It’s a bit like an early Christmas present that astronomers have just discovered in our skies. Because this galaxy is surrounded by a huge band of gas. And because it could allow them to learn a little more about the formation and evolution of galaxies in the universe.
NGC 4632 is a spiral galaxy. It is located on the side of the constellation Virgo. About 56 million light-years from the Milky Way. And by pointing one of the Australian Square Kilometer Array Pathfinder (Askap) radio telescopes at it, astronomers from the Commonwealth Scientific and Industrial Research Organization (CSIRO, Australia) recently made an unexpected discovery.
This spiral galaxy is surrounded by a huge band of gas. A hydrogen ring forming a very tilted angle to the galactic plane of NGC 4632. And it’s not a diffuse ring. We are talking about a structure that makes up about half the mass of the system. If astronomers have never observed it before, that’s because it turns out to be invisible across most of the spectrum. It only appears clearly at radio wavelengths.
A rare polar ring galaxy?
Astronomers also had to harness the power of 3D visualization and even virtual reality to separate the gas in the galaxy’s main disk from that of the ring, so that the color gradient in the images reveals their orbital motion.
NGC 4632 thus joined the very closed class of polar ring galaxies. Very closed? Maybe not so much. As work published in the Monthly Notices of the Royal Astronomical Society suggests that there may be more galaxies of this type. While optical telescopes have revealed hydrogen bands in only about 0.5% of galaxies near our Milky Way, in reality they could be found in about 1 to 3% of galaxies.
The Widefield ASKAP L-band Legacy All-sky Blind (Wallaby) pilot study, in which astronomers discovered NGC 4632’s hydrogen ring, has already discovered another galaxy with a polar ring: NGC 6156. Another among about 600 galaxies mapped to date. But the Wallaby study ultimately aims to map the distribution of gas in more than 200,000 galaxies across the Southern Hemisphere. Enough to hope to find many other galaxies with polar rings.
This galaxy and its ring could provide valuable clues about the universe
How do these strange polar ring galaxies form? Two theories are put forward. The hydrogen ring observed around NGC 4632 may consist of material that traveled along strands of dark matter through the cosmic web before being captured by the galaxy’s gravity. It is more likely that this gas came from a galaxy that came a little too close and whose hydrogen would have been sucked up by NGC 4632.
Therefore, beyond their spectacular appearance, these galaxies are also of great scientific interest. Because they could provide clues about how galaxies form and evolve over time. But they also provide some crucial information for understanding the shape of the dark matter halo that is thought to surround most galaxies.
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