3.6. Rings in Bulge-dominated Systems and Hoag-like Galaxies
A recent study of an early-type system that is almost certainly a collisional ring system is the study of AM 1724-622 by Wallin and Struck-Marcell (1994). This pair of galaxies was given the name "The Sacred Mushroom" by Arp and Madore (1987), since the ring forms the cap of the mushroom shape, and the bridge to the highly elongated companion appears as the mushroom's "stalk". In the AM catalog there are a number of such "mushroom-shape" ring galaxies, and northern examples may include Arp 284. Perhaps Arp 148 is also an early stage of the same phenomenon. AM 1724 has a smooth morphology, which led Wallin and Struck-Marcell to speculate that it may be an example of an early-type ring system. This indeed proved to be the case, based on the photometry of the ring. Unfortunately only small parts of the ring could be measured to determine the color of the ring material because of the large number of foreground stars contaminating the galaxy, which lies at a galactic latitude of -15 degrees. Nevertheless, tentative colors for the ring confirmed that its colors are red (U - B = 0.67, B - V = 0.87) compared with the ring sample of Appleton and Marston (1995). Profiles of ring sections suggested sharp-edged caustics, predicted in a purely stellar ring. Ironically, this southern galaxy may be one of the few truly stellar ring systems studied so far which conforms to the models of Lynds and Toomre (1976), being free of the complicating effects of gas.
Of possible relevance to collisional ring galaxies in early type galaxies are galaxies like Hoag's object (Hoag 1950; Schweizer et al. 1987). These galaxies are characterized by having a central nuclear bulge surrounded by a smooth, thick and extremely regular ring, often almost perfectly circular. Optical spectra of Hoag's object taken by O'Connell et al. (1974) showed the galaxy to be dominated by late-type stars in the core, and this is confirmed by the red colors of the galaxy (Brosch 1985). However, more recent observations by Schweizer et al. (1987) show emission lines of H and [OIII] 5007 in the ring and the detection of significant amounts of HI emission from the galaxy. These observations suggest that the ring contains a young population of stars, despite the overall red color of the system. Ever since its discovery, the origin of the almost perfect ring has been controversial. Hoag was unsure of its origin, but suggested that it might be a gravitational lens, although this would require an exceptionally large mass of the central bulge. Brosch (1985) suggested that the ring was produced at a resonance by a weak bar, similar to the ringed galaxies of Buta (1994). However, deep imaging by Schweizer et al. failed to show any evidence for a bar, apparently ruling out the formation of the ring by this mechanism. On the other hand, it is possible that any ring-making bar may have recently dissolved. The authors discuss the possibility that the galaxy is a collisional ring, but do not favor this interpretation because the central bulge has the same radial systemic velocity as the ring. If the central bulge was a small elliptical galaxy caught in the act of penetrating the disk and generating a ring, the authors argue that it should have a high radial velocity compared with the ring. The origin of these galaxies would still appear to be a mystery.
We believe that, although the case for a collisional origin for Hoag's object is not strong, it cannot yet be completely ruled out. For example, it is possible that the argument about the small velocity of the companion relative to the ring can be circumvented if the companion has been strongly decelerated by dynamical friction with a massive halo in the target galaxy and is about to fall back onto the target or has already merged (see Section 3.4 for more discussion of this possibility). This would be consistent with the heavy halo around the galaxy postulated by Schweizer et al. based on the kinematics of the ring. We note that, if the ring in Hoag's object was formed collisionally, its relatively red colors and low ring star formation rates argue for an early-type target disk. Early-type galaxies may naturally contain a more dominant dark-matter component and bulge capable of significantly slowing and eventually absorbing a small intruder galaxy. However, such a scenario, whilst explaining some of the properties of Hoag's object, does require a combination of remarkable circumstances. For example, we would have to be viewing the collision from a special position (exactly along the symmetry-axis of the disk and target trajectory) and at a special time (either just at the moment when it has reached its furthest distance from the target and has zero relative velocity to the ring, or just after it has merged). Since such circumstances would be rare, we suggest that spectroscopic studies of the other Hoag-like galaxies would be worthwhile to increase the statistics on the velocity of the central objects relative to the ring.