On the Nature of Hoag-Type Galaxy NGC 6028 and Related Objects

Surface photometry has been done on the nearest and brightest Hoag-type galaxy, NGC 6028 (RSAB0^+^;RC2), which is known to be a peculiar galaxy consisting of a luminous core surrounded by a faint, apparently detached ring. Contrary to Hoag's object, the core appears to be not round but elongated (oval), reminiscent of a faint bar embedded in a lens, and it has a shoulder on its luminosity profile. The ring appears as a bump with a peak brightness of μ_B_ = 23.9 mag arcsec^-2^ and fades away to the detection limit with no sharp boundary. There is luminous material between the core and the ring. Giovanelli, Chincarini, and Haynes detected 21 cm H I emission from NGC 6028 with a spectrum of a double-horn profile of a full width of 276 km s^-1^, implying rotation of the ring. These observed properties are consistent with those of early-type barred galaxies with outer rings. Most of the Hoag-type galaxies are also found to have oval-shaped cores. It is suggested that the outer rings of both NGC 6028 and Hoag-type galaxies may be formed through mechanisms related to the oval structures of the central cores. A countereffect of the ring formation relating to a possible dissolution of barred structures is discussed briefly. The accretion hypothesis for the origin of outer rings has been reviewed, and a comment is given on possible interactions between the accreting gas and the hot, X-ray-emitting gas around elliptical galaxies.