Nearby Cocoon Galaxy has a rare double-nucleus structure

The system is about 20% the size of the Milky Way, located in the Northern Hemisphere and about 30 million light-years from Earth.

Using the data gathered from optical and radio telescopes, astronomers recently examined a galaxy known as NGC 4490 (nicknamed the “Cocoon Galaxy”). They found that not only the galaxy has a unique shape, but also has a rare double-nucleus structure. One nucleus is observable in optical wavelengths, whereas the other is hidden in dust and only visible in infrared and radio wavelengths.

First author, Allen Lawrence from Iowa State University, said, “Both nuclei are similar in size, mass, and luminosity. And, it says the double nucleus structure could also explain why an enormous plume of hydrogen surrounds the galaxy system.”

Lawrence started the study in 2013 while taking astronomy classes at the University of Wisconsin-Madison. He had the chance to study one of two galaxy systems and picked NGC 4490, which is interacting with a smaller galaxy, NGC 4485. The system is about 20% the size of the Milky Way, located in the Northern Hemisphere and about 30 million light-years from Earth.

He said, “I saw the double nucleus about seven years ago. It had never been observed – or nobody had ever done anything with it before.”

Allen Lawrence, a 77-year-old who earned an Iowa State master's degree in astrophysics in 2018, is first author of a paper revealing a rare double-nucleus structure in a well-known, nearby galaxy. Larger photo. Photo by Christopher Gannon.
Allen Lawrence, a 77-year-old who earned an Iowa State master’s degree in astrophysics in 2018, is first author of a paper revealing a rare double-nucleus structure in a well-known, nearby galaxy. Larger photo. Photo by Christopher Gannon.

Authors noted, “The most straightforward interpretation of the observations is that NGC 4490 is itself a late-stage merger remnant” of a much-earlier collision of two galaxies. A merger could drive and extend the high level of star formation necessary to create such a large hydrogen plume.”

“There are other reasons they find the study of this system interesting: Double-nucleus galaxies are very rare, especially in smaller galaxies such as this one. And, astronomers think a double nucleus could contribute to the buildup of supermassive black holes found in the center of some galaxies.”

Charles Kerton, an associate professor of physics and astronomy, said, “This project demonstrates that using multiple wavelengths from space- and ground-based observations together can help us understand a particular object.”

The paper reporting the discovery is now online and has been accepted for publication in the Astrophysical Journal.

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