Archive:Hoag's Object

Hoag's Object is a non-typical galaxy of the type known as a ring galaxy. The sheer beauty of this object has riveted amateur astronomers as much as its uncommon structure has fascinated professionals. The galaxy was discovered in 1950 by astronomer Art Hoag.

Characteristics

A nearly perfect ring of young hot blue stars circle the older yellow nucleus of this ring galaxy ~600 million light-years away in the constellation Serpens. The diameter of the 6″ inner core of the galaxy is about 17 ± 0.7 kly (5.3 ± 0.2 kpc) while the surrounding ring has an inner 28″ diameter of 75 ± 3 (24.8 ± 1.1 kpc) and an outer 45″ diameter of 121 ± 4 kly (39.9 ± 1.7 kpc), which is slightly larger than the Milky Way Galaxy. The gap separating the two stellar populations may contain some star clusters that are almost too faint to see. As rare as this type of galaxy is, oddly another ring galaxy can be seen, between the nucleus and the outer ring at the 010 position.

History and formation

Ironically, 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, nor the Catalogue of Galactic Planetary Nebulae.

In the initial announcement of his discovery, Art Hoag proposed the hypothesis that the visible ring was a product of gravitational lensing. This idea was later discarded due to the fact that the nucleus and the ring have the same redshift, and because more advanced telescopes revealed the knotty structure of the ring, something that would not be visible if the ring were the product of gravitational lensing.

Many of the details of the galaxy remain a mystery, 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. This collision produces a density wave in the disk which leads to a characteristic ring-like appearance. However, there is no sign of any second galaxy that would have acted as the "bullet", and the core of Hoag's Object has a very low velocity relative to the ring, making the typical formation hypothesis quite unlikely.

It has been suggested that Hoag's Object might be a product of an extreme "bar instability" which occurred a few billion years ago in a barred spiral galaxy. Schweizer, et. al. (1987) claim that this is an unlikely hypothesis due to the fact that 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. Interestingly, a few galaxies share the primary characteristics of Hoag's Object, including a bright detached ring of stars, but their centers are elongated or barred, and they may exhibit some spiral structure. While none match Hoag's Object in symmetry, this handful of galaxies are known to some as Hoag-type galaxies.

While there is much still to be explained about the formation of Hoag's Object, it is suggested that a major "accretion event", in other words a collision or a capturing of one galaxy by another, caused the formation of the galaxy we see today. The event likely took place at least 2-3 billion years in the past, and may have resembled the processes that form polar-ring galaxies.

External links

• Hubble Space Telescope image of Hoag's Object.
• http://nedwww.ipac.caltech.edu/level5/Rings/Rings17_2.html
• http://oposite.stsci.edu/pubinfo/PR/2002/21/index.html

References

• Hoag, A. A. (1950), AJ, 55, 170
• Schweizer, Francois; Ford, W. Kent Jr.; Jedrzejewski, Robert; and Giovanelli, Riccardo (1987). The structure and evolution of Hoag's Object. The Astrophysical Journal 320, 454-463.