NGC-7217 - A SPHEROID-DOMINATED, EARLY-TYPE RESONANCE RING SPIRAL GALAXY

NGC 7217 is a well-known northern spiral galaxy which is characterized by flocculent spiral structure and a series of three optical ringlike zones: a nuclear ring 21 '' in diameter, a weak inner ring 63 '' in d iameter, and a striking outer ring 2.'6 in diameter. The rings all hav e nearly the same shape and position angle in projection. The appearan ce of the galaxy suggests that it may be more axisymmetric than the ty pical spiral galaxy, since there is little evidence for the presence o f a bar, oval, or stellar density wave. This makes the origin of the r ing features uncertain. In an effort to understand this kind of ringed galaxy, which is by no means typical, we have obtained multicolor CCD BVRI images, accurate surface photometry, mappings of the CO and HI g as distributions, and rotational velocities from H alpha and HI spectr al line data. Our deep surface photometry has revealed an important fe ature of NGC 7217 that was missed in previous studies: The region occu pied by the rings of the galaxy is surrounded-by an extensive, nearly circular luminous halo. This halo cannot be merely an extension of the disk component because it is much rounder than the inner regions. Ins tead, we believe the light represents either the outer regions of the bulge or a separate stellar halo component. We are able to successfull y model the luminosity profile in terms of an r(1/4) ''spheroid'' and an exponential disk with a spheroid-to-total disk (including rings) lu minosity ratio of 2.3-2.4. This makes NGC 7217 one of the most spheroi d-dominated spirals known, and the finding has important implications for the recent discovery by Merrifield and Kuijken of a significant po pulation of counter-rotating stars in the galaxy. Although the spiral structure of NGC 7217 is flocculent in blue light, there is a definite two-armed stellar spiral in the region of the outer ring. This ring i ncludes about 4.4% of the total blue luminosity and is the locus of mo st of the recent star formation in the galaxy. The ring is also where we find the HI gas to be concentrated. The galaxy is very gas poor (M( HI) = L(B)(0) = 0.024 M.IL.,(B)) for its morphological type. The HI ro tational velocities agree well with published and our new H alpha-valu es. Fourier analysis reveals a very weak possible oval distortion in t he stellar mass distribution. Using the I-band light distribution to d efine the potential, we carried out simulations of gas streaming with no self-gravity. A model with a bulge-to-disk mass ratio of 2.4 reprod uces the observed optical ring morphology very well. This suggests to us that in spite of the extreme weakness of the observed nonaxisymmetr y of this galaxy, this nonaxisymmetry is still sufficient to torque th e gas into the usual resonance rings identified in other, more obvious ly barred galaxies. An additional noteworthy feature that we have iden tified in a B-I color index map is a symmetric, nuclear dust ring 17 ' ' in angular diameter. Other dust lanes are seen mainly on the near si de of the galaxy.