THE 3 RINGS OF THE ISOLATED GALAXY NGC-7217

We present WSRT HI line observations, together with CCD-BVRI photometry, of NGC 7217, which is known to be an isolated galaxy with an inner ring, an inner pseudoring and an outer ring, but for which no clear bi-symmetric distortion is immeadiately apparent. Assuming, as is known to be the case for barred galaxies, that the outer ring corresponds to the outer Lindblad resonance, we have derived the expected locations for the other resonances using a combined optical/H I rotation curve. Our result is that the observed inner ring coincides with the inner Lindblad resonance and the inner pseudoring with the ultraharmonic (4:1) resonance. The associated pattern speed is 86.0 km s(-1) kpc(-1) However, it is less clear which feature is actually setting up this pattern. The outer ring, which has a size of similar or equal to 6.3 x 5.9 kpc, contains roughly two-thirds of the total H I mass, and has bluer colours and more intense Her emission than the main disk. A Fourier analysis of the B - I colour along this ring suggests that it is composed of 9 blobs, indicating the existence of a bead instability. This is in agreement with a simple calculation showing that the number of Jeans lengths along the ring is also 9, and that self-gravity is probably important here. Clumps also exist in the inner pseudoring, but they are less well defined, and there is no HI concentration along it. This ring has redder colours than the outer ring. The blue inner ring is incomplete, coincides with a complete and intense Ha ring, and is surrounded by a redder ring. A spiral-like structure extends from the inner ring out to the inner pseudoring, with the same winding direction as the outer flocculent arms. We have constructed a mass model, from which we obtain a mass-to-I-band luminosity ratio of 5.1 for the bulge, and 1.8 for the disk. The core radius of the halo is 11.0 kpc, and its central density 0.062 Mo pc(-3). The ratio of halo core radius to optical radius is thus of order unity.