The Story of Hoag's Object
by Ray A. Lucas (STScI)

It was 1950 when astronomer Art Hoag published the first description of this object which has come to be named after him. Although he thought it most likely resembled a planetary nebula in some ways, he was skeptical because the size of the object and the properties of its nucleus in comparison to the rest of the object were not typical of planetary nebulae. And, unlike the outer shell of gas or so-called "planetary" part of planetary nebulae, the "halo" as he termed it, was not glowing with light emitted at specific wavelengths typical of atomic elements normally found in planetary nebulae. Also, the object was at a higher galactic latitude than the typical location of most known planetary nebulae in our galaxy. Therefore, he thought that it was perhaps some new example of a "pathological" galaxy.

Though he thought it possible that there was some optical diffraction effect or perhaps even a gravitational lens system on display, he estimated that the mass would have to be much greater than the mass of a normal galaxy, and therefore a gravitational lens was very unlikely. Other astronomers calculated that the grain size would also have to be very unusual compared to other examples known in nature if this was an optical diffraction effect like a ring around the moon.

In 1974, Bob O'Connell and colleagues studied the object further, using spectroscopy to find a distance for it very similar to our present estimate of 600 million light years, and estimating the size of the object. They estimated a diameter of 5 kiloparsecs or kpc (~16,000 l.y.) for the core (though later observations by Schweitzer et al. showed that there was luminous material in the apparent gap between the "core" and the ring which extended as far outward as the ring and probably almost as far as the outer part of the ring), and they estimated the inner and outer diameters of the ring as 23 kpc (~75,000 l.y.) and 37 kpc (~121,000 l.y.). So, it was then indeed found to be a peculiar or "pathological" galaxy rather than an object like a planetary nebula within our galaxy.

Noah Brosch studied the object both optically and at radio wavelengths to look for neutral hydrogen, but he did not detect it in the radio, nor did he detect the kind of optical emission lines which would be important for signifying star formation in the bluish ring. Although Brosch wondered if the ring might be one which could be formed at the end of a central bar in a spiral galaxy, no bar was found. Brosch summarized his work and that of others for the public and gave a good accounting of the state of understanding of Hoag's Object at that time in the November/December 1987 issue of Mercury, a publication of the Astronomical Society of the Pacific. In particular, he also detailed work of that same year by Francois Schweizer and colleagues.

Notably, Schweizer et al., with additional observations from Rogier Windhorst and David Koo, had used the Palomar 200-inch (5-meter) telescope in conjunction with the 4-Shooter camera (a predecessor of HST's WFPC-I) to image Hoag's Object in several colors, had taken spectra of the object with a spectrograph on the 5m telescope, and had also used the Arecibo radio telescope to look once again for the signature of neutral hydrogen gas. This time, with the use of the larger telescopes and new instruments, and with a wider radio beam, optical emission lines of a type associated with star formation (Hydrogen-Beta and Oxygen III at 5007 Angstroms) had been detected in the spectrum of the ring, and neutral hydrogen had been detected as well, perhaps beyond or outside the ring itself. These facts indicated that there probably is active star formation going on in the ring of Hoag's Object. The distance to the object would not allow individual HII regions or small clusters of very hot stars to be visible, rather the knotty bluish appearance of the rings would more likely be due to supermassive clusters of younger, hotter stars burning brightly in the ultraviolet and blue portions of the spectrum. In their paper in the 15 September, 1987 Astrophysical Journal, Schweizer and colleagues postulated that Hoag's Object is a product of the accretion of another galaxy some 2-3 billion years ago, with a spheroidal central galaxy within the ring. They said that the fact that there are no evident tidal tails etc. from such an interaction argues for the time scale of the interaction or accretion having been 2-3 billion years ago.

However, even with their better data, Schweizer et al. could not determine for sure the exact nature of the central galaxy inside the ring. More specifically, if Hoag's Object is the result of an accretion event in which the central galaxy's gravitational field captured and shredded a passing or companion galaxy, then it may be related to polar ring galaxies, although we are not sure of the relative orientation of the rotation axes of the central galaxy and the ring. If in the central galaxy we are seeing an elliptical or bulge-dominated spiral galaxy like an S0 from its polar axis, then the ring would be equatorial rather than polar, although some evidence also suggests that it could be inclined at up to 25 degrees or less to the plane of the sky and the equatorial plane of the central galaxy. This rotation in or near the equatorial plane of the central galaxy would make it more kin to galaxies known as dust-lane ellipticals. If, on the other hand, we are seeing an elliptical galaxy from more or less its equatorial plane, the central galaxy would have to be of a very spherical type - an E0 galaxy, and the ring would then most likely be a polar ring, orbiting somewhere in the range of 75 to 90 degrees with respect to the central galaxy's polar axis of rotation, or in other words, nearly orthogonal to it. Schweizer et al. also noted that the very brightest knots in the ring are not in an exactly centered circular pattern, but appear to be somewhat off-center, a bit like a celestial hula hoop, nearer the outer edge of the ring in the west and the inner edge of the ring in the east, a fact which may also help to eventually suggest some clues to help solve the mystery that is Hoag's Object.

The key facts from Schweizer's study are that there is luminous material in the apparent gap between the nucleus and the ring, that the core or nucleus is a normal spheroidal galaxy with a normal radial light profile for its type (a so-called r^1/4 profile), that the nucleus and ring have essentially the same recessional velocity or redshift and are therefore physically associated, that there is evidence of star formation in the ring, and that the likelihood of this object resulting from an accretion event and having the geometry of something like or related to a polar ring galaxy or perhaps an elliptical galaxy with a luminous stellar ring is quite high, although a study published in 1986 by Y. Taniguchi and collaborators found no polar ring galaxies with face-on rings, which may mean that ellipticals with luminous stellar rings, such as Hoag's Object may be, are possibly even more rare, as pointed out later in an article published in 1990 by Ken-ichi Wakamatsu.

For such reasons, when Brad Whitmore and myself and other colleagues were putting together a catalog of known polar ring galaxies and other possibly related objects in the late 1980s, we included Hoag's Object in our list of category D objects. For classification as a definite polar ring, there had to be firm spectroscopic evidence of objects having components rotating in 2 orthogonal planes. Categories B and C were composed of objects which looked more like polar ring galaxies, but which lacked the conclusive spectroscopic evidence, and category D was a broader category which incorporated more objects of varied types which were possiby related to polar rings. It was due to its somewhat ambiguous nature based on what was known at the time that we placed Hoag's Object in our category D list when we published our polar ring catalog in November 1990. We also included the likely case that such rings could be kinematically related to the so-called "Magellanic Stream" of material orbiting our own Milky Way galaxy. The Magellanic Stream appears to be related to the Magellanic Clouds, two smaller, nearby satellite galaxies of the Milky Way.

Empty Rings and Displaced Nuclei of Ring Galaxy AM0644-741

In earlier days, most galaxies were not considered "pathological", but were seen as being of certain well-defined morphological types, and our picture of the universe was of a somewhat less dynamic, less interactive place when we thought of galaxies. It was generally thought that galaxies formed and evolved in "splendid isolation". Today, with the sharper vision of instruments such as the Hubble Space Telescope and other telescopes in space and on the ground, we know that the universe of galaxies is a very dynamic place, with many, if not perhaps most, galaxies undergoing some interaction or merging activity within their lifetime. To paraphrase or quote another astronomer who has spent a lifetime studying galaxies, Vera Rubin, "galaxies are a bit like people. They may look normal enough at first sight, but when you get to know them better, they're all a little different and strange in some way!" With HST, the existence and nature of massive, young, hot blue star clusters has become easier to detect and study in greater detail over the spatial extent of galaxies, allowing us to map the concentration and location of such clusters as an aid to our attempts to understand the dynamics and formation mechanisms for such clusters and their relation to the possible history of interactions and mergers or accretions for any galaxy. Despite its greater distance and the consequent fact that we can only detect the most massive star clusters in the ring, such new understanding is possible at some level for Hoag's Object as well, since we can now map, with better resolution, the detailed spatial distribution of these very massive star clusters.

There are a small number of other galaxies which appear similar to Hoag's Object, but it remains the prototype, and still appears rare from our perspective in at least one important way. Although most rings in galaxies appear to be the result of a dynamical process within those galaxies, if we are considering the possibility of a dynamical ring formation as opposed to the collisional or accretion hypotheses, there are other galaxies with bright, seemingly detached rings, but their central components almost always appear elongated and are most likely barred, according to Schweizer et al. They identified only a few more objects which appeared not to have a central bar. Wakamatsu's study, published in 1990, found that, of known similar objects, in fact, Hoag's Object is the only one with a very round central component. All of the others have central objects with an oval profile. Wakamatsu pointed out that Ron Buta, a researcher who has studied the dynamical ring phenomena in galaxies extensively, has said that even an oval central profile rather than a bar should be enough to lead to formation of a ring in many cases, but the case of Hoag's Object itself is still more mysterious and unusual since the central component appears so round.

Also in 1990, C. Horellou and colleagues published a study of a number of ring galaxies in which they attempted to detect CO emission. They did not detect Hoag's Object. This suggests that any gas is diffuse and spread over a much wider area than the beam of the telescope, much as Brosch's earlier finding when he could not detect Hoag's Object with the Westerbork Synthesis Radio Telescope (WSRT) in Holland. As Horellou et al. pointed out, this would also fit the model proposed by Schweizer et al., that the ring is in fact an accreted disk made of material from another galaxy which may have come too near the central (spheroidal) galaxy in Hoag's Object, and which may have been subsequently ripped apart, captured, and kept in orbital "limbo" around the central object as new, bluish star clusters were formed from the available, diffuse, neutral hydrogen gas in the wake of the interaction. If such is the case, Hoag's Object may indeed be much more like the polar ring galaxies and dust-lane ellipticals or ellipticals with a stellar ring (although we're not sure of the exact shape and rotation of the central galaxy) than some other kinds of ring galaxies such the Cartwheel Galaxy, which is another different kind of ring galaxy - a collisional ring galaxy, in which one galaxy may have scored a more direct "hit" on another galaxy or giant gas cloud and may have splashed through it, shocking and compressing the gas to form a star-forming ring which is expanding outward like ripples from a pebble thrown into a pool of water.

V.P. Reshetnikov and collaborators also observed Hoag's Object and NGC 6028 (another Hoag-like galaxy) along with a sample of polar ring galaxies and found the photometric properties of the rings of the two galaxies to be in close agreement with those of polar rings. Although some famous collisional ring galaxies such as the Cartwheel were also not detected in CO by Horellou et al., there are other factors which argue against Hoag's Object being a collisional ring galaxy, not least of which is that there seems to be no nearby companion which might have been the culprit in an event of that sort. So, it seems most likely today that Hoag's Object may indeed have been formed by a kind of grazing or disruptive interaction in which a passing galaxy was shredded and its material captured in orbit around the central spheroidal galaxy of Hoag's Object. But, we still need better dynamical (spectroscopic) information to more clearly understand the exact nature of this central spheroidal galaxy and more details of its inclination, dynamics, and rotation, etc.

Cartwheel Galaxy

Other questions remain. Is the ring really 2-3 billion years old? If so, would we expect to see a larger number of reddish star clusters in the ring instead of just so many bluish ones? (In fact, the stars in the ring of Hoag's Object are not incredibly blue by astronomical standards, although they are definitely much bluer than the central spheroidal galaxy which is much redder in comparison to the ring. The central spheroidal galaxy is of a very typical color for galaxies of its kind. The stars in Hoag's Object's ring are a little less blue than the least blue stars in the Cartwheel's ring, and much less blue than the bluest stars in the Cartwheel ring, some of which are probably on the leading edge of the expanding shock wave in the Cartwheel, and are extremely blue. This means that the bulk of the bluish stars in the ring of Hoag's Object are probably somewhat older than even the oldest stars in the Cartwheel's ring. But we're still likely talking about the difference between somewhat young and somewhat blue and extremely young and extremely blue, rather than red, so our question above remains.) What could make the ring appear to form so well with such apparently clean and relatively sharp edges so far from the center of the light profile of the central galaxy? Wakamatsu discussed related issues a bit more in his 1990 paper and made some of the following arguments. He said that ellipticals with luminous stellar rings and so-called dust-lane ellipticals both exist, although the former may be very rare, since no examples with a face-on view have been found, even among polar rings. In the case of polar rings, the stellar ring and the dust lane are in the same plane, and Wakamatsu wondered what could cause the difference between them if both were formed by accretion events. He suggested that accretions onto ellipticals might differ from accretions onto S0 galaxies which tend to lack gas, and that differences in the shape of the gravitational potentials might play a crucial role, as might the existence or lack of any hot X-ray-emitting gas. Wakamatsu also argued that a cold gas accreting onto a hot gas around an elliptical galaxy might cause some kind of strong interaction as opposed to the accretion of stars, and that accreting gas might interact with hot X-ray-emitting gas by forming shock waves, causing the accreting gas to "lose its angular momentum and kinetic energy and form a thick gaseous ring with filamentary structures," and that "some unknown mechanisms may prevent or delay extensive star formation in the gaseous ring." These galaxies, he suggested, could be considered dust lane ellipticals. He also stated that "if the cold accreting gas has fallen onto an elliptical galaxy with a specific angular momentum large enough to rotate at a large distance from the nucleus, it can be neither heated up nor destroyed without interacting with the hot X-ray-emitting gas," and that "if subsequent star formation occurs in this rotating ring, it may be identified as an elliptical galaxy with a luminous stellar ring, as in the case of Hoag's Object." He concluded that "the stellar rings around elliptical galaxies should have large diameters in order to avoid interaction with the hot X-ray-emitting gas," and that "detailed theoretical studies of the interactions of accreting gas with the hot X-ray-emitting gas around ellipticals are urgently needed." Last, Wakamatsu also pointed out that several polar ring galaxies which supposedly formed 2-3 billion years ago still show signs of tidal structures related to the interaction, with the implication being that we should still see some sign of this in the case of Hoag's Object as well, even if, in fact, the interaction took place 2-3 billion years ago. So, as Wakamatsu summarized it: "the accretion hypothesis certainly seems attractive; however there remain problems to be solved."

This new image from the Hubble Heritage project is displayed in such a way as to emphasize the differences between the nucleus and the ring, and to allow the display of faint reddish objects in the apparent gap. Due to the distance of this galaxy, if the accretion hypothesis of Schweitzer et al. is true, although there may still be some individual globular clusters among the remains of an earlier (now shredded) interloper galaxy in orbit around the central component, we are undoubtedly not seeing objects that small even in this latest Hubble Heritage image. May the small, faint, reddish objects which are visible in the vicinity of the inner component perhaps be old massive clusters of stars which were remnants of the original stellar population, or perhaps massive superclusters of older stars that are remnants of the earlier stages of an accretion event as mentioned above? As far as the ring itself, what we are seeing are most likely massive clusters or superclusters of younger, hotter, bluer stars which were formed in the swirl of neutral hydrogen gas that may have been accreted from the hypothetical passing galaxy which was torn apart in the same event, as mentioned above. It is also interesting to note that some other objects which are likely much more distant can be seen in the area between the central galaxy and the ring, and even perhaps through the ring itself. There is even one object which appears almost as if another Hoag-type object - a reddish central object surrounded by a bluish ring, with no evidence of more structure outside the ring, although it also appears a bit different in that its central component seems somewhat elongated rather than round. Could we be seeing another more distant Hoag-type galaxy through the central region of the prototype? Its hard to tell for sure, but that would seem to be quite a cosmic irony and also an unexpected and very unlikely find indeed! Schweitzer et al. estimated that the fraction of Hoag-type galaxies among all galaxies should be only about 0.001. Therefore, it is most likely not another Hoag-like object.

Like many new images of celestial objects made in ways which were not possible before, this Hubble Space Telescope WFPC2 image provides new clues and also raises more questions, since some important details of Hoag's Object and its formation still remain a mystery. And that is a big part of the fascination of science!

Here is a selected bibliography of some articles related to Hoag's Object. Most are journal articles in professional astronomy publications, but the Mercury article is one written for the general public which outlines an understanding of the object as of 1987, the year it was written.

Brosch, N., 1985, A&A 153, 199
Brosch, N., 1987, Mercury Vol. XVI, Number 6 (Nov/Dec), 174
Buta, R., 1986, ApJ Suppl. 61, 609
Hoag, A.A., 1950, AJ 55, 170
Horellou, C., Casoli, F., Combes, F., and Dupraz, C., 1995, A&A 298, 743
O'Connell, R.W., Scargle, J.D., and Sargent, W.L.W. 1974, ApJ 191, 61
Reshetnikov, V.P., Hagen-Thorn, V.A., and Yakovleva, V.A., 1995, A&A 303, 398
Schweizer, F., Ford, W.K., Jr., Jedrzejewski, R., and Giovanelli, R., 1987, ApJ, 320, 454
Taniguchi, Y., Shibata, K., and Wakamatsu, K., App. Space Science, 118, 529
Whitmore, B.C., McElroy, D.B., and Schweizer, F., 1987a, ApJ 314, 439
Whitmore, B.C., Lucas, R.A., McElroy, D.B., Steiman-Cameron, T.Y., Sackett, P.D., and Olling, R.P., 1990, AJ 100, 1489 (PRC = Polar Ring Galaxy Catalog)
Wakamatsu, K., 1990, ApJ 348, 448