55 million light-years away, scientists spot something unusual: A strange galactic collision may have left this spiral galaxy with a centre that spins sideways

Indeed, the image of the galaxy NGC 4698 photographed by the Hubble Space Telescope appears to be a rather calm one. One may see the bright core of the galaxy shining against the background, and some blue stars and dust clouds located within the spiral arms of the galaxy.

However, the structure of the galaxy NGC 4698 is different from the structure of the Milky Way and other spiral galaxies. The elongated form of the galactic bulge and the nearly perpendicular rotation of the inner stars and gas in the galaxy are specific to the galaxy under discussion.

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Some special phenomena took place in the history of the galaxy.

For instance, the galaxy was possibly interacting with some outer gas clouds or another galaxy, which led to some disturbances in the central region of the galaxy.

The distance between the galaxy NGC 4698 and the Earth is equal to 55 million light-years.

Hubble reveals an unusual spiral galaxy

NGC 4698 forms part of the general category of spiral galaxies, and like the Milky Way, it has a disk consisting of a flattened collection of stars, gases and dust, and a bright bulge of stars at the centre.

The outer region of its disk has spiral arms characterised by dark clouds of dust and groups of young bright blue stars. The spiral arms, however, are not very prominent near the centre but are more clearly seen around the outer edges of the galaxy in a formation resembling a ring.

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The centre of the galaxy consists of a long bulge.

Most of the light emitted by the bulge originates from old, cool, low-mass stars. This gives it an off-white colour different from the blue stars in the outer disk.

In the middle of the bulge is a supermassive black hole thought to have the mass of several million suns. Some observations suggest that the black hole is still growing since it continues to pull in gas.

The most prominent feature of this galaxy, however, is neither the black hole nor the bulge.

The galaxy’s central stars seem to spin sideways.

Typically, in most known spiral galaxies, there is alignment between the central bulge and the surrounding disc. However, it appears that NGC 4698 does not follow that trend.

Here, the central bulge protrudes from the main disc at nearly 90 degrees. While the deviation may not be very evident from the Hubble image, the faint glow of the galaxy’s central stars seems to rise above and below the dusty disc.

Observations also show that gases and stars in the immediate vicinity of the central region seem to rotate in a direction that is close to being perpendicular to the rotation of the outer disc.

Therefore, NGC 4698 becomes one of a small group of known spiral galaxies exhibiting such structures.

This peculiar orientation may be because of the independent formation of the central bulge or due to a disturbance within the galaxy after its formation.

The structure and orientation of a galaxy are not permanent and continue to evolve due to accretion of gases and merging with other nearby galaxies.

A past galactic encounter may explain the tilted core

One explanation involves the movement of gas from outside the galaxy.

If NGC 4698 drew in gas from an external source, that material may have settled into a new disk around the centre. If the incoming gas was moving at a different angle from the original galactic disk, the stars formed from it could have developed a separate orientation.

Over time, that process could produce a central structure that rotates differently from the rest of the galaxy.

Another possibility is a minor galactic merger. Astronomers have identified indications that NGC 4698 may once have interacted with a smaller companion. A short stream or tail of hydrogen extending from one side of the galaxy could be a remnant of that encounter.

Such features can remain after a merger, although they do not always provide conclusive proof on their own.

A smaller merger would not necessarily destroy the entire galaxy. Instead, it could disturb the gas, change stellar orbits and create a new central component while leaving the outer spiral disk largely intact.

Further observations are needed to establish which explanation best fits NGC 4698’s history.

The Virgo Cluster offers clues about galaxy evolution

NGC 4698 belongs to the Virgo Cluster – a collection of thousands of galaxies.

Thanks to its proximity to Earth, it allows for a detailed investigation of the galaxies within the cluster.

This study involves investigating some characteristics of the cluster’s members – such as the presence of star clusters, nebulae, dust lanes and unusual galaxy structures.

The aim of the research is to gain insights into how a galaxy’s environment impacts its evolution.

In particular, the galaxies in a cluster may lose gas due to interactions with the external environment, have collisions with other galaxies, gravitational perturbations or variations in their capacity to create stars.

NGC 4698 is an example of how such phenomena may leave an imprint on a galaxy’s structure.

The tilted centre of NGC 4698 is not only aesthetically pleasing.

Moreover, it can be useful for studying the movement of gases and stars for several billion years back in time and how it reflects previous interactions of a galaxy with another object in its environment.

Thus, in the calm image of NGC 4698, there is a galaxy whose stars contain traces of a past collision.

Frequently asked questions

1. What makes NGC 4698 unique?

NGC 4698 has an off-centre bulge extending almost vertically (at an angle of about 90 degrees) to the plane of the rest of the galaxy. The matter around the galaxy's centre also moves perpendicular to the outer disc.

2. How distant is NGC 4698?

NGC 4698 is situated about 55 million light-years away from our planet in the constellation of Virgo.

3. How could the unusual galaxy structure have developed?

Scientists think that the galaxy might have gotten its off-centred structure due to gas acquisition from a different orientation from the galaxy or because of minor galaxy merging.

4. Does NGC 4698 have a black hole?

Yes. At the galaxy’s centre, there is a supermassive black hole containing several million times the mass of the Sun. It still seems to be growing now.