Retrieved 2018-03-31. ↑ "Revised NGC Data for NGC 4638". spider.seds.org. Retrieved 2018-03-31. ↑ "New General Catalog Objects: NGC 4600 - 4649". cseligman.com. Retrieved 2018-03-31. ↑ Binggeli, B.; Sandage, A.; Tammann, G. A. (1985-09-01).