Identification of an Extensive Luminous Halo Around the Ringed Spiral Galaxy NGC 7217

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Session 79 -- Structure of Galaxies
Display presentation, Friday, January 14, 9:30-6:45, Salons I/II Room (Crystal Gateway)

[79.04] Identification of an Extensive Luminous Halo Around the Ringed Spiral Galaxy NGC 7217

R. Buta (U. Alabama), W. van Driel (U. Tokyo), J. Braine (Max-Planck Inst.), and F. Combes (DEMIRM)

The isolated spiral galaxy NGC 7217 is characterized by flocculent spiral structure and three optical ring-like zones: a stellar nuclear ring, a weak inner pseudoring, and a bright patchy outer ring. The rings all have nearly the same shape and position angle in projection. To understand this kind of ringed galaxy, we have obtained deep CCD $BVRI$ surface photometry and mapping of the CO and HI gas distributions and kinematics. Our images reveal something that was missed in previous studies: a large, nearly round halo of light extending far beyond the outer ring. We interpret this as bulge light which comes back to dominate the luminosity distribution at large radii. Ellipse fits to isophotes out to 240$^{\prime\prime}$ radius reveal a minimum axis ratio of 0.83 just outside the outer ring at 90$^{\prime\prime}$, and then a rise to 0.96 at about 140$^{\prime\prime}$. The luminosity profiles are well-fitted by a combined r$^{1\over 4}$ bulge and exponential disk model. In all filters, the bulge dominates at all radii, and the bulge-to-total disk ratio is about 2.3 ($B$). If the minimum axis ratio of 0.83 approximates the apparent flattening of the disk, then NGC 7217 is remarkably axisymmetric. Nevertheless, the $I$-band image reveals a tightly-wrapped, two-armed spiral pattern in the outer ring region. The outer ring includes 4.5\% of the total $B$ luminosity and is the locus of most of the recent star formation in the galaxy; it is also where the HI gas is concentrated. An additional noteworthy feature is a circumnuclear dust ring 1.2 kpc in diameter. Other dust lanes are seen only on the near side of the galaxy.

The rings of NGC 7217 could be resonances with a very weak internal perturbation. We are attempting to simulate the structure using the $I$-band light distribution to help define the potential. But most interesting is the recent discovery of a substantial population of counter-rotating stars in the galaxy (Kuijken 1993, PASP, 105, 1016). One possible explanation for these stars is that the bulge is more important than previous studies indicated. Our surface photometry tentatively supports this idea.

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