Previously Known Gas Rings

HI rings are not common. Briggs (1982) reported the discovery of an elliptical ring of gas enclosing the otherwise normal spiral galaxy NGC 628. Unlike the gas in the LGG 138 ring, the gas in NGC 628 extends from around three times the optical diameter all the way in to the optical bulge. The velocity contours of the gas imaged by Briggs (1982) and Shostak & van der Kruit (1984) suggested the HI to be distributed in two different orbital planes, with an outer ring of HI inclined at with respect to the inner HI disk. The total HI mass of the disk and ring appears to be

. Like the LGG 138 ring, NGC 628 is embedded in a group of galaxies, containing three nearby dwarf galaxies. Whilst their luminosities and hence masses indicate that they would have little effect on NGC 628 even in a close encounter (Briggs, 1982), it is conceivable that the pair of galaxies UGC 1171 and 1176 were nearby NGC 628 of order 1 Gyr in the past. More recent VLA observations of the system (Kamphuis & Briggs, 1992) have studied the high velocity cloud complexes to the southwest of the galaxy, and an extended HI arc which may be the tidal remains of an HI-rich object that has merged into the outer disk. Kamphuis & Briggs (1992) conclude that the galaxy is well modeled by an extended warped disk, and that the HI structure seen in NGC 628 is most likely that of a large isolated system settling towards a flattened disk following many massive accretion events.

An HI ring that is perhaps more comparable to the LGG 138 ring was found in the M96 region by Schneider (1985) in follow-up observations of an intergalactic cloud found serendipitously by Schneider et al. (1983). The M96 ring has an HI mass of order . However, the rotation period of the M96 ring is over ten times that of the LGG 138 ring, being . Using deep Arecibo observations, Schneider (1985) determined that the ring was composed of several clumps of HI rather than a single contiguous cloud of HI. The broken ring has projected within it NGC 3379/3384/3389 and the faint dwarf galaxy Leo dw A, and extends across

kpc on the sky at its greatest extent, roughly five times the projected size of the LGG 138 ring. It is the lack of neutral hydrogen detected in the galaxies interior to the projected M96 ring which make a comparison with the LGG 138 ring worthwhile. Furthermore, both rings exhibit an HI mass typical of that bound to an early type spiral galaxy, and very low, if any, luminosity is associated directly with the ring. Even though the M96 ring is disjoint, its regular structure suggests it has probably existed for at least one full revolution, and more likely a few full revolutions ( Gyr; Schneider, 1989).

A third example of an HI ring is the flat ring surrounding the elliptical galaxy IC 2006. A faint ring of stars around this galaxy prompted Schweizer et al. (1989) to obtain VLA images of the HI emission associated with the galaxy, in the hope that the discovery of an HI ring or disk would lead to the first confident determination of the mass to light ratio of an elliptical galaxy. The ring has a radius of at a distance of , and an HI content of at least . Under the assumption that the ring is intrinsically flat and circular, kinematical models show the system to have a dark halo containing of order twice as much mass as the luminous matter indicates to be enclosed by the ring. Schweizer et al. (1989) suggest that the gas in the ring has been gradually accreted from the companion galaxy (ESO 359- G005 at a projected distance of ), or instead is the result of a Spitzer-Baade, or gas-sweeping, collision. For the latter proposal, the ring would be in a state of expansion at present, and consequently the mass to light ratio determined by Schweizer et al. (1989) would be over-estimated. Franx et al. (1994) present new HI data for the ring, and detect HI smoothly distributed around the entire ring, ie. it is not broken. Analysis of the (very regular) velocity structure of the ring shows that the galaxy is embedded in a nearly axisymmetric massive halo.

The IC 2006 ring bears a remarkable similarity to the LGG 138 ring: the radii of the two rings, their estimated rotation speeds, and their narrow annular widths are comparable. Furthermore, while the LGG 138 ring contains nearly ten times as much neutral gas as IC 2006, the HI mass to blue luminosity ratios of the two systems are nearly identical, and both rings have tenuous luminous tails, or even spiral arms, extending into the HI ring.

© Copyright Astronomical Society of Australia 1997