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View Full Version : NGC 7673-77 A galaxy odd couple



RickJ
2011-Oct-02, 09:20 PM
The galaxy pair of NGC 7673 and NGC 7677 is another Sakib Rasool brought to my attention. They are located inside the Great Square of Pegasus about 140 to 150 million light-years distant by redshift measurement. NGC 7673 (left) is highly disturbed. It is classed as (R')SAc?pec HII Sbrst. Starburst activity can be triggered by two or more galaxies interacting in some way. Though we now know this isn't necessary. Still, its torn up condition as well as starburst nature would seem to argue for some sort of interaction. Some say the culprit is NGC 7677. That galaxy is classed as SAB(r)bc: Sbrst so is also a starburst galaxy. While the starburst activity of NGC 7673 is scattered in star-clouds as well as the core it is mostly found in the core of NGC 7677. Others say that while the two may have had some interaction in the past the main reason NGC 7673 is so torn up is that it recently merged with a smaller galaxy. http://adsabs.harvard.edu/full/1999IAUS..193..743H Some just can't make up their mind. http://iopscience.iop.org/0004-637X/522/1/199/pdf/39289.web.pdf Maybe it is a bit of both. A merger augmented by the interaction of the two NGC galaxies.

NGC 7673 has been imaged by Hubble using red, green and UV filters. This filter choice results in some odd green areas in the color image. The text and image can be found here: http://www.spacetelescope.org/images/heic0205a/ . The article mentions the two reddish galaxies seen on either side of it in both the HST and my image. This blames redshift for the color. Without redshift data on these it is impossible to say with much certainty if this is the case or not. It would require them to be at least 4 billion light years away. I hate to argue with the HST folks but it seems, to me, more likely intergalactic dust and gas is doing the reddening. In fact it could be dust thrown out by NGC 7673 itself due to the interaction it experienced. Also these could be red spiral galaxies, a rather new class of galaxies. A combination of all of these could be involved.

Not seen in the HST image is the huge plume of stars that extends out beyond these two reddened galaxies. If you look closely you can see a plume that extends well west of NGC 7673 ending in a large but faint tidal star field. It is on the right edge of the enlarged, cropped image. The papers cited above refer to these as ripples rather than plumes.

NGC 7677 is also strangely shaped. It appears to have a very faint bar that extends to arms that make a complete ring about the core but is brighter in two arc starting where the wide arms appear to attach to the ring. The area between the wide arms and the ring appears rather reddened indicating it is very dusty and not empty space. Unfortunately I found no HST images of NGC 7677.

My image was taken September 9, 2010. So I'm now about 13 months behind.

14" LX200R @ f/10, L=4x10' RGB=2x10', STL-11000XM, Paramount ME

Rick