Weakly Barred Early-Type Ringed Galaxies. IV. The Double-ringed S0 + Galaxy NGC 7702

Abstract

The southern galaxy NGC 7702 is an interesting example of a late S0 galaxy showing a high-contrast inner ring surrounding a bright bulge, with no conventional bar crossing the ring as in typical SB(r)-type galaxies. It also has a low-contrast outer ring whose peak surface brightness is only ~6% of the night sky brightness in blue light. This paper presents UBVRI surface photometry of NGC 7702 as a means of evaluating its structure and properties with a view toward understanding the origin of the rings and how they relate to problems of internal dynamics. The photometry reveals the typically complex structure of a double-ringed galaxy both in terms of profile properties and color distribution. The inner ring is a subtle blue enhancement compared to its surroundings, and in addition shows azimuthal variations in color and surface brightness, due in part to internal extinction and tilt and also probably to an intrinsically oval shape. Long-exposure spectra taken along the major and minor axes reveal mainly a continuous spectrum in the inner ring region, and average colors are consistent with a feature that may have experienced a burst of star formation less than 2 Gyr ago which abruptly cut off after 10 Myr. The bulge of the galaxy shows a distinctive oval shape and may be characterized as a nuclear bar ~1 kpc in diameter; it is reminiscent of the triaxial bulges observed in some SB galaxies. Fourier analysis also suggests that the inner ring lies near the boundary of an oval distortion or lens. The nonaxisymmetric component (excluding the nuclear oval) contributes about 14% of the total B band luminosity. Although NGC 7702 is not a conventional barred galaxy, the two rings possess many properties in common with barred galaxy rings. It is possible that NGC 7702 once had a stronger, more important bar which lasted long enough to generate the rings but which is now mostly dissolved. The triaxial bulge may be a relic of such a past feature.