ARP 281/NGC 4631-4627 is the first of 6 entries under his category for infall and attraction. The two NGC galaxies that make up Arp 281 are in the process of merging and are gravitationally bound. Nearby NGC 4656 is also gravitationally bound to this system and likely interacted with it hundreds of million years ago by most accounts. The system is likely about 25 million light-years distant though I found no good consensus on its distance. Redshift, unreliable this close, says 40 million light-years. Tip of the red branch estimates at NED show a distance of about 25 million light-years while Tully fisher measurements show a closer distance of about 16 million light-years. Some papers say 30 million. Non redshift measurements of NGC 4627 gives a distance of 30 million light-years compared to 19 million for all for NGC 4631. Balancing these and others I decided on 25 million light-years which gives a size of NGC 4631 of about 116,0000 light-years. Some sources say it is a starburst galaxy but NED didn't note this in its classification. Still it seems quite likely after its interaction with two other NGC galaxies. NGC 4627 is 17,000 light-years in size including its plumes using the same distance.

The pair was discovered by William Herschel on March 20, 1787. NGC 4631 is in the original Herschel 400 observing program. My comments using a 12.5" f/6 scope at up to 150x on an excellent night in the New Mexico on April 27, 1984 desert reads: "WOW! edge on with many faint knots but no dust lane, Round companion appears larger than in photos but maybe I'm not seeing the full length with NGC 4631. In same low power field with NGC 4656." Arp's comment: "Knots resolved with 48-inch. Diffuse counter tail on companion."

NGC 4631 is often called the Whale Galaxy though I've also heard it called the submarine galaxy. The annotated image lists all galaxies that NED had redshift data on.

Arp's image: http://ned.ipac.caltech.edu/level5/Arp/Figures/big_arp281.jpeg

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