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This image, taken with the Hubble Space Telescope, shows the galaxy NGC 4490. The scattered and warped appearance of the galaxy are the result of a past cosmic collision with another galaxy, NGC 4485 (not visible in this image). The extreme tidal forces of the interaction between the two galaxies have carved out the shapes and properties of NGC 4490. Once a barred spiral galaxy, the outlying regions of NGC 4490 have been stretched out, resulting in its nickname of the Cocoon Galaxy. This interacting pair with the smaller irregular galaxy NGC 4485 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. Virtually no trace of its past spiral structure can be seen from our perspective.
NGC 4490 has a high star formation rate (SFR) and is surrounded by an enormous HI feature stretching about 60 kpc north and south of the optically visible galaxies. Both the driver for the high SFR in NGC 4490 and the formation mechanism of the HI structure are puzzling aspects of this system. Mid-infrared Spitzer data showed that NGC 4490 has a double nucleus morphology. One nucleus is visible in the optical, while the other is only visible at infrared and radio wavelengths. The optical nucleus and the potential infrared visible nucleus have similar sizes, masses, and luminosities. Both are comparable in mass and luminosity to other nuclei found in interacting galaxy pairs and much more massive and luminous compared with typical non-nuclear star-forming complexes. The double nucleus structure could also explain why the galaxy system is surrounded by an enormous cloud of hydrogen. It is likely that NGC 4490 is itself a merger remnant, which is now interacting with NGC 4485. This earlier encounter provides both a possible driver for extended star formation in NGC 4490, and multiple pathways for the formation of the extended HI plume.
Image Credit: ESA/Hubble and NASA
Acknowledgements: D. Calzetti (UMass) and the LEGUS Team, J. Maund (University of Sheffield), and R. Chandar (University of Toledo)
Image enhancement: Jean-Baptiste Faure
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