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NGC 7217: A Spheroid-dominated, Early-Type Resonance Ring Spiral Galaxy
Buta, R.; van Driel, W.; Braine, J.; Combes, F.; Wakamatsu, K.; Sofue, Y.; Tomita, A.
Astrophysical Journal v.450, p.593 (ApJ Homepage)
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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 diameter, and a striking outer ring 2'.6 in diameter. The rings all have nearly the same shape and position angle in projection. The appearance of the galaxy suggests that it may be more axisymmetric than the typical spiral galaxy, since there is little evidence for the presence of a bar, oval, or stellar density wave. This makes the origin of the ring 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 H I gas distributions, and rotational velocities from Halpha and H I spectral line data.

Our deep surface photometry has revealed an important feature of NGC 7217 that was missed in previous studies: The region occupied 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. Instead, we believe the light represents either the outer regions of the bulge or a separate stellar halo component. We are able to successfully model the luminosity profile in terms of an r114 "spheroid" and an exponential disk with a spheroid-to-total disk (including rings) luminosity ratio of 2.3-2.4. This makes NGC 7217 one of the most spheroid-dominated spirals known, and the finding has important implications for the recent discovery by Merrifield and Kuijken of a significant population 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 includes about 4.4% of the total blue luminosity and is the locus of most of the recent star formation in the galaxy. The ring is also where we find the H I gas to be concentrated. The galaxy is very gas poor (MH I/L0B = 0.024 Msun/Lsun, B for its morphological type. The H I rotational velocities agree well with published and our new Halpha-values.

Fourier analysis reveals a very weak possible oval distortion in the stellar mass distribution. Using the I-band light distribution to define 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 reproduces the observed optical ring morphology very well. This suggests to us that in spite of the extreme weakness of the observed nonaxisymmetry of this galaxy, this nonaxisymmetry is still sufficient to torque the gas into the usual resonance rings identified in other, more obviously barred galaxies.

An additional noteworthy feature that we have identified 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 side of the galaxy.

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