The galaxy NGC 4618 discovered by William Herschel is one of only three one-armed galaxies in the constellation of Orion, about 1/3 of the diameter of the Milky Way. In the process of studying the reason for its one-armed, scientists have proposed that the one-armed may be caused by the interaction of neighboring galaxies with it, but there is still no evidence as a conclusive conclusion, which needs to be further studied.
There are all kinds of strange things in the sky. These things ignore our naming conventions and our attempts to understand them. For example, NGC 4618, one-armed galaxy.
A spiral galaxy with only one spiral arm may not top the list of cosmic weirdness, but it does attract the attention of curious astronomers. The obvious question is: Why does it have only one spiral arm? No one is quite sure yet.
William Herschel discovered NGC 4618 in 1787, six years after the discovery of Uranus. He built powerful telescopes that allowed him to make these discoveries. Herschel also made many other discoveries.
Herschel also speculated that the vague clumps that astronomers see in the sky, known as nebulae in the Messier catalogue, are actually groups of individual stars, but they are too far away to discern. Now we know he's right, every schoolboy knows about galaxies, and we live in them.
This one-armed galaxy is about 21 million light-years away in the constellation of Scorpio. It is about one-third the diameter of the Milky Way. In the Atlas of Strange Galaxies, NGC 4618 is one of only three one-armed spiral galaxies.
Why does it have only one spiral arm?
This is not an easy question to answer. Obviously, we can start by looking at neighboring galaxies to see if they are engaged in gravitational interaction. It is well known that such interactions promote the morphology of some galaxies. We have a large number of such examples.
Here's an image of an antenna galaxy taken by Hubble, a pair of interacting galaxies that have been interfering with each other for hundreds of millions of years. These interactions are shaping the two galaxies. Gas clouds are pink and red, while the regions where stars are born are blue. Image credit: ESA/Hubble, CC BY 4.0.
NGC 4618 does have a neighbor whose name is NGC 4625. It is known as a twisted dwarf galaxy, also known as the Sm galaxy. This means that according to the de-Wocurer galaxy morphological system, it "has an irregular appearance" and "has no uplift component".
NGC 4625 is a neighbor of this one-armed galaxy. Astronomers considered whether the gravitational interaction between the two galaxies would give NGC 4618 a one-armed form. Image credit: Hubble Space Telescope
But while they are sometimes referred to as a pair of interacting galaxies, at least one study has concluded that this may not be the case, and that no interaction can explain this one-armed galaxy. The paper is titled "High-Resolution H-I Distribution and Multi-Wavelength Analysis of Magellan Spirochetes NGC 4618 and NGC 4625". It was published in the Journal of Astronomy in 2012.
In that paper, the authors said: "The goal of this work is to revisit whether these two galaxies actually interact and what effect this interaction might have on NGC 4618 and NGC 4625, particularly in terms of star formation." ”
They also said: "The statement that NGC 4618 and NGC 4625 are interacting is consistent with the idea that the asymmetry of the Magellan spiral is the result of interaction with near neighbors (e.g., Odewahn 1994), and the apparent proximity of these galaxies to each other and the overlap of the H−i distribution highly suggest the interaction." The height hint is good, but it's also not conclusive either. ”
Much of the argument that supports the interaction of these two galaxies is based on the gas bridges that connect them. But the authors say that, based on their observations, "the gas that is thought to be the bridge connecting the two galaxies actually has two well-defined, independent components of motion." This observation is a powerful demonstration of the ongoing interaction between the galaxies NGC 4618 and NGC 4625..."
They concluded: "Although there is a significant overlap in the outer disks of the two galaxies, we found that they were kinematically different, suggesting that NGC 4618 and NGC 4625 did not interact." The structure of NGC 4618, particularly the nature of its outer ring, is highly suggestive of an interaction, but the timing and nature of this interaction remain unclear. ”
So we came to that conclusion. In NGC 4618, a one-armed galaxy, there must be some intrinsic process that causes it to have only one spiral arm. But to find out what this is needs further research.
BY: Evan Gough
FY: Charlene
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