SIMBAD references

Near-infrared observations of NGC 2997 and NGC 6951 reveal complete rings of symmetrically placed hot spots, which are star-forming complexes. The masses of the complexes range from a few times 10⁴ to a few times 10⁵ M_☉ in both galaxies. The regular hot-spot spacings of about 200 pc in a ring 55 pc thick in NGC 2997 and 380 pc in a ring 150 pc thick in NGC 6951 are consistent with formation by a large-scale gravitational instability in the rings. Visual extinction is estimated to be up to 3 mag around the ring in NGC 2997 and up to 5 mag in NGC 6951, as dust lanes spiral in and cross the ring. Hubble Space Telescope observations reveal smaller super-star clusters within the large complexes. These clusters have power-law luminosity functions with an exponent of approximately -2 in both galaxy rings, the same as for star-forming complexes in the disks of normal spiral galaxies.