NGC 772 featured here is a "peculiarly" impressive spiral galaxy, with a prominent outer arm that has been drawn out by tidal interactions with a dwarf elliptical galaxy above it (on the right) catalogued as NGC 770. At a distance of about 100 million light-years, the galaxies have a projected separation of about 100,000 light-years. The oddball features of NGC 772 led Halton Arp to include it as the 78th entry in his Atlas of Peculiar Galaxies published in 1966.

The unusual spiral has an unusually large disk about 200,000 light-years in diameter with a fascinating morphology; to my eyes, the disk looks like it has been shredded on the far side. The peculiar arm with its bright blue star-forming regions, the extensive tidal streams spilling out of both galaxies, and the bridge between them, are all tell-tale signs of a past close encounter. The age of the youngest star clusters in the arm has been estimated at about 15 million years, which may give some indication of when the encounter took place.

The peculiar arm and the vaporous tidal streams brought to mind a scene in the classic 1953 Looney Tunes cartoon Bully for Bugs, when Bugs Bunny tells an ornery bull with hot breath: "Stop steamin' up my tail .... What are you trying to do, wrinkle it?!" (the full cartoon is a riot!).

The image spans about 21'x17' at a plate scale of about 0.47"/pixel, and is the result of just over 21 hours of integration, split roughly evenly between luminance and RGB colour, taken over the course of nine nights this month. I threw out an additional 20+ hours of exposures according to an aggressive set of quality cuts, especially on the FWHM of star profiles in the luminance channel, in order to obtain the sharpest and deepest image possible with an acceptable level of noise.

Arp 78 played a part in a curious “controversy” that originated in the 1970s and that today is largely forgotten. This had to do in part with a tidal stream emanating upwards from the left side of NGC 772, which seems to be in line with a small spiral galaxy catalogued as LEDA 212884, as clearly seen in this image. On the basis of that feature alone, one could reasonably infer that the two galaxies are linked, just as a tidal bridge links NGC 772 and NGC 770. However, LEDA 212884 has a redshift about eight times larger than that of NGC 772 (6.6% of the speed of light for the former, compared with 0.8% for the latter) which, according to Hubble’s Law, implies that it is about eight times further away; consequently the galaxies are unrelated, and the apparent tidal link is simply due to a chance alignment in directions.  

The controversy was generated by a small group of astronomers led by Halton Arp (and that included Fred Hoyle, who coined the term "Big Bang"), who argued strenuously that there were many examples of apparent tidal connections between low-redshift galaxies and high-redshift "neighbours" (mainly quasars), which they claimed to be too numerous to be due to chance; if true, then the galaxies in those cases would be at the same distance, and the high-redshift would violate Hubble's Law [†]. This amounted to a direct challenge to Big Bang cosmology. Arp published a huge body of work on this topic, including this 1970 paper on the specific case of Arp 78. I have an earlier post on one of the most dramatic systems that was cited by Arp as evidence for his theory, the quasar Markarian 205, at a distance of about 1 billion light years, according to Hubble's Law, seen through the arms of spiral NGC 4319, which is at a distance of only 80 million light years; Arp claimed that a tidal tail connected the two galaxies (my post includes an image of the system and a measurement of the quasar's spectrum).

Today there is overwhelming evidence for the Big Bang, while large-scale quasar surveys have catalogued more than 750,000 entries, such that there is no doubt that apparent connections between quasars and low-redshift galaxies occur as a matter of chance. Arp devoted much of his career to the redshift controversy, eventually becoming estranged from the broader astrophysical community. Following his death at the end of 2013, Arp was the subject of a moving obituary by Dennis Overbye in the New York Times. He has also been profiled in a popular account of cosmology by Timothy Ferris.

[†] Proponents of the "discordant" redshifts theory argued that the high-redshift galaxies in question had been ejected from their claimed low-redshift partners, so that the large redshifts were not related to the expansion of the universe. But they also had to rule out redshift due to motion through space of the ejected galaxies (since no large blueshifts were found, which would be just as likely for high-speed ejected objects); instead, they invoked some previously unknown and highly-speculative new physics to account for the redshifts. Hope this wordy post makes some sense! 🙃