NGC 3081: Surface Photometry and Kinematics of a Classic Resonance Ring Barred Galaxy

Abstract

This paper presents a detailed photometric and kinematic study of the well-known Seyfert 2 galaxy NGC 3081, one of the best examples of a resonance ring barred galaxy in the sky. Improved optical images compared to previous studies reveal that NGC 3081 is a classic R_1R´_2 galaxy, a type that shows a distinctive outer ring/pseudoring pattern at large radii that can be linked to orbit families at the outer Lindblad resonance (OLR). Together with an exceptionally strong inner ring and a blue nuclear ring, NGC 3081 has the rare distinction of having all four of the main types of resonance rings that have been predicted by test-particle models of barred spirals. Near-infrared imaging of NGC 3081 reveals clear old rings connected to the inner ring and the R_1 outer ring. Objective comparison of the B- and H-band positions of the inner ring indicates no significant difference in shape, major-axis position angle, or major-axis radius between the two passbands, in spite of the different stellar populations each band emphasizes. Imaging Fabry-Perot interferometry provides an intriguing picture of star formation in the galaxy and of the dynamics of the system. H∝ emission is strong in the inner ring and is confined to a bounded elliptical annulus of diffuse emission whose ellipticity increases from the inner edge to the outer edge. A few H II regions are connected to the strong R_1-type outer ring, particularly just of the major axis of the inner ring where "dimples," typical of the R_1 morphology, are found. There is bright emission in an oval zone in the inner 10" radius, but no enhancement of emission is associated with the blue-light nuclear ring. Since most of the emission is connected with the intrinsically oval inner ring and the bright center, the global dynamics of the galaxy are uncertain. Nevertheless, using a combination of the gravitational potential derived from the H-band light distribution and a simple halo model, we estimate the pattern speed and the location of all major resonances in the disk of NGC 3081. We find that NGC 3081 fits extremely well into recent ideas of barred galaxy dynamics.