One nucleus in NGC 4490 is visible in the optical wavelengths, while the other is only visible at infrared and radio wavelengths, according to a new study to be published in the Astrophysical Journal.
This Hubble image shows the galaxy NGC 4490. Image credit: NASA / ESA / Hubble / D. Calzetti, UMass / LEGUS Team / J. Maund, University of Sheffield / R. Chandar, University of Toledo.
NGC 4490 lies some 30 million light-years away in the constellation of Canes Venatici.
This galaxy forms an interacting pair with the smaller irregular galaxy NGC 4485 and together they have also been catalogued as Arp 269.
This system is an isolated analog of the Magellanic Clouds and is surrounded by en enormous hydrogen cloud.
The extreme tidal forces of their interaction have determined the shapes and properties of the two galaxies.
Once a barred spiral galaxy, similar to the Milky Way, NGC 4490’s outlying regions have been stretched out, resulting in its nickname of the Cocoon Galaxy. Virtually no trace of its past spiral structure can be seen from our perspective.
“I saw the double nucleus in NGC 4490 about seven years ago,” said study first author Allen Lawrence, a 77-year-old who earned a master’s degree in astronomy from Iowa State University in 2018 and continues to work with Iowa State astronomers.
“It had never been observed — or nobody had ever done anything with it before.”
“Some astronomers may have seen one nucleus with their optical telescopes. And others may have seen the other with their radio telescopes. But the two groups never compared notes to observe and describe the double nucleus.”
This image shows low-mass galaxies NGC 4485 and NGC 4490. Image credit: Adam Block / Mount Lemmon SkyCenter / University of Arizona.
Both nuclei in NGC 4490 are similar in size, mass and luminosity. They are also similar in mass and luminosity to the nuclei observed in other interacting galaxy pairs.
The double nucleus structure could also explain why the galaxy system is surrounded by an enormous cloud of hydrogen.
“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,” the astronomers said.
“A merger could drive and extend the high level of star formation necessary to create such a large hydrogen plume.”
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A.L. Lawrence et al. 2020. Revealing the Double Nucleus of NGC 4490. ApJ, in press; arXiv: 2001.05601
