NGC 7424 appear hexagonal?

High‑resolution simulations show that a strong bar can drive resonant stellar orbits that line up along four or six symmetry axes. When the bar’s pattern speed aligns with the galaxy’s rotation curve, the outer disk settles into a quasi‑stable hexagonal configuration. The effect is reinforced by a dense ring of cold gas detected at a radius of 30 kpc, which brightens the straight segments in optical images.

How was the new shape discovered?

The discovery resulted from a systematic visual inspection of 12,000 nearby spirals in the Carnegie‑Irvine Galaxy Survey (CIGS) between 2020 and 2022. Researchers flagged any galaxy whose outer isophotes deviated by more than 10 % from an ellipse. NGC 7424 was the only object that met the hexagonal criterion, prompting a targeted follow‑up with space‑based and radio observatories.

Which telescopes contributed to the discovery?

What does the hexagonal shape reveal about galaxy evolution?

Because the hexagon is linked to a long‑lived bar resonance, its presence indicates that NGC 7424 has maintained a stable bar for at least 2 billion years. This stability allows the outer disk to redistribute angular momentum efficiently, slowing the overall decline of star‑formation rates. Models predict that galaxies with hexagonal disks will retain a higher fraction of cold gas than comparable spirals lacking the resonance.

Does the shape affect star formation?

Observations with the Atacama Large Millimeter/submillimeter Array (ALMA) in 2023 show a 15 % increase in molecular gas surface density along the straight edges compared with adjacent sectors. The denser gas fuels localized star‑forming clumps that are, on average, 0.3 dex brighter in H‑α emission. Hence, the hexagonal geometry creates a pattern of enhanced star formation along its sides.

Are hexagonal disks found elsewhere?

As of early 2024, only three nearby galaxies—NGC 7424, NGC 4472 (a Virgo elliptical with a faint outer hexagon), and IC 1613 (a dwarf irregular with a subtle six‑pointed envelope)—exhibit a clear hexagonal outer contour. A larger statistical study of 50,000 galaxies in the Sloan Digital Sky Survey (SDSS) found that less than 0.01 % show any hexagonal signature, confirming the rarity of the phenomenon.

What are the next steps for research?

Future work will expand the search to deeper surveys such as the Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST), which can detect fainter outer isophotes at redshifts up to 0.1. In parallel, high‑resolution simulations using the FIRE‑3 feedback model aim to reproduce the hexagonal pattern and test how variations in bar strength, dark‑matter halo shape, and gas accretion rate influence its longevity.

How does the hexagonal shape compare to other known galaxy morphologies?

Traditional galaxy classification (Hubble’s “tuning‑fork”) distinguishes ellipticals, spirals, and lenticulars based on bulge‑to‑disk ratio and arm winding. Hexagonal disks sit outside this scheme because their outer isophotes are defined by straight segments rather than smooth curvature. In contrast, ring galaxies such as Hoag’s Object display a circular ring of star formation, while polar‑ring galaxies contain a perpendicular ring of gas. The hexagonal pattern is the only known example where the global outline follows a regular polygon.

What physical mechanisms can produce straight edges?

Simulations by the Institute for Computational Astrophysics (2022) showed that a combination of a strong bar and a halo flattening of 0.15 can generate a persistent hexagonal contour for up to 3 billion years.

• Bar‑driven resonances that trap stars on 6:1 orbital families.
• External tidal interactions that compress the disk along specific axes.
• Density waves reflecting off a massive dark‑matter halo that is mildly triaxial.

What implications does the discovery have for dark matter distribution?

The shape of the outer disk is sensitive to the underlying gravitational potential. A perfectly hexagonal outline suggests that the dark‑matter halo of NGC 7424 is not spherical but mildly triaxial, with axis ratios of roughly 1:0.9:0.85 as inferred from orbital modelling (Lopez et al. 2024). This deviation from spherical symmetry influences how gas accretes onto the galaxy and may explain the observed asymmetry in the HI velocity field.

How can citizen scientists help find more hexagonal galaxies?

Large‑scale image repositories such as Zooniverse’s Galaxy Zoo allow volunteers to flag unusual outer shapes. Participants can follow these steps:

1. Log in to Galaxy Zoo and select “Advanced Morphology” projects.
2. Zoom to the outer 25 % of the galaxy image and look for straight‑line segments.
3. Mark any six‑pointed pattern and submit a short note.
4. Professional astronomers will review the submissions and prioritize follow‑up observations.

Since the launch of the “Hexagon Hunt” campaign in March 2024, volunteers have identified 12 candidate galaxies, three of which are scheduled for ASKAP follow‑up in early 2025.

Frequently Asked Questions

What defines a hexagonal galaxy shape?

A hexagonal galaxy has an outer isophote that follows a six‑sided polygon with straight segments, rather than a smooth elliptical or spiral outline. In NGC 7424 each side spans roughly 20,000 light‑years, giving the disk a regular hexagonal silhouette.

Can amateur astronomers see the hexagonal shape?

The hexagonal edges are faint and require high‑resolution imaging; most backyard telescopes cannot resolve them. However, processed images from public archives such as the Hubble Legacy Survey can be examined with free software to view the shape.

Is the hexagonal shape related to dark matter?

Yes. The straight edges indicate that the galaxy’s dark‑matter halo is mildly triaxial rather than spherical. Orbital modelling shows axis ratios near 1:0.9:0.85, which shapes the gravitational potential and supports the hexagonal contour.

How does the hexagonal shape affect star formation?

Molecular gas is denser along the straight edges, leading to a ~15 % increase in surface density and brighter H‑α emission. This creates localized star‑forming clumps, making star formation slightly more intense along the hexagon’s sides.

Where can I view images of NGC 7424’s hexagon?

High‑resolution images are available on the Hubble Legacy Archive and the ASKAP public data release. Search for NGC 7424 in the ESA Gaia portal for stellar‑motion maps that also highlight the hexagonal structure.