NGC 1232
A face-on spiral galaxy with perturbed, flocculent arms.
NGC 1232, sometimes called the Eye of God Galaxy, is an intermediate spiral galaxy located roughly 50 million light-years away in the constellation Eridanus. It was first spotted by William Herschel on 20 October 1784.
The galaxy is made up of millions of bright stars and dark dust, arranged in spiral arms that rotate around the center. Along these arms are open clusters of hot, blue stars, separated by dark lanes of dense interstellar dust. Less obvious are dimmer stars and interstellar gas, which together have enough mass to control the inner galaxy’s dynamics. Invisible is dark matter, an unknown substance needed to explain how visible material moves in the outer parts. The galaxy spans about 150,000 light-years across. Its spiral arms are not smooth but are bent and disturbed, leading some to think it collided with a dwarf galaxy, though other studies question this idea.
NGC 1232 and its apparent companions may belong to the Eridanus Cluster of galaxies, along with NGC 1300.
The galaxy is seen face-on and is technically a grand-design galaxy, often considered a model for multi-arm spirals. Its central bulge is small, and though classified as intermediate, the bulge shows hints of a bar.
The spiral arms are bright and flocculent, winding counterclockwise from the center. They contain many HII regions. Far from the center, the arms either branch into long, diffuse segments or connect with other arm pieces. They are not smooth as expected; instead, they bend sharply and vary in pitch—so much so that one arm cannot be described by a single pitch angle. Astronomer Halton Arp suggested this came from a galaxy interaction. The many star-forming regions make NGC 1232 a good place to study how stars form.
Most arms and arm segments widen as they move outward, except for one segment, called "E," which narrows. This may be because star formation drops off at greater distances, or because the spiral pattern starts to co-rotate with orbiting material.
NGC 1232 and NGC 1232A might both be part of the Eridanus cluster, but at about 2.2 million parsecs from the cluster’s center, they may not be gravitationally bound to it.
At first, NGC 1232A was thought to be a satellite galaxy interacting with NGC 1232 and causing disturbances in its disk, but this is probably not true.
Quick Facts
- Epoch
- J2000
- Ra
- 03 · 09 · 45.4322
- Dec
- -20 · 34 · 44.443
- Z
- 0.005347
- H Radial V
- 1603 ± 1 km/s
- Dist Ly
- 15.432 ± 0.870 Mpc
- Appmag V
- 10.9
- Constellation Name
- Eridanus
- Notes
- Discrepant galaxy pair with NGC 1232 A
- Names
- PGC 11819, Arp 41
Facts from the source article.
Lore & Background
NGC 1232 was discovered by German-British astronomer William Herschel on 20 October 1784. It is a face-on spiral galaxy that can be technically considered a grand-design galaxy and is regarded as a prototype for multi-arm spiral galaxies. Its galactic bulge is small, though it shows hints of a galactic bar. The spiral arms are bright and flocculent, winding counterclockwise from the center, and contain numerous HII regions. At large distances from the center, the arms either branch out and disperse or connect with arm segments, but they are not smooth; they bend abruptly and show significant deviations from a constant pitch angle. Astronomer Halton Arp suggested this results from a galaxy interaction. Most arms widen with distance from the center, except for arm segment 'E', which narrows, possibly due to less star formation or co-rotation of the spiral pattern.
The galaxy and its apparent companions may be part of the Eridanus Cluster, along with NGC 1300, though at a distance of about 2.2 million parsecs from the cluster's center they may not be bound to it. A debated collision with a dwarf galaxy has been proposed. Originally, NGC 1232A was thought to be an interacting satellite, but redshift measurements show NGC 1232A is about 68 million light-years away while NGC 1232 is about 65 million light-years away, and their redshifts do not match, making the pair a striking example of discrepant redshifts. X-ray studies suggested an interaction with a different dwarf galaxy, evidenced by a massive X-ray gas cloud superimposed on NGC 1232, possibly the first case of such a collision visible only in X-rays. However, a later X-ray study found the hot gas luminosity not very high, concluded the cloud does not exist, and that the disk is not very warped, arguing no interaction occurred at all.
Reader's Guide
NGC 1232 is notable as a prototype for multi-arm spiral galaxies and as a laboratory for star formation studies, containing numerous HII regions—over 970 identified in one study. Its perturbed spiral arms have sparked debate about a possible collision with a dwarf galaxy, though evidence remains disputed. The galaxy's role in the Eridanus Cluster is uncertain, as it may not be gravitationally bound. The discrepant redshifts between NGC 1232 and NGC 1232A have made the pair a well-known example in studies of galaxy pairs. X-ray observations have alternately supported and refuted the idea of a past collision, highlighting the complexity of interpreting galactic interactions. The galaxy's face-on orientation and rich star-forming regions make it a valuable object for understanding spiral structure and star formation processes.
Did You Know?
- NGC 1232 is also called the Eye of God Galaxy.
- It was discovered by William Herschel on 20 October 1784.
- The galaxy is approximately 150,000 light-years across.
- Its spiral arms are not smooth and show abrupt bends, possibly from an interaction.
More in Galaxy Clusters and Groups, Part 4 1-24
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