It's only realized now because while one nucleus can be seen in optical wavelengths, the other is hidden in dust and can only be seen in infrared and radio wavelengths.
The work started when first author Allen Lawrence was taking undergraduate 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 percent of the size of the Milky Way, located in the Northern Hemisphere and about 30 million light years from Earth.
"I saw the double nucleus about seven years ago," Lawrence said. "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 he said the two groups never compared notes to observe and describe the double nucleus.
SDSS g mosaic image of Arp 269. The optical extent of the two galaxies is shown using contours
at magnitude 24.5 and 27 (SDSS Pogson (Pogson 1856) magnitude system). The large cyan cross marks the location of the optical nucleus (RA = 12h30m36s:2767, DEC = +41◦3803700 :082, J2000), and the small cyan cross (RA = 12h30m35s:6596, DEC = +41◦3805400 :275, J2000) shows the location of the dust-shrouded infrared nucleus. The 2 kpc line length is based on a distance of 9.2 Mpc. The mosaic was constructed using montage (Berriman&Good 2017).
The new paper says both nuclei are similar in size, mass and luminosity. It says both are similar in mass and luminosity to the nuclei observed in other interacting galaxy pairs. And, it says the double nucleus structure could also explain why the galaxy system is surrounded by an enormous plume 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 authors wrote. A merger could drive and extend the high level of star formation necessary to create such a large hydrogen plume.
Double-nucleus galaxies are very rare, especially in smaller galaxies such as this one.
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