Object name: ARP320

Designation(s): ARP320, NGC3746, NGC3745, NGC3748, NGC3750, NGC3751, NGC3753, NGC3754, PGC036010,

Arp 320/Copeland\'s Septet is a group of 7 galaxies though you\'ll see 9 main galaxies in the center of the field. A finder chart of the 7 that are members of the \"official\" septet http://www.ngcicproject.org/dss/n/3/n3753.jpg
Note the blob above the label for NGC 3754 is a plate defect so you won\'t see it in my image. It is also known as Hickson 57. The main galaxy in the group is NGC 3753 also known as ARP 320. The reason for so many names is just that each astronomer has some reason to study it and puts it in their catalog. Before long there are a ton of catalog entries for the same thing. Besides NGC 3753 is also known as: UGC 06602, VV282a, KUG1135+222, CGCG127-012S, CGCG1135.4+2216, MCG+04-28-010, PGC03616, UZC J113753.8+215852 and [BDG09] J113753.9+215853 to name a few. The group is also known as Copeland\'s septet, HCG 057, WBL 343-005, USGC U404 and UZC-CG 142, RSCG 43 and others. Confused yet?

The group is about 410 to 440 million light years distant and somewhat red. I suspect this is due to lots of dust between us and the group though one galaxy does show some blue color. But two of the redder galaxies have \"retired\" nuclei. This means star formation has virtually ceased though some emission lines of LINER galaxies are still seen. This usually is interpreted to mean they are about to become red and dead galaxies. Several papers mention this color problem but none I saw gives an answer or even a guess as to why the group but for NGC 3754 is so red. Some of it was discovered by Ralph Copeland on February 9, 1874 (NGC 3746, 3750 and 3753) and the rest on April 5, 1874 (NGC 3745, 3748, 3751 and 3754). He got their position rather wrong by about 1.5 minutes of Right Ascension and about 15 arcmin of NPD. Enough that my field would have missed it using Copeland\'s position. But since no other group is in the area and its description fits his identification is certain. As to why 4 were found in April and 3 in February, I couldn\'t discover.

Just under NGC 3748 is PGC 36010 an S0/a galaxy that some consider part of Arp 320 but Copeland didn\'t see so it is not Copland\'s Octet. It is part of HCG 57 however. Look around the annotated image and you will see quite a few other galaxies, mostly dwarfs compared to the seven in the septet, also at the 410 to 440 million light-year distance indicating this is quite a large group.

It is quite obvious NGC 3753 is interacting with NGC 3750 by the huge plumes it has. I suppose the vertical one off the west end may be coming from NGC 3750. NGC 3753\'s southeastern plume almost looks like it is sticking its tongue out. Its dust lane is quite disturbed. Often a sign it ate something recently. While SLOAN shows a point source object just northwest of its upper end it doesn\'t show any entry for the plume. I thought it might be some low surface brightness dwarf that just happened to line up with NGC 3753 but apparently, it is just a plume. Assuming it is a plume and including the northwestern plume, I measure the full size at 300,000 light-years. Measuring just the bright disk I get 110,000 light-years.

NGC 3748 shows plumes as well indicating it did interact with one of the galaxies in the group but which one? Including plumes, it is 162,000 light-years across. The disk is much smaller at 55,000 light-years. NGC 3746 I measure at 135,000 light-years in size. I measure NGC 3750 at 140,000 light-years, NGC 3751 at 145,000 light-years and NGC 3745 at 90,000 light-years including the faint plume, mainly to the east. Omitting the plume I get 40,000 light-years making it the smallest of the 7.

The asteroid to the lower left (southeast) is (30376) 2000 JE65 at magnitude 17.7.

Arp\'s image: http://ned.ipac.caltech.edu/level5/Arp/Figures/big_arp320.jpeg

14\" LX200R @ f/10, L=4x10\' RGB=2x10\'x3, STL-11000XM, Paramount ME