Editing Hoag's Object (section)

==History and formation==

Even though Hoag's Object was clearly shown on the [[Palomar Star Survey]], it was not included in either the ''[[Morphological Catalogue of Galaxies]]'', the ''[[Catalogue of Galaxies and Clusters of Galaxies]]'', or the ''[[catalogue of galactic planetary nebulae]]''.<ref name="OConnelletal1974" />

In the initial announcement of his discovery, Hoag proposed the hypothesis that the visible ring was a product of [[gravitational lensing]].  This idea was later discarded because the nucleus and the ring have the same [[redshift]], and because more advanced telescopes revealed the ring's knotty structure, which would not be visible if the galaxy were a product of gravitational lensing.<ref name="Schweizeretal1987">{{Cite Q|Q56113478}}</ref>

Many of the galaxy's details remain mysterious, foremost of which is how it formed. So-called "classic" ring galaxies are generally formed by the collision of a small galaxy with a larger disk-shaped galaxy, producing a density wave in the disk that leads to a characteristic ring-like appearance. Such an event would have happened at least 2–3 billion years ago,<ref name="Schweizeretal1987" /> and may have resembled the processes that form [[polar-ring galaxy|polar-ring galaxies]]. However, there is no sign of any second galaxy that would have acted as the "bullet", and the likely older core of Hoag's Object has a very low velocity relative to the ring, making the typical formation hypothesis implausible.<ref name="Schweizeretal1987" /> Observations with one of the most sensitive telescopes have also failed to uncover any faint galaxy fragments that should be observable in a collision scenario. However, a team of scientists that analyzes the galaxy admits that "if the carnage happened more than 3 billion years ago, there might not be any [[detritus]] left to see."<ref>{{Cite web|url=https://www.newscientist.com/article/dn20885-astrophile-saturn-lookalike-galaxy-has-a-murky-past/|title=Astrophile: Saturn-lookalike galaxy has a murky past|website=www.newscientist.com|access-date=2019-10-31}}</ref>

Noah Brosch suggested that Hoag's Object might be a product of an extreme "bar instability" that occurred a few billion years ago in a [[barred spiral galaxy]].<ref name="Brosch1985">{{Cite Q|Q68206828}}</ref> Schweizer ''et al''<ref name="Schweizeretal1987" /> claim this is an unlikely hypothesis because the nucleus of the object is [[spheroidal]], whereas the nucleus of a barred spiral galaxy is disc-shaped, among other reasons. However, they admit evidence is somewhat thin for this particular dispute to be settled satisfactorily.