How did Hoag’s Object form?
We normally think of galaxies as falling into two main morphological types, spiral and elliptical. Spiral galaxies have spiral arms, which wind about a central nucleus, all embedded in a halo containing stars, globular clusters, and dark matter. Elliptical galaxies are ellipsoidal in shape, appearing oval from all angles; stellar orbits in ellipticals are less ordered and more random. Not all galaxies fit into these categories, of course, and are sometimes classified as lenticular (an intermediate type between spiral and elliptical), peculiar (a type denoting some unusual property) or simply irregular.
Peculiar galaxies are often the most interesting, and certainly the most unique. They usually result from interactions between two galaxies, such as a galactic merger. The result of such an interaction depends on the structure and properties of each component involved, and therefore each interaction results in something new and unexpected.
Hoag’s Object is one of these peculiar galaxies. It is a ring galaxy, consisting of a luminous central core surrounded by a ring of gas and dust where new, young blue stars are forming. There are other ring galaxies; the most famous might be the Cartwheel Galaxy, which evolved to its current state when another, smaller galaxy collided with it. This is expected to be how most ring galaxies form. However, observations over several decades have, it seems, ruled out this possibility in the case of Hoag’s Object.
The galaxy was discovered in 1950, by Art Hoag (Hoag (1950)). He described its appearance as that of a “perfectly spherical” planetary nebula; the near-perfect symmetry was quite surprising. However, Hoag felt that the object wasn’t a nebula, based on an analysis of its size and location. He put forth a few hypotheses:
1. That this was an example of gravitational lensing, which would require a fairly massive object.
2. That this was an effect of the instrument’s optics.
3. That this was a new type of “pathological” galaxies.
Hoag felt that of these, the galaxy solution was the most likely. and further observations by other groups supported this theory. However, an important question remained: How did Hoag’s Object form?
The accretion hypothesis
In 1987, Schweizer et al. attempted to address the problem. They used two telescopes: A 5-meter telescope at the Palomar Observatory for optical photometry and optical spectroscopy, and the 305 radio telescope at Arecibo Observatory, to search for the 21-cm H I line. Their observations established that the ring and core have the same radial velocity and distance, meaning that they are associated with one another. Additionally, they found that both structures rotate in the same direction, another key connection.
This data seems to rule out the collisional hypothesis. Repeated observation showed that there is no other galaxy nearby that could have caused the formation of such a ring. The authors pointed out that the only possible candidate is the core itself — in other words, that the core and ring are the remnants of two separate galaxies — but the nearly identical radial velocities make this impossible.
A different hypothesis (Brosch (1985)) was that the galaxy was originally a barred spiral galaxy, and that an instability in the bar a few billion years ago destroyed the inner structure. However, Schweizer et al. determined that the core was spheroidal, not disk-like (as would be required for such an instability to take place). They also concluded that it was unlikely for the core to have evolved from a disk to a spheroid in the intervening time.
Schweizer et al. put forward a new and novel hypothesis, that Hoag’s Object evolved to its current state after what they termed a “major accretion event”. Essentially, an interaction with another galaxy — though not a merger or collision — could have transferred large amounts of mass to Hoag’s Object. The accretion would most likely result in the formation of a ring, rather than another shape. The lack of any other structural remnants, like tidal tails, puts the time of this event at least 2–3 billion years in the past.
Current open questions
While we have a large selection of ring galaxies, Hoag’s Object still stands apart. One unexplained property is its extreme roundness. The core is likely spheroidal — not even ellipsoidal — and the ring itself is quite circular. This is rather strange, and not what we see in most other ring galaxies. It’s possible that the shape is tied to the formation of Hoag’s Object.
Another open question is the age of the galaxy. Most of the current hypotheses put the disruptive event at 2–3 billion years in the past. Can an analysis of star clusters and gas in the ring give us a better idea of the age? Will they confirm the accretion hypothesis? It’s possible, although it’s also possible that more observations will beget more hypotheses.
