Above: Arp 220 (a very famous Hubble photograph you probably recognize).
Arp 220 has yet another remarkable and miraculous feature: An On-site Demonstration of the Origin of Galaxies.
This ULIRG is reputed to be one of the most luminous galaxies known and extremely active both optically and in X- rays. Investigation of the brighter X-ray sources immediately around the galaxy have identified quasars and quasar candidates (Arp et al. 2001). Most striking are a pair of quasars shown here in Fig. 7 which are exactly aligned across the central active galaxy. By now many such pairs have been identified across active galaxies and they tend to have similar redshifts. But the striking result in Fig. 7 is that this pair have almost identical redshifts of z = 1.26 and 1.25.
From Universe: The Cosmology Quest
Halton Arp to Fred Hoyle: "Matter of fact, I took this Arp 220 pair into the resident expert on gravitational lenses at Max Planck and, well I didn't show it to him I just asked him, I said, 'Uh, I have 2 quasars that are 15 arc minutes apart with a galaxy in the center, uh, can this be a gravitational lens?' and he said, he started calculating in his head and he said, 'Well no', he said, 'it would have to have a dispersion of 6 or 7 hundred kilometers per second and that would be too much.' And uh, he said, 'that couldn't be a gravitational lens.' And then he said, 'did you say 15 arc minutes? I thought you said 15 arc seconds.'"[Laughs]
Fred Hoyle to Halton Arp: "I was going to say, yeah, yeah yeah, yeah, [laughing] that's right. Yeah yeah obviously not."